[Illustration: FIG. 337.—Wings of a carpenter bee. A, The pair of wings

separated; _a_, position of the hooks: B, the same wings when united by the

hooks. C, Portions of the two wings: _a_, the series of hooks; _b_,

marginal hairs; _c_, portion of edge of front wing, of which the other part

has been broken away in order to show the hooks.]

The studies of the internal anatomy of Hymenoptera are at present by no

means numerous or extensive. The alimentary canal (Fig. 69) possesses a

crop, gizzard, and chylific stomach in addition to the oesophagus and

intestine. The social Hymenoptera have the power of disgorging matter from

the alimentary canal for the purpose of supplying food for their young.

{495}[Illustration: FIG. 338.—Central nervous system (supra-oesophageal

ganglion and ventral chain) of a worker ant, _Camponotus ligniperdus_.

(After Forel.) _a_, Cerebral hemisphere; _b_, primordial cerebral lobe or

pedunculate body (depressed so as to show other parts); _c_, olfactory lobe

(raised from natural position); _d_, nerve to labrum; _e_, antennary nerve;

_f_, scape of antenna; _g_, eye; _h_, optic nerve; _i_, longitudinal

commissures connecting the hidden sub-oesophageal ganglion with _k_, the

prothoracic ganglion; _l_, mesothoracic, _m_, metathoracic ganglion; _s_,

ganglion of the petiole; _n_, nerve from petiole to other part of abdomen;

_r_, _q_, _o_, 2nd, 3rd, 4th abdominal ganglia; _p_, terminal nerve to

cloaca; _t_, bases of legs.]

The crop—which is situated in the anterior part of the abdomen—is the

reservoir that contains this matter. The mode of disgorgement is believed

to be pressure exerted on the crop by contraction of the abdomen. Salivary

glands are present in remarkable variety. The tracheal system possesses, in

the higher winged forms, large saccular dilatations situated at the side of

the abdomen. The nervous system is of peculiar interest on account of the

high intelligence of many of the members of this Order; and on this point

of the anatomy, Brandt[414] has made rather extensive investigations,

having examined it in the adult of seventy-eight species, and in the larva

of twenty-two. In the adult there are two cephalic—the supra- and the

sub-oesophageal—two or three thoracic, and from three to seven abdominal

ganglia. The bees, wasps, and some other of the Aculeata have only two

thoracic ganglia, while some ants have three. The supra-oesophageal

ganglion is very large. The most remarkable fact revealed by Brandt's

investigations is the great difference that exists between the sexes and

the worker caste in the same species. The {496}pedunculate bodies of the

supra-oesophageal ganglion are considered to be in their development

correlative with that of the intelligence or instinct. In the workers of

the social Hymenoptera these bodies are very large, while in the males and

females they are small. The workers and females of _Bombus_ have six

abdominal ganglia, while the males have only five; and the worker of the

honey-bee has five abdominal ganglia, while the male and the queen-bee have

but four. In the leaf-cutting bee (_Megachile_) the male has four abdominal

ganglia and the female five, and in the wasps the workers have five, the

males and females six. The nervous system in the larvae shows but little

difference between the ganglia, which are thirteen in number, eight being

abdominal. In the embryo of the bee Kowalewsky has observed seventeen

ganglia. The changes that take place from the embryonic to the imago

condition are therefore directed to the reduction in number of the ganglia,

which is accomplished by the fusion of some of them. In the adult

Hymenopterous Insect it would appear that the first abdominal ganglion is

always joined with the last thoracic.

SUB-ORDERS.—The distinction in the form of the abdominal articulation,

previously alluded to (p. 492, Fig. 336, A, B), divides the Hymenoptera

into two great sub-Orders, the members of which are very different in their

habits and life-histories. The Sessiliventres are plant-eaters; their

larvae (Fig. 343, A) are provided with legs, and are able to procure their

vegetable food for themselves. The larvae of the Petiolata are maggot-like

and helpless, and are dependent for food on supplies afforded them by their

parents or companions. It is said by Dewitz that although the larvae of the

Petiolata appear to be legless, there are thoracic legs within the body.

The metamorphosis, so far as it is known, and the early life-history of the

Sessiliventres are very similar to those of butterflies and moths, except

that the pupa is soft and has no hard external skin. A few of these

plant-eating Sessiliventres become carnivorous in the perfect state—a

change of habit that is most unusual in Insects, though the reverse

occurrence is common. The larvae of the Petiolata exhibit, in the cases

that have been examined, the peculiarity that the alimentary canal has not

any outlet posteriorly until the termination of the larval stage of

existence is approaching. In some cases there is no anal orifice; in others

this orifice exists, {497}but there is no communication between the stomach

and the posterior intestine.

Packard informs us[415] that in _Bombus_ the larva, after it is full fed,

passes into the pupa state (Fig. 331, A, B) by a series of transformations

accompanied by moultings of the skin. Packard's statements have been

confirmed by others, but details have not been fully given, so that the

number of the moults, their intervals and other particulars, are still

unknown. We have remarked that the pupal instar is very like the perfect

instar, except that it is colourless and soft, and that each of the members

is wrapped in a very delicate skin; the colour appears gradually. This

metamorphosis exhibits important differences from that of the Lepidoptera.

Packard calls the Insect, during the stages of transformation from the

full-fed larva to the pupa, the semi-pupa; the later stages of the pupa,

when the colouring has appeared, he terms the subimago. Altogether he

considers there is a series of at least ten moultings of the skin. His

ideas were apparently derived from examination of a series of specimens

after death rather than from observation of the development in living

individuals. The parasitic forms of Hymenoptera have apparently

extraordinary metamorphoses of very varied kinds.

PARTHENOGENESIS.—One of the most remarkable facts connected with this Order

is the prevalence of parthenogenesis in a considerable number of widely

separated species. In many of these Hymenoptera it is not a mere occasional

occurrence, but plays an important part in the continuity of the species;

indeed, it is believed that in some members of the Order the reproduction

is entirely parthenogenetic. We shall give particulars as to some of these

cases in subsequent chapters, and will here make some remarks on the

different forms of parthenogenesis existing in the Order. The three forms

of parthenogenesis mentioned on p. 141 all occur in Hymenoptera. In the

gall-making Cynipidae parthenogenesis is frequently accompanied with

alternation of generations, a generation consisting of the two sexes being

followed by another consisting entirely of females, which in its turn gives

origin to a bisexual generation. In this case deuterotokous parthenogenesis

is established as a part of the normal economy of the species. This same

form of parthenogenesis also occurs in other species of Cynipidae

unaccompanied by alternation {498}of generations. Thus in _Rhodites rosae_

the generations resemble one another, and the male is very rare, but is

still occasionally produced,[416] and the same condition exists in other

Cynipidae. According to the observations of Adler, we may assume that the

male, in the latter cases, is useless, the continuation of the species

being effected by virgin females although males exist. Deuterotokous

parthenogenesis also occurs in the sawflies, but as a comparatively rare

phenomenon.[417]

Thelyotokous parthenogenesis is common in sawflies, and it also occurs in

some Cynipidae. There are several species of this latter family in which no

males have ever been found.[418] The phenomena in _Rhodites rosae_ we have

mentioned, give rise to the idea that in that species deuterotokous

parthenogenesis occurs as an exception, the species being usually

thelyotokous. A most remarkable case of thelyotokous parthenogenesis is

said to exist in the case of the parasitic ant _Tomognathus_. This species

is said to be monomorphic, only the female existing, and reproducing by

uninterrupted parthenogenesis.

Arrhenotokous parthenogenesis—_i.e._ parthenogenesis in which the progeny

is entirely of the male sex—occurs in several species of sawflies. We find

it also occurring in the case of the social Hymenoptera; the workers of

ants, bees, and wasps occasionally produce eggs parthenogenetically, and

the progeny in these cases is always of the male sex. In the honey-bee the

queen sometimes produces eggs before she has been fertilised, and the

parthenogenetic young are then always of the male sex.

Some species of Hymenoptera exhibit two forms of parthenogenesis. In

_Nematus curtispina_ the parthenogenetic generation is generally of the

male sex, but a female is occasionally produced;[419] while in _Hemichroa

rufa_ parthenogenesis may result in either deuterotokous or thelyotokous

progeny. No case is yet known of a species exhibiting the three forms of

parthenogenesis. From this review we may conclude that parthenogenesis does

not favour the formation of one sex more than another; but it is clear that

it decidedly favours the production of a brood that is {499}entirely of one

sex, but which sex that is differs according to other circumstances.

PRODUCTION OF SEX.—It is believed that a very peculiar form of

parthenogenesis exists in the honey-bee, and it is confidently stated that

the drones, or males, of that species are always produced from unfertilised

eggs. These views are commonly called the Dzierzon theory, and are widely

accepted. They assume that the eggs are male till fertilised, and then

become female. After the queen-bee is fertilised most of the spermatozoa

soon find their way into a small chamber, the spermatheca, near the

posterior orifice of the body; it is believed that each egg may be

fertilised as it passes the door of this chamber, and that the eggs that

produce females (_i.e._ workers or queens) are so fertilised, but that the

eggs that produce drones are not fertilised. Hence it is supposed that the

sex is determined by this act of fertilisation, and Cheshire has described

what he calls an apparatus for differentiating the sexes. It is also

confidently stated that no male honey-bee ever has a father.

The facts we have stated as to the sexes resulting from parthenogenetic

reproduction in Hymenoptera generally, are extremely opposed to the

Dzierzon theory, in so far as this relates to the production of sex. There

have always been entomologists[420] who have considered this view

unsatisfactory, and the observations of several recent French

naturalists[421] are unfavourable to the idea that the sex of an egg is

determined by its fertilisation.

There can be no doubt that the queen honey-bee frequently produces males

parthenogenetically, and the error of the views we are alluding to consists

in taking the parthenogenesis to be the cause of the sex of the individual.

It must be recollected that the laying of an unfertilised egg by a

fertilised female may be different physiologically from the laying of an

egg by an unfertilised female; for, though both have as result an

unfertilised egg, it is possible that the fertilisation of the female may

initiate processes that modify the sex of the eggs produced by the ovaries,

so that though these may produce previous to fertilisation only male eggs,

yet after fertilisation they may produce eggs of the opposite sex or of

both sexes. In other {500}words, the act of fertilisation may initiate a

different condition of nutrition of the ovaries, and this may determine the

sex of the eggs produced.

POLYMORPHISM, OR CASTES.—The question of the causes of the modified

individuals forming the various castes of the social Hymenoptera has been

much discussed. These individuals are many of them very different in size

and structure from either of their parents, and are also different in their

habits and instincts. This difficult subject is far from being completely

elucidated. In the case of the honey-bee it is well established that an egg

of the female sex can, after deposition, be made either into a queen or a

worker-bee by the mode of nutrition—using that word in the largest sense.

On the other hand, Dewitz thought that in the case of the ant _Formica

rufa_, the caste—whether worker or winged female—is already determined in

the Insect before leaving the egg.[422] Weismann and others associate the

caste with some hypothetic rudiments they consider to exist at the very

earliest stage of the embryonic, or oogenetic process.

Herbert Spencer says:[423] "Among these social Insects the sex is

determined by degree of nutrition while the egg is being formed," and

"after an egg, predetermined as a female, has been laid, the character of

the produced Insect as a perfect female or imperfect female is determined

by the nutrition of the larva. _That is, one set of differences in

structure and instincts is determined by nutrition before the egg is laid,

and a further set of differences in structures and instincts is determined

by nutrition after the egg is laid._"

Spencer's generalisation is not inconsistent with the facts hitherto

brought to light, though it is possible that the progress of knowledge may

show some variety as to the periods of the development at which the

commencements of the modifications occur.

Fig. 339 represents the chief castes, or adult forms, existing in a

community of one of the most highly developed of the species of social

Hymenoptera, the leaf-cutting ant, _Atta cephalotes_. We shall, when

dealing with Formicidae, enter into some details as to these and other

cases of polymorphism.

{501}[Illustration: FIG. 339.—Adult forms of _Atta (Oecodoma) cephalotes_,

taken from a nest in Trinidad by Mr. J. H. Hart, 25th June 1895. A, male;

B, winged female; C-F, various forms unwinged; C, so-called soldier; D,

large worker; E, smaller worker; F, smallest worker or nurse. All equally

magnified (one and half times).]

{502}Our object at present is to bring to the eye of the reader the great

diversity of outer form that is believed, rightly or wrongly, to result

from the mode of treatment of the young. And we will also take this

opportunity of more fully illustrating the remark we made on p. 85 as to

the profound distinctions that exist between ants and white ants, or

Termites, notwithstanding the remarkable analogies that we shall find to

exist in many of their social arrangements.

The analogies we allude to, coupled with the fact that there is a certain

general resemblance in outer form between the workers of Termites and ants,

and even between the extraordinary castes called soldiers in the two

groups, have given rise to the idea that there is a zoological relationship

between the social forms of Neuroptera and Hymenoptera. The two are,

however, zoologically amongst the most different of Insects. The external

skeleton in Termites is remarkable for its imperfect development, the

sclerites being small and isolated, while the segmental differentiation of

the body is low (Fig. 225, etc.), so that there is no difficulty in

counting the segments. In ants the reverse is the case as regards both

these facts, the various segments being most unequal, so that their

homologies have only been detected after prolonged studies, while the

chitinisation and articulation of the various parts is so complete that the

ant may be described as cased in armour, fitting together so exactly that

it is difficult anywhere to introduce the point of a needle into its

chinks. The wings of the two kinds of Insects are also extremely different.

The differences between the modes of growth and development of the two sets

of Insects are as profound as the distinctions in their anatomy. Termitidae

belong to the division of Insects in which the wings are developed outside

the body; Hymenoptera to the division in which they are developed inside

the body. In Termites the growth of the individual is slow, and the final

form is reached gradually. In the ants the growth is carried on with great

rapidity, and during it the Insect is a helpless maggot absolutely

dependent on the attentions of its seniors, while the difference in form

and structure between the ant-larva and the ant are enormous. Both anatomy

and ontogeny are profoundly different in ants and Termites. To these

distinctions must be added, as of much importance, the fact that in

Hymenoptera only the female sex {503}is modified for the division of

labour, while in Termites both sexes undergo this change. Hence it is

impossible to suppose that the remarkable analogies that exist between the

societies of ants and those of Termites are due to any common origin. It is

probably to some similar physiological susceptibilities in the ancestors,

at an extremely remote epoch, of both groups that we must look for an

explanation of the interesting resemblances in the social lives of ants and

Termites.

The Hymenoptera are no doubt one of the largest Orders of Insects, the

species of the parasitic tribes being apparently innumerable. No doubt

250,000 species of the Order exist, and possibly the number may prove to be

very much larger. Up to the present time 25,000 or 30,000 have been

discovered. No remains of Insects of this Order, of older age than the

Lias, have been brought to light; it is indeed doubtful whether the fossils

considered to be Hymenopterous of the period referred to are really such.

The Order, as already mentioned, consists of two very distinct sub-Orders,

viz.:—

1. _Hymenoptera Sessiliventres._—Insects with the abdomen broad at the

base, its first segment not completely amalgamated with the thorax.

2. _Hymenoptera Petioliventres_ or _Petiolata_.—The abdomen connected

with what appears to be the thorax by a slender joint, the posterior part

of the apparent thorax consisting of an abdominal segment.

HYMENOPTERA SESSILIVENTRES.—This group has been variously called

_Hymenoptera phytophaga_, _H. securifera_, _H. sessiliventres_, _H.

serrifera_, _H. symphyta_. We prefer an old term, taken from a character

that enables us to recognise at a glance which group a species belongs to.

The division or sub-Order may be formally defined as follows:—

_Abdomen nearly continuous in outline with the thorax, the two parts

having a broad connexion instead of a small highly mobile articulation.

Anal lobe of hind wings usually of considerable size. Trochanters

ditrochous (transversely divided into two, Fig. 345). Extremity of body

of female furnished with saws or boring instruments, usually concealed,

in some cases visible in part. Larvae with complex mouth-parts; three

pairs of thoracic legs (imperfect in Cephidae and {504}Siricidae), and

frequently with numerous abdominal legs, which are destitute of hooks.

Food vegetable._

The Insects of this sub-Order never exhibit the highly specialised habits

and activity of the better known petiolate Hymenoptera. Though the food in

the larval stages is always vegetable, there is considerable variety in the

larvae and their habits; some feed in galls, some in the twigs of plants,

some in the hard wood of trees and shrubs. The majority, however, live on

the leaves of plants. Those that live in wood (Fig. 342, C) resemble in

appearance Coleopterous larvae that have similar habits, and those that

live on leaves (Fig. 343, A) resemble Lepidopterous larvae that do

likewise. There are four families included in the sub-Order, viz. Cephidae,

Oryssidae, Siricidae, Tenthredinidae.

The British Sessiliventres—under the name Phytophagous Hymenoptera—have

recently been monographed by Mr. Peter Cameron in a series of vols.

published by the Ray Society.[424] These contain many figures and many

details relating to natural history, in addition to the descriptions of

genera and species.

FAM. I. CEPHIDAE—STEM SAWFLIES.

_Slender Insects, with weak integument; free, more or less elongate

pronotum; one spine on the front tibia. Larvae living in the stems of

plants or in the tender shoots of trees and shrubs._

The obscure little Insects composing this family have slender antennae of

peculiar form, composed of eighteen to thirty joints, two of which are

short and stout; then come several long joints, with more or less power of

movement, the terminal portion consisting of an elongate club of many

joints with little power of movement. The pronotum is longer than is usual

in the Hymenoptera, and instead of being very closely connected with the

mesonotum, it is free and mobile, although its base overwraps the front of

the mesonotum. The median plate (_i.e._ the dorsal plate connecting the

thorax and abdomen) is divided to the base along the middle, the divisions

being separated by a membranous piece broader behind; the anal lobe of the

posterior {505}wings is small but distinct. The female bears a saw at the

extremity of the body, but it is covered by two flaps; these form a short,

terminal projection. Although too much neglected, the Cephidae are really

of great interest as being of more imperfect or primitive structure than

any of the other families of Hymenoptera. The larval history has been

traced in several species. _C. pygmaeus_ is sometimes very injurious to

corn crops on the continent of Europe, and even in our own country its

effects in this respect are considered to be occasionally serious. The egg

is laid in the stem of the corn plant; the larva soon hatches and eats its

way upwards in the stem. It is a soft grub, apparently footless, but really

possessing six small projections in place of thoracic legs. It occupies all

the summer in feeding, and when full fed and about to prepare for its

metamorphosis, it weakens the stem by a sort of girdling process below the

ear; it then descends in the stem to near the root, where it constructs a

transparent cocoon, in which it passes the winter as a larva, changing to a

chrysalis in the month of May, and completing its development by appearing

as a perfect Insect shortly thereafter. The girdling operation is very

injurious, and causes the corn stem, when ripe or nearly so, to break in

two under the influence of a strong wind, so that the ears fall to the

ground.

[Illustration: FIG. 340.—_Cephus pygmaeus._ Upper figure, larva; lower,

female imago. Britain. (After Curtis.)]

The history of _C. integer_ has been given by Riley. This Insect attacks

the young shoots of willows in North America. Riley states[425] that by a

wonderful instinct the female, after she has consigned her egg to the twig,

girdles the latter, preventing it from growing any further, and from

crushing the egg by so doing. The larva after hatching eats downwards,

sometimes destroying a length of two feet of the twig; when full grown it

fills the bottom of the burrow with frass, and then previous to making its

cocoon eats a passage through the side of the shoot about a quarter of an

inch above the spot where the cocoon will be placed, thus making it easy

for the perfect Insect to effect its {506}escape; it leaves the bark,

however, untouched, and is thus protected in its retreat. A delicate

transparent cocoon is then spun in which the larva passes the winter,

changing to a pupa in the following March, and emerging as a perfect Insect

about six weeks thereafter.

Somewhat less than 100 species of this family are at present known; the

great majority are found in the Mediterranean region, but there are several

in North America. As a single species is known from Mexico and another from

Japan, it is probable that the family may prove to have a wider

geographical extension than at present appears to be the case.

FAM. II. ORYSSIDAE.

_The median plate behind the metanotum entire, not divided in the middle;

antennae inserted below the eyes immediately above the mandibles, under a

sharp edge._

This family consists of the genus _Oryssus_, and includes only about twenty

species, but is nevertheless very widely distributed over the world. They

are very rare Insects, and little is known as to their habits; one species,

_O. abietinus_, was formerly found in England. Should any one be so

fortunate as to meet with it, he can scarcely fail to recognise it on

noticing the peculiar situation of the base of the antennae. In this

respect the Chrysididae somewhat resemble _Oryssus_, but in that group of

Hymenoptera the hind body or abdomen is remarkably mobile, so that the

Insects can coil themselves up by bending at this joint; whereas in

_Oryssus_ the hind body is very closely amalgamated with the thorax—more

so, in fact, than in any other Hymenopterous Insect—and has no power of

independent movement.

[Illustration: FIG. 341.—_Oryssus sayi._ North America. A, The female

Insect; B, head seen from the front.]

{507}_Oryssus abietinus_ very closely resembles _C. sayi_ (Fig. 341); it

has indeed been recently suggested by Mr. Harrington that the two supposed

species may really be identical.

FAM. III. SIRICIDAE OR UROCERIDAE.

_Pronotum closely connected with the mesonotum, perpendicular in front;

the anterior lobe of the latter not separated by the lateral lobes from

the posterior lobe: the median plate (behind the metathorax) is divided

longitudinally along the middle. The female is provided at the extremity

of the body with an elongate, cylindrical boring instrument. The larvae

live in the wood of trees._

[Illustration: FIG. 342.—_Tremex columba._ North America. A, Imago, female:

B, pupa, female, ventral aspect: C, larva; _a_, imperfect legs: D,

parasitic larva of _Thalessa_. (B and D after Riley.)]

The Insects of this family are usually of large size and of bright

conspicuous colours; these, however, frequently differ greatly in the sexes

of the same species, and may be very variable even in one sex. The antennae

are filiform and usually elongate; the head is usually contiguous with the

thorax, but in one division, Xyphidriides, it is exserted and separated

from the thorax by a well-marked neck. The pronotum is attached to the

mesonotum, and possesses very little, if any, freedom of movement; it

varies in its size, being sometimes conspicuous {508}from above; in the

Xyphidriides it is smaller, and in the middle is entirely vertical in its

direction. The mesonotum is moderate in size, and its divisions are

delimited by broad vague depressions. The prosternum appears to be entirely

membranous, but the prosternal plates (pleura) are large, and meet together

accurately in the middle, so as to protect the greater part of the

under-surface of the neck. The abdomen is cylindrical or somewhat flattened

above; it has seven dorsal plates in addition to the spine-bearing terminal

segment. The trochanters are double, the outer division being, however,

short; the anterior tibia has only one spur; the anal lobe of the posterior

wings is large. The "borer" or ovipositor of the female is a remarkable

organ; it is held projecting directly backwards from the extremity of the

body, and has the appearance of being a powerful sting. The apparatus is

much longer than it appears, for it proceeds not from the apex of the body,

but from the under-surface far forwards, so that the part exposed is only

about one-half of the total length; it consists of a pair of elongate

sheaths, which are easily separable though they wrap together, and enclose

a slender tube. This tube is rigid and quite straight; though appearing

solid, it is really composed of two very perfectly adjusted laminae and a

third arched piece or roof. The two lower laminae are called the spiculae;

they are serrated or grooved in a peculiar manner near the tip, and

although so closely adjusted to the borer or upper piece of the tube as to

appear to form one solid whole with it, they are said to be capable of

separate motion. In addition to these parts, the termination of the abdomen

bears above a shorter piece that projects in a parallel plane, and forms a

sort of thick spine above the ventral pieces we have described; this

process is very strong, and has in the middle of its under-face in _Sirex

gigas_ a membranous cavity, replaced in _S. juvencus_, according to

Westwood, by a pair of minute pilose styles. The Insect, by means of this

powerful apparatus, is enabled to deposit her eggs in the solid wood of

trees, in which the larva sometimes penetrates to the depth of eight

inches.

_Sirex gigas_ is one of the most remarkable of our British Insects, but is

little known except to entomologists, being usually rare. On the continent

of Europe it is, however, an abundant Insect, especially in the

neighbourhood of forests of fir-trees, and is a cause of considerable

terror. As the Insect is not {509}capable of inflicting much injury to the

person, it is probable that the peculiar ovipositor is believed to be a

sting. The eggs are laid—it is said to the number of 100—in the solid wood

of fir-trees, but not in perfectly healthy wood; the reason for this, it is

thought, being that in a healthy tree the great affluence of sap caused by

the burrows and presence of the Insect would be injurious to the latter.

The _Sirex_ will, however, attack a perfectly healthy tree immediately

after it has been felled. The larva, small at first, enlarges its burrows

as itself grows larger, and thus the wood of a tree may be rendered

completely useless for trade purposes, although there may be very little

outward indication of unsoundness. The larva (Fig. 342, C, larva of

_Tremex_) is a pallid, maggot-like creature, with six projections

representing thoracic legs; there are no other legs behind these, but some

slight protuberances take their place; the terminal segment is enlarged,

and bears a hard spine. There is a difference of opinion as to the duration

of the life of the larva, Kollar saying that in seven weeks after the

deposition of the egg the maggot is full fed, while others consider that it

takes two years to attain this condition; the latter statement is more

probably correct, it being the rule that the life of wood-feeding larvae is

more than usually prolonged. After becoming full fed, the Insect may still

pass a prolonged period in the wood before emerging as a perfect Insect. As

a result of this it not infrequently happens that the Insect emerges from

wood that has been carried to a distance, and used for buildings or for

furniture. A case is recorded in which large numbers of a species of

_Sirex_ emerged in a house in this country some years after it was built,

to the great terror of the inhabitants. The wood in this case was supposed

to have been brought from Canada.

Fabre has studied[426] the habits of the larva of _Sirex augur_, and finds

that it forms tortuous galleries in the direction of the longitudinal axis

of the tree or limb, and undergoes its metamorphosis in the interior,

leaving to the perfect Insect the task of finding its way out; this the

creature does, not by retracing its path along the gallery formed by the

larva, but by driving a fresh one at right angles to the previous course,

thus selecting the shortest way to freedom. By what perception or sense it

selects the road to the exterior is quite unknown. Fabre is not {510}able

to suggest any sort of perception that might enable the larva to pursue the

right course, and considers it must be accomplished by means of some

sensibility we do not possess. Fabre's observation is the opposite of what

has been recorded in the case of _S. gigas_, where the larva is said to

prepare the way for the exit of the perfect Insect.

Individuals of _Sirex_ are often found in dried and solid wood, encased by

metal. When the Insect finds itself so confined, it gnaws its way through

the metal, if this be lead, and escapes. The perseverance displayed by the

Insect in these circumstances seems to indicate a knowledge of the

direction in which liberty is to be found.

About 100 species of Siricidae are known. They form two sub-families:—

1. _Siricides_: back of head nearly or quite contiguous with the

pronotum.

2. _Xyphidriides_: back of head separated from the pronotum by an

elongate neck.

We are reputed to possess in Britain two species of each of these

sub-families, but it is doubtful whether more than one Siricid is truly

native. _Sirex gigas_ is frequently brought over in timber, and certainly

breeds at times freely in Britain. Mr. Leech has recorded the occurrence of

the larvae in abundance in fir-trees in the neighbourhood of Dublin. _Sirex

juvencus_ is more rarely met with. _Xyphidria camelus_ is doubtless a

native, though now apparently rare. It used to occur about old willows,

near London, in the New Forest, and, I believe, also in the neighbourhood

of Cambridge.

FAM. IV. TENTHREDINIDAE—SAWFLIES.

_Hymenoptera Sessiliventres, having the pronotum small, accurately

adapted to the mesonotum; the anterior lobe of the latter is widely

separated from the posterior; there are two spurs on the anterior tibiae.

The larvae usually live on leaves after the manner of caterpillars, but a

few inhabit galls._

The sawflies are an important family of Insects, their species being

numerous, while some of them are, in the larval state, very destructive to

vegetables and fruit. Being quiet creatures, rarely seen on the wing, they

are, though common Insects in this {511}country, but little known, and few

persons recognise a sawfly as such. They are usually of small or moderate

size, and the numerous species have a great family resemblance. This remark

requires some qualification in the case of the Cimbicides, they being

Insects of larger size—usually surpassing the honey-bee—of more robust

structure, and with greater powers of flight.

[Illustration: FIG. 343.—_Lophyrus pini._ Britain. A, Larva; B, ventral

aspect of pupa; C, imago, male. (After Vollenhoven.)]

The antennae are remarkably variable in form and structure. Cameron

considers that nine should be taken as the normal number of their joints;

but there are only three in _Hylotoma_, while in _Lyda_ there may be forty

or more. The head is usually held closely applied to the thorax, but is

really borne on a neck capable of much elongation (Fig. 332). The pronotum

forms a part of the alitrunk, but is not soldered thereto. Usually the

prosternum is more or less completely concealed by the side-pieces, but in

Cimbicides it is larger and conspicuous, the side-pieces being in this

group smaller than usual. The dorsal pieces of the mesothorax have their

relative proportions different to what we find them in the other families

of Sessiliventres, and even in most of the other Hymenoptera. There is

first an antero-median lobe of triangular shape projecting, like a wedge,

far backwards, into the great lateral lobes. These latter form the larger

part of the area of the mesonotum; they meet together in the middle line,

and behind are separated by a deep depression from the posterior lobe, or

scutellum of the mesothorax, which is frequently divided into two parts,

the anterior being the so-called scutum. The pieces of the metanotum are

short and obscure, owing to the great unevenness of their parts; on each

side of the middle there is a small membranous space of pallid colour. The

cenchri, as these spaces are called, are, in _Lyda_, {512}delicate,

membranous, depressed spaces, in front of each of which there stands up a

flap of membrane. The function of the cenchri is quite unknown. The median

plate is fastened to the hind margin of the metanotum, and looks quite like

one of the dorsal plates of the following abdominal segments, from which,

however, it is separated by a more or less conspicuous membrane. In the

majority of the Tenthredinidae the median plate is divided along the

middle, but in the Cimbicides this is not the case. The mesosternum is very

large, and the metasternum small, so that the middle and hinder pairs of

coxae are placed close together. The abdomen consists of nine segments,

there being eight dorsal plates in addition to the median plate, and seven

ventral plates besides the terminal armature. There is a pair of short

cerci, each of a single segment. The trochanters are divided; each tibia

bears two spurs at the extremity, and the tarsi are 5-jointed.

[Illustration: FIG. 344.—Saws of _Cimbex sylvarum_. A, The pair spread out

and placed in a horizontal position; _a_, the lower margin of the saw

proper; _b_, the upper margin of the support: B, two teeth of the saw more

highly magnified.]

The most characteristic and interesting of the structures with which the

Insects of this family are provided is the apparatus from which the name of

sawfly is derived. As long as two centuries ago these instruments excited

the admiration of Vallisnieri and of Réaumur, who described them at length;

and it is truly astonishing that any part of a living being should be

changed into tools so mechanically perfect as these saws are (Fig. 344).

They serve the purpose of assisting the female in depositing the eggs in a

suitable situation, the place selected being frequently the tender stems of

shrubs or other plants, or the interior of leaves. These organs are

therefore of course possessed only by the female. They are placed on the

{513}lower aspect of the hinder extremity of the body, where they are

enclosed and protected by a pair of sheaths, from which they can be made to

protrude by a little pressure exercised on the parts immediately in front

of them. Each female possesses a pair of these saws; they consist of thin

laminae of very hard consistence, and are not only toothed at their edge,

but in many cases each tooth is itself serrate; at the same time the outer

face of the saw is sculptured or plicate in a remarkable manner, so that

the saw in this way acts as a file or rasp. The Insect having selected a

suitable place, uses the saws by placing the extremity of the abdomen

against a twig or leaf, protruding the blades, which, moving with an

alternate motion, one being thrust forward while the other is retracted,

act on the plant so as to make an incision. Each saw is directed in its

movement by the support, the pair of supports being united at the base by

membrane as shown in Fig. 344. In the case of some species,—_Hylotoma

rosae_, the common sawfly of our rose-bushes, for instance—there is no

difficulty in observing the operation; indeed old Réaumur, when speaking of

the placid disposition of the sawflies, suggests that it was given them so

that we may easily observe their charming operations. We cannot but regret

that in these days we are unable to take so complacent a view of the

arrangements of nature. There is much variety in the details of the

structure of these saws; so much indeed that it is possible to identify

most of the species by means of the saw alone. According to certain

observers, the eggs are laid by some kinds on, not in, the leaves, so that

we may conclude that in these cases the saws are not used by their

possessors. An incision having been made, an egg is placed in it, and also

a drop of some liquid matter. The egg is at first small, but soon increases

till it becomes twice or three times its former size, and the development

of the embryo commences.

The larvae of the Tenthredinidae exhibit great variety, and are indeed in

this respect more interesting than the perfect Insects. The usual rule is

that the larvae much resembles those of Lepidopterous Insects, and feed

exposed on plants in the same way as Lepidopterous larvae do. But the

exceptions are numerous; sometimes the larva is covered with slime, and

thus protected from various enemies. In other cases it is very depressed, a

broad creature, of irregular outline, living closely {514}attached to the

leaf, somewhat after the fashion of a huge scale-Insect. Some larvae mine

between the layers of a leaf, others roll up leaves; a few live in the

stems of plants, and one or two inside fruits. Even this does not complete

the list of their habits, for a few species of _Nematus_ live in galls

caused by the deposition of the egg. A species of _Lyda_ forms for itself a

case out of bits of leaves, and carries this habitation about with it after

the fashion of the Phryganeidae. The number of legs in these larvae is

unusually great, varying from eighteen to twenty-two—that is, three pairs

of thoracic legs and eight of abdominal or pro-legs. This character offers

a ready means of distinguishing, in the majority of cases, these larvae

from those of the Lepidoptera in which the number of legs varies, but is

only from ten to sixteen; moreover, the pro-legs in sawflies are destitute

of the circles of hooklets that exist in Lepidoptera. This mode of

identifying the immature stages of the Tenthredinidae is not, however,

always satisfactory, as there are some of these larvae that have no

pro-legs at all, but only the three thoracic pairs. Another point of

distinction exists, inasmuch as the larvae of the sawflies have only one

ocellus on each side of the head, whereas in the Lepidopterous caterpillars

the rule is that there are several of these little eyes on each side. In

addition to this, we should mention that the Lepidopterous larva never has

any pro-legs on the fifth body-segment, whereas in the sawflies when

pro-legs are present there is always a pair on the segment in question.

These larvae are of various colours, but the patterns and markings they

exhibit are not quite like those of the Lepidoptera, though it would be

difficult to make any correct general statement as to the nature of the

differences. The variety of their postures is very remarkable; and in

respect of these also Tenthredinidae differ considerably from Lepidoptera.

Some of them hold the posterior part of the body erect, clasping the leaf

by their anterior legs; others keep the posterior part of the body curled

up (Fig. 343, A), and some combine these methods by curving the posterior

part of the body and holding it away from the food. These attitudes, like

the general form, are characteristic for each species. The _Nematus_ larvae

that inhabit galls possess all the characteristics of those that feed

externally. As a rule the skin of the larva is naked and free from hair,

but it is often minutely tuberculate, and in a few species it is armed

{515}with remarkable forked spines. These spines may exist during part of

the larval life, and completely disappear at one of the moults. The

creatures are as a rule very sluggish, and move about much less than

Lepidopterous larvae; many of them, when alarmed, have the power of exuding

a disagreeable liquid, either from the mouth or from pores in the skin; in

the latter case it may be sent as a sort of spray to some little distance

from the body. This operation is said to be very efficacious as a means of

protecting the larvae from the attacks of parasitic flies that are desirous

of laying eggs in their bodies. One peculiarity as to their colour has

attracted the attention of Réaumur and subsequent naturalists, namely, that

in the case of many species a great change takes place in the colour during

the life of the larva, and more especially at the period of the last moult.

The change to the pupal state usually takes place in a cocoon, and some

species have the peculiar habit of forming a double cocoon, the outer one

being hard and coarse, while the inner is beautifully delicate. The cocoon

is sometimes formed in the earth, and in that case it may be to a large

extent composed of earthy matter. The Insect frequently remains a long time

in its cocoon before emerging as a perfect Insect; however long this time

may be, it is nearly all of it passed in the larval state; when the Insect

does change to a pupa it speedily thereafter emerges as a perfect Insect.

In the pupa the parts of the imago may be seen enveloped in a very

delicate, transparent skin.

In Brazil _Dielocerus ellisii_, a sawfly allied to _Hylotoma_, constructs a

nest in which the cocoons of many specimens are crowded together, being

packed side by side like the cells in the comb of the bee, while the whole

mass is protected by a thick outer wall. It is not known in what manner

this communal work is carried out, but it is interesting to note that the

cocoons assume to a considerable extent the hexagonal form of the cells in

the comb of the bee. Some doubt was expressed as to the interpretation put

on this structure by Curtis, but his observations have been confirmed by

Smith and Peckholt.

Several species of sawflies are known to be very injurious to crops. One of

these—the sawfly of the turnip, _Athalia spinarum_ (_centifoliae_

Panz.)—sometimes commits excessive depredations on the turnip crops in this

country as well as on the continent of Europe; its life-history and anatomy

were described by Newport {516}in an essay published by the Entomological

Society in 1838. The eggs, it appears, are laid singly at the edges of the

leaves in the month of May, as many as 200 or 300 being deposited by one

female; as the parent flies are usually gregarious, appearing in large

numbers in fields of turnips, it is not difficult to form an idea of the

serious nature of their depredations. The egg grows very considerably; the

development of the embryo is rapid, occupying, even in unfavourable

weather, only seven or eight days, while in quite congenial circumstances

it is probable that the eggs may hatch about the fourth day after their

deposition. The young grub immediately begins to feed, and in about five

days changes its skin for the first time; it repeats this operation twice

at similar or slightly longer intervals, the third moult thus occurring

when the larva is three or four weeks old; it is then that the larva begins

to be most destructive. Sunshine and warm weather are very favourable to

it, and under their influence it grows so rapidly that in a few days a

field may be almost completely stripped of its foliage. This larva is of a

sooty black colour, and will live on other Cruciferous plants quite as well

as on the turnip. When full grown it buries itself to a slight depth under

the surface of the earth, and forms an oval cocoon of a firm texture, and

with many particles of earth closely adherent to it. The perfect fly

emerges towards the end of July, and a second brood will be produced in the

same season if circumstances are favourable; in that case the resulting

larvae enter the ground for the formation of their cocoons in September or

October, and pass the winter in their cocoons, but still in the larval

state; changing to pupae in the following spring, and appearing as perfect

Insects in May. From this account it appears not improbable that the

offspring of a single female existing in the April of one year may amount

by the following May—three generations having been passed through in the

interval—to as many as 27,000,000 larvae. Fortunately the creatures are, as

Frauenfeld observed, destroyed in very large numbers by a parasitic fungus

and by a Nematode (_Filaria_).

We have, earlier in the chapter, alluded to the fact that the phenomena of

parthenogenesis prevail somewhat extensively among sawflies. It is the rule

in the family that males are very much less numerous than females, and

there are some species of which no males have been discovered. This would

not be of {517}itself certain evidence of the occurrence of

parthenogenesis, but this has been placed beyond doubt by taking females

bred in confinement, obtaining unfertilised eggs from them, and rearing the

larvae produced from the eggs. This has been done by numerous observers

with curious results. In many cases the parthenogenetic progeny, or a

portion of it, dies without attaining full maturity. This may or may not be

due to constitutional weakness arising from the parthenogenetic state.

Cameron, who has made extensive observations on this subject, thinks that

the parthenogenesis does involve constitutional weakness, fewer of the

parthenogenetic young reaching maturity. This he suggests may be

compensated for—when the parthenogenetic progeny is all of the female

sex—by the fact that all those that grow up are producers of eggs. In many

cases the parthenogenetic young of Tenthredinidae are of the male sex, and

sometimes the abnormal progeny is of both sexes. In the case of one

species—the common currant sawfly, _Nematus ribesii_—the parthenogenetic

progeny is nearly, but not quite, always, entirely of the male sex; this

has been ascertained again and again, and it is impossible in these cases

to suggest any advantage to the species to compensate for constitutional

parthenogenetic weakness. On the whole, it appears most probable that the

parthenogenesis, and the special sex produced by it, whether male or

female, are due to physiological conditions of which we know little, and

that the species continue in spite of the parthenogenesis, rather than

profit by it. It is worthy of remark that one of the species in which

parthenogenesis with production of males occurs—_Nematus ribesii_—is

perhaps the most abundant of sawflies.

Although many kinds of Insects display the greatest solicitude and

ingenuity in providing proper receptacles for their eggs, and in storing

food for the young that will be produced, there are extremely few that

display any further interest in their descendants; probably, indeed, the

majority of Insects die before the eggs are hatched, one generation never

seeing the individuals of another. It is therefore interesting to find that

a fairly well authenticated case of maternal attachment, such as we have

previously alluded to as occurring in earwigs, has been recorded in _Perga

lewisii_, an Australian sawfly of the sub-family Cimbicides. The mother,

having deposited about eighty eggs on the leaf of a Eucalyptus, remains

with them until they hatch, {518}after which she sits over her brood with

outstretched legs, and with admirable perseverance protects them, so far as

she is able, from the attacks of parasites and other enemies; she quite

refuses to be driven away from her charges. Mr. Lewis, to whom we are

indebted for this account,[427] states that the sawfly does not recognise

her own special brood, but will give equal attention to another brood if

she be transferred thereto; and he adds that many of the batches of larvae

were destitute of any maternal guardian.

There are about 2000 species of sawflies known. A large majority of them

are found in the European and North American regions; still, a good many

are known to live in South America, and _Perga_—one of the genera of the

family containing many species of large size—is peculiar to the Australian

region. Although the family includes so many species, very few anomalies of

structure have been detected in it; one species, _Pompholyx dimorpha_

Freymuth, is described as being apterous in the female, and as having the

thorax curiously modified in its form. There are no very small Insects in

the family, and none over the middle size. Nearly 400 species have been

detected in Britain; this number could certainly be increased by

persevering researches. The palaeontological record has hitherto given only

a very meagre evidence about sawflies. Several species have been preserved

in amber, and three or four are known from Tertiary strata in Europe and

North America.

{519}CHAPTER XXIII

HYMENOPTERA PETIOLATA–PARASITIC HYMENOPTERA–CYNIPIDAE OR GALL-FLIES–

PROCTOTRYPIDAE–CHALCIDIDAE–ICHNEUMONIDAE–BRACONIDAE–STEPHANIDAE–

MEGALYRIDAE–EVANIIDAE–PELECINIDAE–TRIGONALIDAE.

We now pass to the consideration of the Hymenoptera of the sub-Order

Petiolata, or Apocrita, as they are styled by Brauer. We should make use of

the term Petioliventres, for it contrasts naturally by its termination with

Sessiliventres, were it not that the word is so uncouth that we think it

better to adopt the shorter and more euphonious expression, Petiolata.

The members of this sub-Order, without exception, have the hind body

connected with the thorax by means of a deep constriction, so that the base

of the abdomen (Fig. 336, B, _b_) is very narrow; the articulation between

the two parts is effected by means of a complex joint allowing great play,

and facilitating the operations of boring and stinging, processes that are

of extreme importance in the economy of the great majority of the species.

The petiole is sometimes extremely short, but it may be so long that it

appears like a stalk, at whose extremity is borne the remaining part of the

abdomen (Fig. 369). When the petiole is very short the abdomen reposes

close to the back of the thorax (Fig. 331, C), and in this case the abdomen

is usually described as sessile; while, when it is evidently stalked, it is

said to be petiolate. These terms are, however, unsuitable, as the words

sessile and petiolate should be reserved for the conditions characteristic

of the two sub-Orders. We shall therefore use the terms pseudo-sessile and

pedicellate for the two conditions of the Petiolata.

The Hymenoptera Petiolata comprises an enormous majority {520}of the Order.

Although it includes many of the most interesting and important of Insects,

its classification is but little advanced, for a great many of the forms

are still rare or unknown. Three series may be adopted for the purposes of

nomenclature.

1. _Parasitica._—Trochanters of two pieces, female with an ovipositor.

2. _Tubulifera._—Trochanters undivided; abdomen consisting of only three,

four, or five visible segments.

3. _Aculeata._—Trochanters undivided; abdomen consisting of six or seven

visible segments; female furnished with a retractile sting.

In the absence of any clear distinction between sting and ovipositor, these

groups are merely conventional. The character furnished by the trochanters

is unfortunately subject to some exceptions, there being a few parasitic

forms in which the trochanters are not divided, and a few aculeates in

which the reverse is more or less distinctly the case; moreover, the

division, when it exists, is in some cases obscure, and the two pieces are

of unequal size. Ratzeburg calls the upper division, which is frequently

much larger than the other, the trochanter, and the lower division the

apophysis. There is much reason for believing that the apophysis is really

merely a secondary division of the femur. The Tubulifera are a

comparatively small group, and will probably be merged in one of the other

two, when the anatomy and morphology of the abdomen have been more

thoroughly elucidated.

[Illustration: FIG. 345.—Divided (ditrochous) trochanter of an Ichneumon:

_a_, coxa; _b_, the two divisions of the trochanter; _c_, femur. (For

monotrochous trochanter see Fig. 335, A, _c_.)]

HYMENOPTERA PARASITICA OR TEREBRANTIA.

This is one of the most extensive divisions of the class Insecta. There can

be little doubt that it contains 200,000 species, and possibly the number

may be very much greater than this. It is, however, one of the most

neglected of the great groups of Insects, though it is perhaps of greater

economic importance to mankind than any other.

{521}Insects derive their sustenance primarily from the vegetable kingdom.

So great and rapid are the powers of assimilation of the Insect, so

prodigious its capacity for multiplication, that the Mammal would not be

able to compete with it were it not that the great horde of six-legged

creatures has divided itself into two armies, one of which destroys the

other. The parasitic Hymenoptera are chiefly occupied in destroying the

tribes of vegetarian Insects; the parasites do this by the simple and

efficient device of dwelling in the bodies of their hosts and appropriating

the nutriment the latter take in. The parasites do not, as a rule, eat the

structures of their host,—many of them, indeed, have no organs that would

enable them to do this,—but they absorb the vegetable juices that, in a

more or less altered state, form the lymph or so-called blood of the host.

The host could perhaps starve out his enemies by a judicious system of

abstention from food; instead, however, of doing this, he adopts the

suicidal policy of persistent eating, and as the result of his exertions,

furnishes sufficient food to his parasites, and then dies himself,

indirectly starved. Ratzeburg considers that the traditional view that the

larvae of parasitic Hymenoptera live by eating the fat-body of their host

is erroneous. They imbibe, he considers, the liquid that fills the body of

the parasitised Insect.[428]

The wide prevalence of Insect parasitism is appreciated only by

entomologists. The destructive winter moth—_Cheimatobia brumata_—is known

to be subject to the attacks of sixty-three species of Hymenopterous

parasites. So abundant are these latter that late in the autumn it is not

infrequently the case that the majority of caterpillars contain these

destroyers. Although Lepidoptera are very favourite objects with parasitic

Hymenoptera, yet other Insects are also pertinaciously attacked; there is

quite a host of Insect creatures that obtain their sustenance by living

inside the tiny Aphididae, or "green-flies," that so much annoy the

gardener. A still larger number of parasites attack eggs of Insects, one or

more individuals finding sufficient sustenance for growth and development

inside another Insect's egg. As Insects have attacked Insects, so have

parasites attacked parasites, and the phenomena called hyperparasitism have

been developed. These cases of secondary parasitism, in which another

{522}species attacks a primary parasite, are extremely numerous. It is also

pretty certain that tertiary parasitism occurs, and Riley is of opinion

that even quaternary destruction is not outside the range of probability.

The physiological problems connected with Insect parasitism are of great

interest to the entomologist; the modes of nutrition and respiration of

these encaged creatures could not fail to be most instructive were we fully

acquainted with them. It is obvious that when an Insect-egg is laid inside

another Insect's egg, and the parasite has to undergo the whole of its

growth therein, it is in the strangest condition as regards nutrition. It

is unnecessary for the intruded egg to have yolk of its own; moreover, the

embryonic mode of nutrition may be continued during what would, with other

Insects, be the larval period. And it seems to be the case that both these

conditions are actually met with in the lives of egg-parasites. The

embryology and post-embryonic development of parasitic Hymenoptera have

already been ascertained to be of the most extraordinary nature. Great

variety, however, will no doubt be found to exist, as will be readily

understood if we tabulate the conditions of the early life of various

parasitic Hymenoptera.

1. The egg may be laid outside a larva, and the embryonic and larval

developments may both be passed on the exterior.

2. The egg may be laid and the embryonic development passed through,

outside the host, but the parasite on hatching may enter the host, so that

the post-embryonic development is passed in the lymph of the host.

3. The egg may be laid inside the host, both embryonic and post-embryonic

developments being gone through in the fluids of the host.

4. The egg may be laid inside another egg, the embryonic and post-embryonic

developments being passed therein.

We shall find that all these conditions exist in the Insects we are about

to consider.

We shall treat the series as composed of ten families; but we must remind

the student that this great subject is still in a very unadvanced state;

the combined efforts of generations of naturalists will be required to

perfect it. Of the ten families five are comparatively insignificant in

number of species. Many of the Cynipidae are not parasitic in habits, but

live in galls. {523}After what we have said as to the mode of nutrition of

parasites it will be understood that the physiological conditions of life

may not be so different in a gall-dweller and a parasite as would at first

be supposed; and it is perhaps not a matter for much surprise that good

characters cannot be found to separate the gallicolous from the parasitic

forms.

FAM. I. CYNIPIDAE—GALL-FLIES.

_Wings with very few cells, with no dark patch (stigma) on the anterior

margin; pronotum fixed to the mesonotum, and at each side extending back

to the point of insertion of the front wing. Antennae not elbowed but

straight, composed of a moderate number (12-15) of joints. Early stages

passed either in galls or as parasites in the bodies of other Insects._

[Illustration: FIG. 346.—_Neuroterus lenticularis._ Britain.]

The Cynipidae are always small, frequently minute, Insects; usually black

or pitchy in colour. The simple structure of the antennae and the number of

their joints are of importance as an aid in identifying a Cynipid. The

mesonotum is usually remarkably convex, and has, behind, a prominent

scutellum, which more or less overhangs the small metanotum and the median

segment; these are perpendicular in their direction; the sculpture of these

posterior parts of the alitrunk is usually deep and remarkable. The abdomen

has usually only a short petiole, so as to be pseudo-sessile; but there are

some genera in which this part is rather long. The abdomen is generally so

very much changed in outer form that its structure is not easily

understood. The visible portion is frequently in larger part made up of the

greatly enlarged dorsal plate of the second or third segment, or of both.

These large plates are really chiefly composed of free flaps, and on

lifting them up the large ventral plates are disclosed, although these

appeared previously to be nearly or quite absent. In the female {524}there

is a very slender ovipositor, of which only a small part protrudes,

although the organ is really elongate; it is drawn into the abdomen by

means of a peculiar series of structures, the modified terminal segments to

which it is attached being folded over into the interior of the body in

such a way that the posterior part becomes situated anteriorly. In

conformity with this arrangement, the ovipositor is bent double on itself,

the anterior and the middle portions of the borer being carried into the

body, leaving only a small part projecting beyond the extremity. The

Cynipid ovipositor is an instrument of much delicacy, and is capable of a

great deal of movement; it is usually serrate just at the tip, and although

it looks so very different from the cutting apparatus of the sawflies (Fig.

344), it seems that it is really composed of pieces similar in their origin

to those of the Tenthredinidae.

[Illustration: FIG. 347.—Ovipositor of _Neuroterus laeviusculus_. (After

Adler.) _a_, _a_, The ovipositor partially coiled; _b_, extremity of

posterior plate; _c_, _c_, muscles.]

The wings frequently bear fine hairs; the paucity of nervures and the

absence of the "stigma" are of importance in the definition of the family.

The most important of the cells is one called the radial cell, situate just

beyond the middle of the front part of the wing.

We cannot enter into a consideration of the classification of the family,

as authorities are not agreed on the subject.[429] As regards their habits

Cynipidae are, however, of three different kinds: (1) the true gall-flies,

or Psenides, which lay an egg or eggs in the tissues of a growing plant, in

the interior of which the larva lives after it is hatched; this mode of

life may or may not, according to the species, be accompanied by formation

of a peculiar growth called a gall: (2) Inquilines,[430] or guest-flies;

{525}these lay their eggs in the galls formed by the gall-makers subsequent

to the growth of the galls, of which they obtain the benefit: (3)

Parasites; these live, like most Ichneumon-flies, in the interior of the

bodies of other living Insects; they prey on a considerable variety of

Insects, but chiefly, it is believed, on Aphididae, or on Dipterous larvae.

These parasitic flies belong to the sub-family Figitides.

A great deal of discussion has occurred relative to the nature and origin

of galls, and many points still remain obscure. Considerable light has been

thrown on the subject by the direct observations of modern naturalists.

Previous to Malpighi, who wrote on the subject two hundred years ago, it

was supposed that galls were entirely vegetable productions, and that the

maggots found in them were due to spontaneous generation, it having been an

article of belief in the Middle Ages that maggots in general arose from the

various organic substances in which they were found, by means of the

hypothetical process called, as we have said, spontaneous generation.

Malpighi was aware of the unsatisfactory nature of such a belief, and

having found by observation that galls arose from the punctures of Insects,

he came to the further conclusion that the growth of the gall was due to

the injection by the Insect into the plant of a fluid he termed Ichor,

which had, he considered, the effect of producing a swelling in the plant,

something in the same way as the sting of a bee or wasp produces a swelling

in an animal. Réaumur also made observations on the gall-Insects, and came

to the conclusion that the latter part of Malpighi's views was erroneous,

and that the swelling was not due to any fluid, but simply to irritation

caused by the prick; this irritation being kept up by the egg that was

deposited and by the subsequent development of the larva. Observations

since the time of Réaumur have shown that the matter is not quite so simple

as he supposed, for though in the case of some galls the development of the

gall commences immediately after the introduction of the egg, yet in other

cases, as in the Cynipidae, it does not occur till some time thereafter,

being delayed even until after the hatching of the egg and the commencement

of the development of the larva. Galls are originated {526}by a great

variety of Insects, as well as by mites, on many plants; and it must not be

concluded that a gall has been formed by Hymenoptera even when these

Insects are reared from one. Extremely curious galls are formed by

scale-Insects of the sub-family Brachyscelides on Eucalyptus trees in

Australia; they are much inhabited by parasitic Hymenoptera, and Froggatt

has obtained 100 specimens of a small black Chalcid from a single dead

Brachyscelid.[431] The exact manner in which many of these galls originate

is not yet sufficiently ascertained; but the subject of the galls resulting

from the actions of Cynipidae has received special attention, and we are

now able to form a conception of their nature. They are produced by the

meristematic or dividing tissue of plants, and frequently in the cambium

zone, which is caused to develop to an unusual extent, and in a more or

less abnormal manner, by the presence of the Insect. The exact way in which

a Cynipid affects the plant is perhaps not conclusively settled, and may be

found to differ in the cases of different Cynipidae, but the view advocated

by Adler and others, and recently stated by Riley,[432] seems satisfactory;

it is to the effect that the activity of the larva probably affects the

meristem, by means of a secretion exuded by the larva. The mere presence of

the egg does not suffice to give rise to the gall, for the egg may be

deposited months before the gall begins to form. It is for the same reason

improbable that a fluid injected by the parent fly determines the gall's

growth. It is true that the parent fly does exude a liquid during the act

of oviposition, but this is believed to be merely of a lubricant nature,

and not to influence the development. It is said that the gall begins to

form in some cases before the larva is actually hatched, but the eggs of

some Hymenoptera exhibit remarkable phenomena of growth, so that the egg,

even during development of the embryo in it, may in these cases, exert an

influence on the meristem. It is to reactions between the physiological

processes of the meristem and the growing Insect that the gall and its form

are due.

The investigations of several recent naturalists lend support to the view

that only the meristematic cells of the plant can give rise to a gall.

Riley says that the rate of growth of the gall is dependent on the activity

of the meristem, galls on {527}catkins developing the most quickly; those

forming on young leaves also grow with rapidity, while galls formed on bark

or roots may take months to attain their full size.

[Illustration: FIG. 348.—Bedeguar on rose, cut across to show the cells of

the larvae; in some of the cells larvae are seen.]

It is a curious fact that Cynipid galls are formed chiefly on oaks, this

kind of tree supplying a surprising number and variety of galls. The plants

that furnish Cynipid galls in Europe are not numerous. A list of them is

given by Cameron.[433] Several species, of the genus _Rhodites_, attack

rose-bushes. One of the best known of our British galls is the bedeguar,

found in various parts of the country on both wild and cultivated

rose-bushes (Fig. 348), and caused by _Rhodites rosae_ (Fig. 349). This

gall has the appearance of arising from a twig or stem, but it is really a

leaf gall. Pazlavsky[434] has described the mode of formation of the

bedeguar. The female _Rhodites_ in the spring selects a rose-bud—not a

flower-bud—that should produce a twig and leaves, and pricks this bud in a

systematic manner in three places. The three spots of the bud pricked by

the Insect are the three undeveloped leaves that correspond to a complete

cycle in the phyllotaxis of the plant. The three rudiments do not develop

into leaves, but by a changed mode of growth give rise to the bedeguar.

Usually this gall, as shown in our figure, is of large size, and contains

numerous cells; but abortive specimens are not infrequently met with;

sometimes a small one is seated on a rose-leaf, and it is thought that

these are due to a failure on the part of the Insect to complete the

pricking operation. {528}Cynipidae will not go through their gall-making

operations except under natural conditions. Giraud[435] attempted to obtain

oviposition, on gathered twigs of oak, from flies in confinement; but,

although he experimented with thousands of specimens, they on no occasion

laid their eggs in the fresh shoots placed at their disposal, but

discharged their eggs in little heaps, without attention to the twigs. The

same observer has also called attention to the fact that after being

deposited in a bud the eggs of certain species of _Cynips_ will remain

dormant without producing, so far as can be seen, any effect on the tree

for a period of fully ten months, but when the bud begins to develop and

the egg hatches then the gall grows.

[Illustration: FIG. 349.—_Rhodites rosae_, female. Cambridge.]

The exact mode in which the egg is brought to the requisite spot in the

plant is still uncertain. The path traversed by the ovipositor in the plant

is sometimes of considerable length, and far from straight; in some cases

before it actually pierces the tissues, the organ is thrust between scales

or through fissures, so that the terebra, or boring part of the ovipositor,

when it reaches the minute seam of cambium, is variously curved and flexed.

Now as the canal in its interior is of extreme tenuity, and frequently of

great length, it must be a very difficult matter for the egg to reach the

tissue where it should develop. The eggs of Cynipidae are very remarkable

bodies; they are very ductile, and consist of a head, and of a stalk that

in some cases is five or six times as long as the head, and is itself

somewhat enlarged at the opposite end. Some other Hymenoptera have also

stalked eggs of a similar kind (Fig. 357, A, egg of _Leucospis_). It has

been thought that this remarkable shape permits of the contents of the egg

being transferred for a time to the narrower parts, and thus allows the

broader portion of the egg to be temporarily compressed, and the whole

structure to be passed through a very narrow canal or orifice. It is,

however, very doubtful whether {529}the egg really passes along the canal

of the borer. Hartig thought that it did so, and Riley supports this view

to a limited extent. Adler, however, is of a different opinion, and

considers that the egg travels in larger part outside the terebra. It

should be remembered that the ovipositor is really composed of several

appendages that are developed from the outside of the body; thus the

external orifice of the body is morphologically at the base of the borer,

the several parts of which are in longitudinal apposition. Hence there is

nothing that would render the view of the egg leaving the ovipositor at the

base improbable, and Adler supposes that it actually does so, the thin end

being retained between the divisions of the terebra. Riley is of opinion

that the act of oviposition in these Insects follows no uniform system. He

has observed that in the case of _Callirhytis clavula_, ovipositing in the

buds of _Quercus alba_, the eggs are inserted by the egg-stalk into the

substance of the leaf, and that the egg-fluids are at first gathered in the

posterior end, which is not inserted. "The fluids are then gradually

absorbed from this exposed portion into the inserted portion of the egg,

and by the time the young leaves have formed the exposed [parts of the]

shells are empty, the thread-like stalk has disappeared, and the

egg-contents are all contained within the leaf tissue." He has also

observed that in _Biorhiza nigra_ the pedicel, or stalk, only is inserted

in the embryonic leaf-tissue, and that the enlarged portion or egg-body is

at first external. The same naturalist also records that in the case of a

small inquiline species, _Ceroptres politus_, the pedicel of the egg is

very short, and in this case the egg is thrust down into the puncture made

by the borer, so that the egg is entirely covered.

Some Cynipidae bore a large number of the channels for their eggs before

depositing any of the latter, and it would appear that it is the rule that

the boring of the channel is an act separate from that of actual

oviposition. Adler distinguishes three stages: (1) boring of the canal; (2)

the passage of the egg from the base of the ovipositor, where the egg-stalk

is pinched between the two spiculae and the egg is pushed along the

ovipositor; (3) after the point of the ovipositor is withdrawn, the

egg-body enters the pierced canal, and is pushed forward by the ovipositor

until it reaches the bottom.[436]

{530}About fifty years ago Hartig reared large numbers of certain species

of gall-flies from their galls, obtaining from 28,000 galls of _Cynips

disticha_ about 10,000 flies, and from galls of _C. folii_ 3000 or 4000

examples of this species; he found that all the individuals were females.

His observations were subsequently abundantly confirmed by other

naturalists, among whom we may mention Frederick Smith in our own country,

who made in vain repeated attempts to obtain males of the species of the

genus _Cynips_. On one occasion he collected in the South of England 4410

galls of _C. kollari_ (at that time called _C. lignicola_), and from these

he obtained 1562 flies, all of which were females. A second effort was

attended with similar results. Hartig, writing in 1843, after many years'

experience, stated that though he was acquainted with twenty-eight species

of the genus _Cynips_, he had not seen a male of any one of them. During

the course of these futile attempts it was, however, seen that a possible

source of fallacy existed in the fact that the Insects were reared from

collected galls; and these being similar to one another, it was possible

that the males might inhabit some different gall. Adler endeavoured to put

the questions thus raised to the test by means of rearing females from

galls, and then getting these females to produce, parthenogenetically,

galls on small oaks planted in pots, and thus completely under control. He

was quite successful in carrying out his project, and in doing so he made a

most extraordinary discovery, viz. that the galls produced by these

parthenogenetic females on his potted oaks, were quite different from the

galls from which the flies themselves were reared, and were, in fact, galls

that gave rise to a fly that had been previously considered a distinct

species; and of this form both sexes were produced. Adler's observations

have been confirmed by other naturalists, and thus the occurrence of

alternation of generations, one of the two generations being

parthenogenetic, has been thoroughly established in Cynipidae. We may

mention one case as illustrative. A gall-fly called _Chilaspis lowii_ is

produced from galls on oak-leaves at Vienna at the end of April, both sexes

occurring. The female thereafter lays eggs on the ribs of the leaves of the

same kind of oak, and thus produces a different gall from that which

nourished herself. These galls fall off with the leaves in the autumn, and

in July or August of the following year a gall-fly is produced from them.

It is a different creature from the {531}mother, and was previously known

to entomologists under the name of _Chilaspis nitida_. Only females of it

occur, and these parthenogenetic individuals lay their eggs in the young

buds of the oak that are already present in the autumn, and in the

following spring, when the buds open and the leaves develop, those that

have had an egg laid in them produce a gall from which _Chilaspis lowii_

emerges in April or May. In this case therefore the cycle of the two

generations extends over two years, the generation that takes the greater

part of the time for its production consisting only of females. Adler's

observations showed that, though in some species this alternation of

generations was accompanied by parthenogenesis in one part of the cycle,

yet in other species this was not the case. He found, for instance, that

some gall-flies of the genus _Aphilothrix_ produced a series of generations

the individuals of which were similar to one another, and were all females

and parthenogenetic. In some species of the old genus _Cynips_ no males are

even yet known to occur. A very curious observation was made by the

American, Walsh, viz. that of galls gathered by him quite similar to one

another, some produced speedily a number of both sexes of _Cynips

spongifica_, while much later on in the season the remainder of the galls

gave rise to females only of an Insect called _Cynips aciculata_. It is

believed that the galls gathered by Walsh[437] were really all one species;

so that parts of the same generation emerge at different times and in two

distinct forms, one of them parthenogenetic, the other consisting of two

sexes. It has, however, been suggested that _Cynips spongifica_ and _C.

aciculata_ may be two distinct species, producing quite similar galls.

Turning now to the questions connected with inquiline or guest-flies, we

may commence with drawing attention to the great practical difficulties

that surround the investigation of this subject. If we open a number of

specimens of any kind of gall it is probable that several kinds of larvae

will be found. In Fig. 350 we represent four kinds of larvae that were

taken out of a few bedeguar galls gathered on one day in a lane near

Cambridge. It is pretty certain that No. 1 in this figure represents the

larva of _Rhodites rosae_, and that Nos. 2 and 3 are larvae of inquilines,

possibly of _Synergus_, or of a parasite; while No. 4, which was engaged in

feeding on No. 3 in the position {532}shown, is possibly a Chalcid of the

genus _Monodontomerus_, or may be _Callimome bedeguaris_. It is clear that,

as we cannot ascertain what is inside a gall without opening it, and

thereby killing the tenants, it is a most difficult matter to identify the

larvae; the only safe method is that of observation of the act of

oviposition; this may be supplemented by rearing the flies from galls, so

as to ascertain what variety of flies are associated with each kind of

gall. This last point has been well attended to; but the number of cases in

which oviposition of inquiline gall-flies in the galls formed by the

Psenides has been ascertained by direct observation is still very small;

they are, however, sufficient to show that the inquilines deposit their

eggs only after the galls are formed.

[Illustration: FIG. 350.—Larvae inhabiting bedeguar gall at Cambridge. 1,

_Rhodites rosae_ in cell; 2 and 3, larvae of inquilines; 4, larva of a

parasitic Hymenopteron.]

Bassett recorded the first case of the kind in connexion with a North

American species, _Cynips_ (_Ceroptres_) _quercus-arbos_ Fitch. He says:

"On the first of June galls on _Quercus ilicifolia_ had reached their full

size, but were still tender, quite like the young shoots of which they

formed part. Examining them on that day, I discovered on them two

gall-flies, which I succeeded in taking. They were females, and the

ovipositor of each was inserted into the gall so deeply that they could not

readily free themselves, and they were removed by force."

The great resemblance of the inquiline gall-fly to the fly that makes the

gall both dwell in, has been several times noticed by Osten Sacken, who

says "one of the most curious circumstances connected with the history of

two North American blackberry galls is, that besides the _Diastrophus_,

which apparently is the genuine originator of the gall, they produce

another gall-fly, no doubt an inquiline, belonging to the genus _Aulax_,

and showing the most striking resemblance in size, colouring, and sculpture

to the _Diastrophus_, their companion. The one is the very counterpart

{533}of the other, hardly showing any differences, except the strictly

generic characters! This seems to be one of those curious instances, so

frequent in entomology, of the resemblance between parasites and their

hosts! By rearing a considerable number of galls of _D. nebulosus_ I

obtained this species as well as its parasite almost in equal numbers. By

cutting some of the galls open I ascertained that a single specimen of the

gall frequently contained both species, thus setting aside a possible doubt

whether these Insects are not produced by two different, although closely

similar galls."[438]

The substance of which galls are composed, or rather, perhaps, a juice they

afford, is apparently a most suitable pabulum for the support of Insect

life, and is eagerly sought after by a variety of Insects; hence by

collecting galls in large quantities many species of Insects may be reared

from them; indeed by this means as many as thirty different kinds of

Insects, and belonging to all, or nearly all, the Orders, have been

obtained from a single species of gall. Some galls are sought by birds,

which open them and extract their tenants, even in cases where it might be

supposed that the nauseous flavour of the galls would forbid such

proceedings.

Not more than 500 species of Psenides and Inquiline Cynipidae are known

from all parts of the world; and of described Parasitic Cynipidae there are

only about 150 species. The British forms have recently been treated by

Cameron in the work we have already several times referred to.[439]

A few Cynipidae have been found in amber; and remains of members of the

family, as well as some galls, are said by Scudder to have been found in

the Tertiary strata at Florissant.

FAM. II. PROCTOTRYPIDAE, OR OXYURA.

_Small Hymenoptera, with few, or even no, nervures in the wings: the

pronotum closely adherent to the mesothorax, and at the sides reaching

backwards to the points of insertion of the wings. The abdomen is

pointed, and the pointed apex is frequently deflexed; the ovipositor is

not coiled, but is retractile, and when extruded is of tubular form, and

{534}apparently a continuation of the tip of the body. The earlier stages

are passed in the bodies, or in the eggs, of other Arthropods._

[Illustration: FIG. 351.—_Helorus anomalipes_. Britain.]

The Proctotrypidae is one of the most difficult groups of Hymenoptera to

define; some of its members exhibit a great resemblance to Aculeate

Hymenoptera. This is the case with the Insect we figure (Fig. 351). It,

however, is an undoubted Proctotrypid, but there are other forms that

approach very closely in appearance to the Aculeata, or stinging

Hymenoptera; so that until a better comprehension is reached as to the

distinction between a sting and an ovipositor the separation between

Proctotrypidae and Aculeata must be considered somewhat arbitrary.

There is extreme variety in the family; the wings differ considerably in

shape and neuration; they are not infrequently altogether absent in one or

both sexes. The chief distinction of the family from other parasitic

Hymenoptera is the tubular form of the ovipositor; which part appears to be

a continuation of the tip of the body. This latter is more definitely

acuminate than usual, and has given rise to the term Oxyura, by which name

the Proctotrypidae are distinguished in many books. From the Chalcididae

they are distinguished also by the angles of the pronotum attaining the

tegulae. In this character they agree with the Cynipidae, but the

ovipositor and abdomen are very different in form in these two groups, and

the Proctotrypidae very frequently have a pigmented spot or stigma on the

front wings which is absent in Cynipidae. As if to add to the difficulties

the systematist meets with in dealing with this family, some of its members

have the trochanters undivided, as in the case of the stinging Hymenoptera.

The larvae of all that are known lead a completely parasitic life in the

bodies or eggs of other Insects or of Spiders. Sometimes half a dozen

specimens may find the means of subsistence during the whole of their

development in a single Insect's egg. Usually Proctotrypids pupate in

{535}the position in which they have fed up, enclosed each one in a more or

less distinct cocoon. In Fig. 352 we represent a very remarkable case of

Proctotrypid pupation; a larva of some beetle has nourished many specimens

of a species of the genus _Proctotrypes_, and the pupae thereof project

from the body of the host, a pair of the parasites issuing from each

segmental division in a remarkably symmetrical manner.

[Illustration: FIG. 352.—Pupation of _Proctotrypes_ sp. in body of a beetle

larva.]

Comparatively little is known as to the habits of the members of this

family, but such information as has been obtained leads to the conclusion

that great variety will be found to exist in this respect. We have already

mentioned that numerous species have been ascertained to feed inside the

eggs of Insects or of Spiders; others have been reared from larvae or from

galls of the minute Dipterous midges of the family Cecidomyiidae; others

have been obtained from Cynipid galls, a few from ants' nests and from

green-fly; some species are known to attack Coleoptera. The distinguished

Irish entomologist, Haliday, has written an account of the proceedings of a

species of _Bethylus_,[440] from which it has been supposed that this

Insect carries off living caterpillars, and stores them in a suitable

receptacle as food for its progeny, thus anticipating, as it were, the

habits of the fossorial division of the Aculeata, in which group this

instinct has, as we shall subsequently relate, attained an astonishing

degree of perfection. Haliday's observation was unfortunately incomplete

and has not been subsequently confirmed. The Bethylides are remarkable for

their great approach in structure to the Aculeates, so much so that

entomologists are not agreed as to whether certain Insects are

Proctotrypids or Aculeates. _Pristocera_, with a very wide distribution,

may be mentioned as illustrative of these doubtful forms; but other genera

of the Bethylides are in many respects very similar to the Aculeates, and

it is not matter for surprise that Haliday should have considered the

Bethylides to be a tribe of the stinging Hymenoptera. {536}The genus

_Scleroderma_ consists of small Insects much resembling ants, and, as well

as some of its allies, is of great interest from the remarkable phenomena

of polymorphism presented by certain species. The males in this genus are

winged, the females completely apterous; yet at times winged females are

produced—as exceptional individuals in a brood of wingless specimens—the

females in these cases being not only winged, but possessed of ocelli like

the females of other winged Hymenoptera. Particulars of a case of this kind

have been given by Sir Sidney Saunders,[441] and Ashmead also mentions[442]

the exceptional occurrence of these winged females. Westwood[443] was of

opinion that there are three forms of the female sex. This subject is of

importance in connexion with the production of the various castes in ants.

Although the presence of wings in these Insects is always accompanied by

the existence of ocelli (which, it will be remembered, are normally absent

from the wingless individuals), yet the converse is not always the case,

for a form of the female of _Cephalonomia formiciformis_, without any

wings, yet having ocelli, as well as eyes, well developed, is figured by

Westwood.[444]

[Illustration: FIG. 353.—_Cyclops_ form of larva of _Platygaster_ sp.

(After Ganin.) _a_, Mouth; _b_, antenna; _c_, claw-limb; _d_, lower lip

(the pointing line is a little too short); _e_, doubtful "zapfenförmig"

organ; _f_, wing-like lobe; _g_, branch of the tail.]

The development of some of the Proctotrypids has been partially described

by Ganin and others, and is of an extraordinary character. Ganin's

observations[445] were most complete in the case of a species of

_Platygaster_, which he found in the larva of a very minute Dipteron of the

genus _Cecidomyia_. The _Platygaster_ larva changes its form very much in

the course of its life, resembling at first a minute Crustacean rather than

an Insect-larva; it has a very large rounded anterior portion, while behind

it terminates in two, or more, tail-like processes. By a {537}very peculiar

kind of metamorphosis this _Cyclops_-like larva changes into an almost

unsegmented, oviform larva, destitute of appendages; by a second change

this creature assumes a third condition, in which it is similar to the

ordinary form of parasitic Hymenopterous larvae. Sometimes several of the

_Platygaster_ larvae are found in a single host, but only one of them

reaches this third stage. Afterwards the third larval instar passes into

the pupal stage, which lasts five or six days, and then the perfect Insect

appears. It is worthy of remark that the internal organs undergo quite as

remarkable a change as the outer form does. The metamorphoses of some other

Proctotrypidae have been examined by Ganin, and appear to be of an equally

interesting character.[446]

There is reason to suppose that these _Platygaster_ parasites are of great

economic importance as well as of scientific interest, for _Platygaster

herrickii_ is one of the enemies of the larva of the destructive Hessian

fly, _Cecidomyia destructor_.

The _Proctotrypidae_ are no doubt extremely numerous in species, but as yet

they have been very little studied; a good work on the British species is

much required. A valuable contribution has recently been made to the study

of the family by Ashmead, in the book we have already referred to. This

volume includes much information on the natural history of these Insects,

and the outline figures give some idea of the great variety of external

form.

[Illustration: FIG. 354.—_Alaptus excisus_, Westwood. Britain. (Probable

size about ½ millim.)]

Many entomologists include the Mymarides in Proctotrypidae, but Ashmead

considers that they should be treated as a separate family. _Alaptus

excisus_ Westw. (Fig. 354) has been frequently said to be the smallest

known Insect, the {538}measurement given for it by Westwood[447] being a

length of ⅙ of a millimetre—about 1/150 of an inch. Mr. Enock has recently

examined Westwood's type in the Museum at Oxford, and from his information

we may conclude that this Insect is probably the same as _Alaptus fusculus_

Hal., and that the measurement mentioned by Westwood is erroneous, the

Insect being really about half a millimetre long. The Mymarides are,

however, very minute, some of them not exceeding one-third of a millimetre

in length. Whether any of them are smaller than the beetles of the family

Trichopterygidae, some of which are only one-fourth of a millimetre long,

may be doubted.

The Mymarides are recognisable by their very minute size, and by their

peculiar wings. These are slender, destitute of nervures, fringed with

long, delicate hairs, and stalked at the base. Probably Mymarides may all

prove to be dwellers in eggs of other Insects. The group is remarkable from

the fact that it contains some of the very few Hymenoptera with aquatic

habits. Two species were discovered in their winged condition in the water

of a pond near London by Sir John Lubbock[448]; one of them—_Polynema

natans_ Lubbock—probably, according to Mr. Enock, the same as _Caraphractus

cinctus_ Hal., uses its wings freely for swimming under water, while the

other—_Prestwichia aquatica_—performs this operation by the aid of its

legs. This latter Insect seems to be very anomalous, and its position quite

doubtful. The embryogeny of _Polynema_ is very peculiar, and takes place in

the egg of a dragon-fly—_Calepteryx virgo_—under water. According to

Ganin,[449] in the earliest stages the developments of the embryos of the

_Calepteryx_ and of the _Polynema_ progress simultaneously, but that of the

dragon-fly does not proceed beyond the formation of the ventral plate. The

_Polynema_ appears to leave its own egg at an extremely early stage of the

embryonic development. It would appear, in fact, that there is no definite

distinction between embryonic and larval stages. The information given by

Ganin leads to the conclusion that a complete study of this remarkable mode

of development is necessary before forming any general ideas as to the

nature of Insect embryogeny and metamorphosis.

{539}FAM. III. CHALCIDIDAE.

_Pronotum with some freedom of movement, its angles not extending to the

insertion of the front wings. Antennae elbowed, consisting of from seven

to thirteen joints. Wings without a system of cells; with a single

definite nervure proceeding from the base near the front margin, or

costa; afterwards passing to the costa, and giving off a very short vein

more or less thickened at its termination. The species are, with few

exceptions, of parasitic habits._

[Illustration: FIG. 355.—_Eurytoma abrotani_, male. Britain. Hyper-parasite

through _Microgaster_ of _Liparis dispar_, and according to Cameron,

parasite of _Rhodites rosae_ and other gall-flies in Britain, × 10. (After

Ratzeburg.)]

The Insects of this family—the Pteromalini of Ratzeburg—are frequently of

brilliant colours and of remarkable form; the species are very numerous,

some 4000 or more having already been described. Of this number nearly 3000

are European, and as there is good reason for supposing that Chalcididae

are quite as numerous in the Tropics and in the New World as they are in

Europe, the family will probably prove to be one of the largest in the

class. About twenty sub-families have already been proposed for the

classification of the group; they are based chiefly on the number of joints

in the tarsi, and the details of the antennae and of the ovipositor. This

latter exhibits great variety in external appearance, due chiefly to the

modification in form of the basal, or of the following ventral abdominal

plates, one or more of which may be prolonged and altered in form or

direction, giving rise in this way to considerable diversity in the shape

of the abdomen. Correlative with this is a great variety in the mode of

parasitism of the larva. Many live in galls, feeding on the larvae of the

makers of the galls or on those of the inquilines; others attack

caterpillars, others pupae only; some flourish at the expense of bees or

other Hymenoptera, or of Coccidae {540}and Aphididae, and some deposit

their eggs in the egg-cases of Blattidae. The details of the life-history

are well known in only a few cases.

[Illustration: FIG. 356.—_Leucospis gigas_, female. Gibraltar.]

The career of _Leucospis gigas_ has been investigated by Fabre, and

exhibits a very remarkable form of hypermetamorphosis.[450] This Insect is

of comparatively large size and of vivid colours, wasp-like, black

contrasting with yellow, as in the case of the wasps; and like these it has

the wings folded or doubled. The female bears a long ovipositor, which by a

peculiar modification is packed in a groove on the back of the Insect. This

species lives in Southern Europe at the expense of _Chalicodoma muraria_, a

mason-bee that forms cells of a hard cement for its nest, the cells being

placed together in masses of considerable size; each cell contains, or

rather should contain, a larva of the bee, and is closed by masonry, in the

construction of which the bee displays much ability. It is the mission of

the _Leucospis_ to penetrate the masonry by means of its ovipositor, and to

deposit an egg in the cell of the bee. The period chosen for this predatory

attack is the end of July or the beginning of August, at which time the

bee-larva is in the torpid and powerless condition that precedes its

assumption of the pupal state. The _Leucospis_, walking about leisurely and

circumspectly on the masonry of the nest, tests it repeatedly by touching

with the tips of the antennae, for it is most important that a proper spot

should be selected. The bee's cell is placed in a mass of solid masonry, a

considerable part—but a part only—of whose area is occupied by the group of

cells; every cell is closed by hard mortar, making an uneven surface, and

the face of the masonry is rendered more even by a layer of hardened clay

outside the rougher material; it is the task of the _Leucospis_ to detect a

suitable spot, in the apparently uniform external covering, and there to

effect the penetration so as to introduce an egg into a cell. By what

sensations the fly may be guided is unknown. After a spot has been selected

and the ovipositor brought into play, the masonry is ultimately pierced

{541}by patient work; sometimes a quarter of an hour is sufficient for the

purpose, but in other cases three hours of uninterrupted effort are

required before the end is attained. Fabre expended much time in watching

this operation, and after the Insect had completed it, he marked with a

pencil the exact spot of the masonry that was penetrated, and the date on

which it was done, and he states that he afterwards found that without any

exception a proper spot had been selected, and a cell consequently

penetrated. Admirable as the instinct of the parasite appears from this

point of view, it is nevertheless accompanied by a remarkable deficiency in

two other respects. The first is that though the spot selected by the

_Leucospis_ invariably gives entrance to a cell, yet in the majority of the

cases the selected cell is not a suitable one; a large number of the cells

of the _Chalicodoma_ are not occupied by living larvae on the point of

pupation—though in that case only can the egg of the _Leucospis_ hatch and

successfully develop—but by dead and shrivelled larvae, or by mouldy or

dried-up food. And yet, in each case of penetration, Fabre believes that an

egg is deposited, even though it may be impossible that it can undergo a

successful development. Strange as this may appear, it is nevertheless

rendered less improbable by the second deficiency in the instinct of the

parasite. The Insect has no power of recognising a cell that has been

previously pierced either by itself or by another of its species. One bee

larva can only supply nourishment for a single larva of the parasite, and

yet it is a common occurrence for a cell to be revisited, pierced again and

another egg introduced; indeed Fabre, by means of the cells he had marked,

was able to assure himself that it is no uncommon thing for this to be done

four times; four eggs, in fact, are sometimes deposited in a cell that

cannot by any possibility supply food for more than one larva. The egg of

the _Leucospis_ is a curious object (Fig. 357, A), very elongate oval, with

one end drawn out and bent so as to form a hook; it is not placed at random

in the cell of the bee, but is suspended on the delicate cocoon with which

the _Chalicodoma_ larva is surrounded at the period of pupation.

{542}[Illustration: FIG. 357.—_Leucospis gigas._ A, Egg; B, primary, C,

secondary larva. (After Fabre.)]

Fabre allowed sufficient time to elapse for the hatching of the larvae from

the eggs, and then opened some cells where Leucospis eggs had been

deposited, in order to obtain the larvae; when doing this he was surprised

that he never found more than one _Leucospis_ larva in a cell. Even in

cells where he had observed more than one act of oviposition, and which he

had marked at the time, only one larva existed. This induced him to think

that it was possible that no egg was deposited by the _Leucospis_ at the

second penetration. He accordingly examined cells soon after the eggs were

laid, and thus discovered some that contained more than one egg,—indeed in

one cell he observed no less than five eggs suspended from the cocoon of

the _Chalicodoma_; he was also able to demonstrate that eggs were actually

deposited in some cells that contained no means of support for the larva.

How then could these two facts be reconciled—four or five eggs deposited in

a cell, only one larva present afterwards? It is of course impossible to

observe the operations of a larva shrouded in the obscurity of a cell

formed of masonry, so he transferred some bee larvae with their destructive

companions to glass tubes, in which he was able to note what took place. He

found that the egg deposited by the _Leucospis_ hatches and produces a very

peculiar larva, having little resemblance to the _Leucospis_ larva that he

had found eating the _Chalicodoma_ larva. The primary larva (Fig. 357, B)

of the _Leucospis_ is an arched worm, moderately deeply segmented, a

millimetre or a little more in length, with a remarkably large and

abruptly-defined head. The body bears erect setae, the most remarkable of

which are a pair on the ventral aspect of each of the segments, each of

these ventral setae being borne on a small conical prominence. These

prominences and setae serve as ambulatory organs, and are supplemented in

their function by a protuberance at the posterior extremity. The little

creature has considerable powers of locomotion; it moves, after the fashion

of many other larvae, by contracting and arching the body so as to bring

the posterior part nearer to the anterior; then fixing the hinder part, the

anterior is extended and fixed, the posterior being again brought nearer to

the front. The _Leucospis_ larva when hatched does not at once attack the

bee larva which is to be its future food, but every few hours makes

excursions over its surface, and even explores the walls of the cell;

returning, however, always to the cocoon for repose. The object of these

{543}excursions is, Fabre believes, to ascertain if another _Leucospis_ egg

has been laid in the cell, and in that case to destroy it. For the food, as

we have said, being only enough for one larva, and the mother _Leucospis_

frequently laying more than one egg in a cell, it is necessary that all the

eggs except one should be destroyed. Fabre did not actually observe the act

of destruction, but he found repeatedly in his glass tubes that the

supernumerary eggs were destroyed, being, in fact, wounded by the mandibles

of the first-hatched larva. After several days of this wandering life the

tiny destroyer undergoes a first moult, changing its skin and appearing as

a very different creature (Fig. 357, C); it is now completely destitute of

any means of locomotion, very deeply segmented, curved at one extremity,

with a very small head, bearing extremely minute, scarcely perceptible,

mandibles. The sole object of its existence in this state is to extract the

contents of the _Chalicodoma_ larva, and appropriate this material to the

purposes of its own organisation. This it accomplishes not by wounding,

tearing, or destroying the larva, for that apparently would not answer the

purpose; the contents must be conveyed while still in their vital state to

itself; and this it effects by applying its mouth to the extremely delicate

skin of the victim, the contents of whose body then gradually pass to the

destroyer, without any visible destruction of the continuity of the

integument. Thus the _Leucospis_ larva gradually grows, while the bee larva

shrinks and shrivels, without, however, actually suffering death. The

process of emptying the bee larva apparently does not occupy the

_Leucospis_ more than two or three weeks, being completed by about the

middle of the month of August; afterwards the larva remains in the cell by

the side of the shrivelled skin of its victim for ten or eleven months, at

the end of which time it assumes the pupal condition, and very shortly

thereafter appears as a perfect Insect.

_Monodontomerus cupreus_ is another member of the Chalcididae that lives

parasitically at the expense of bees of the genus _Chalicodoma_. Its habits

have been sketched by Fabre,[451] and exhibit considerable difference from

those of _Leucospis_. It is much less in size, and can accommodate itself

to a greater variety of food; it will, in fact, eat not only the larva of

_Chalicodoma_, but also that of another bee, of the genus _Stelis_, that is

frequently found {544}shut up in the cell of the _Chalicodoma_, at whose

expense the _Stelis_ also lives parasitically. The _Monodontomerus_ bores a

hole through the masonry of the bee and deposits its eggs in the cell after

the fashion of the _Leucospis_; one bee larva is, however, sufficient food

for several individuals of the young of this smaller parasite. There is no

hypermetamorphosis, the early larval condition resembling the later. This

Insect attacks not only _Chalicodoma_ and _Stelis_, as already mentioned,

but also other bees; and a single larva of some of the larger kinds will

afford sufficient food for fifty young of the _Monodontomerus_. They feed

on the bee larva, as the _Leucospis_ does, without wounding it. This fly

has the power of recognising what is suitable provender for its young by

the use of the antennae, even when the conditions are so changed that it is

clear the sense of sight has nothing to do with the recognition. Fabre

relates that he had extracted a number of the bee larvae from their cells

of masonry, and that as they were lying on his table enclosed in their

cocoons, the _Monodontomerus_ recognised the latter as containing the

desired provender for its young by examining them with its antennae; after

which, without hesitation, the _Monodontomerus_ pierced the cocoon with its

ovipositor and deposited the eggs in a suitable position. This observation,

together with those made on _Leucospis_, seem to indicate that it is

neither by sight nor smell that these Insects discover the desired object,

but by some sense we do not understand, though its seat is clearly in the

antennae of the Insect.

Newport discovered a _Monodontomerus_, which he described as _M.

nitidus_,[452] in the cells of the bee _Anthophora retusa_, and

demonstrated that the alimentary canal, as is usual in Petiolate

Hymenoptera, is closed behind until the Insect is about to enter the pupal

state, when it becomes perforated and faecal matters are for the first time

passed from it. "These matters were composed of the refuse of digestion and

of epithelial cells accumulated during the period of feeding, and retained

in the digestive sac until the period of its perforation. In this way the

food and abode of the Insects are maintained pure and uncontaminated, and

the digestive apparatus is completed, and the refuse of nutrition ejected

only when the whole of the food has been consumed."

{545}In the cells of the same bee Newport discovered another curious

parasitic Chalcid, _Anthophorabia retusa_.[453] The male has short wings,

and the compound eye is replaced by an ocellus on each side of the head,

the female having fully developed wings and eyes. A variation may occur in

the metamorphosis of this Insect, inasmuch as when the growth is completed

during the month of August, the Insect changes to a pupa, the imago appears

ten or twelve days thereafter, and the perfect Insect then hibernates for

seven or eight months; but should the completion of growth be deferred till

after the end of August, hibernation takes place in the larval condition. A

large and brilliant Chalcid _Eucharis myrmeciae_, has been described by

Cameron as preying on the formidable Australian ants of the genus

_Myrmecia_.

The development of _Smicra clavipes_ has been partially described by

Henneguy.[454] This Insect lives in the interior of the aquatic larva of

_Stratiomys strigosa_, a Dipterous Insect. As many as fifty eggs of the

parasite are found in one larva, but a large number of embryos die during

development, so that he has never found more than two or three well-grown

larvae in one _Stratiomys_ larva. It has been ascertained that the eggs of

many of these parasitic Insects are deficient in yolk, and the ovum of

_Smicra_ is said to obtain the nutritive materials necessary for the

development of the embryo from the blood of its host by endosmosis. For a

long time after the assumption of the larval condition, the larva appears

to nourish itself only at the expense of the blood of its host. The

segmentation of the ovum is total, and a single embryonic membrane appears

at an early period, before the formation of the embryo, by a process very

different from that giving origin to the amnion of the majority of Insects.

A very interesting sketch of the development of _Encyrtus fuscicollis_ has

been given by Bugnion.[455] This small parasite passes its earlier stages

in the interior of the larva of _Hyponomeuta cognatella_ or other

Lepidoptera. The female _Encyrtus_ deposits her eggs in the interior of a

caterpillar, in the form of a series of 50 to 100 or more eggs enclosed in

a sac; the origin {546}of the sac is obscure, but the embryonic development

and the early part of the larval life are passed in the sac, which contains

a supply of nutritive matter. The larvae of the _Encyrtus_ are at first

entirely confined to this sac, but when they have consumed all the

nutritive matter in it, they leave it and pass the remainder of their

larval and pupal existence in the body-cavity of the caterpillar. They live

at first on the lymph (blood) of the Insect, and apparently do it no harm;

nevertheless the strength of the caterpillar is so much enfeebled that it

fails to undergo the transformation to a pupa; the parasites then devour

its interior, and use the empty skin as a nidus for their own pupation;

they form cocoons which divide the area into compartments. Usually the

individuals disclosed from one _Hyponomeuta_ are all of one sex, which may

be either male or female. Unfortunately the most interesting points of this

development, viz. the history of the common sac for the larvae, the nature

of the eggs, the earlier embryonic stages, and the nutriment in the sac,

are still without elucidation. The account given by Bugnion raises a great

desire for information on these points.

We have in a previous page described the remarkable mode of oviposition of

_Mantis_. Captain Xambeu[456] has made a very curious observation to the

effect that a minute Chalcid, _Podagrion (Palmon) pachymerus_, shelters

itself under the wings of the _Mantis_ so as to be in a position to

oviposit in the eggs of the latter when it shall be forming its peculiar

ootheca.

The genus _Isosoma_ consists of Insects that differ in habits from their

congeners, being phytophagous instead of parasitic. _I. tritici_ and _I.

hordei_ live in the stalks of corn, and in North America, where they are

known to the agriculturist as joint-worms, are frequently very injurious to

crops. They are sometimes obtained in large numbers without any males

appearing, and a wingless as well as a winged form of the female occurs.

Owing to the fact that the allies of these Insects are parasitic, it has

been frequently maintained that this may also prove to be the case with

_Isosoma_, but the observations of Riley[457] and others leave no doubt

that the Insects of this genus are really plant-feeders.

{547}Riley has called attention[458] to some facts in connection with _I.

tritici_ and _I. grande_, that make it clear that these two supposed

species are really alternate generations, and that both generations are

probably in larger part, if not entirely, parthenogenetic. Some species of

the genus _Megastigmus_ are known to be of phytophagous habits.[459]

[Illustration: FIG. 358.—_Blastophaga grossorum._ A, Male, × 22; B, female,

× 15. (After Mayer, _Mitt. Stat. Neapel_, iii. 1882.)]

The most interesting of all the forms of Chalcididae are perhaps those

called fig-Insects. A considerable number of species are now known, and

amongst them we meet with the unusual phenomenon of species with wingless

males, the females possessing the organs of flight normally developed. The

wingless males exhibit the strangest forms, and bear no resemblance

whatever to their more legitimately formed partners (Fig. 358, A, B). Many

of the fig-Insects belong to a special group called Agaonides. Others

belong to the group Torymides, which contains likewise many Chalcididae of

an ordinary kind; possibly some of these may be parasitic on the Agaonides.

Some of these Torymid fig-Insects have winged males, as is normal in the

family, but in other cases winged and wingless forms of the male of one

species may be present.

The most notorious of these fig-Insects is the one known as _Blastophaga

grossorum_ (Fig. 358), this being the chief agent in the custom known as

caprification of the cultivated fig-tree. This process has been practised

from time immemorial, and is at the present day still carried on in Italy

and the Grecian archipelago. The Greek writers who describe it say that the

wild fig-tree, though it does not ripen its own fruit, is absolutely

essential for the perfection of the fruit of the cultivated fig. In

accordance with this view, branches from the wild fig {548}are still

gathered at certain seasons and suspended amongst the branches of the

cultivated fig-trees. The young fig is a very remarkable vegetable

production, consisting of a hollow, fleshy receptacle, in which are placed

the extremely numerous and minute flowers, the only admission to which is

by a small orifice at the blunt end of the young fig; this orifice is lined

with projecting scales, that more or less completely fill it up or close

it; nevertheless inside this fruit the _Blastophaga grossorum_ develops in

large numbers. The males are, as we have seen, wingless creatures, and do

not leave the fruit in which they were bred, but the females make their way

out of the wild fig, and some of them, it is believed, enter the young

fruit of the cultivated trees and lay their eggs, or attempt to do so,

therein; and it has been supposed by various writers that these proceedings

are essential to the satisfactory development of the edible fruit. It is a

curious fact that the _Blastophaga_ develops very freely in the wild fig—so

much so, indeed, as to be a means of preventing it from coming to maturity;

but yet the Insect cannot complete its development in the cultivated fruit.

This is due to the fact that the fly must lay its egg in a particular part

of the fig-ovule, so that when the egg hatches the larva may have a proper

supply of food. In the cultivated fig the structure of the flower differs

somewhat from that of the caprificus, as the wild fig is called, and so the

egg, if deposited at all, does not reach a proper nidus for its

development. Hence the _Blastophaga_ can never live exclusively on the

cultivated fig, and if it be really necessary for the development of the

latter, must be brought thereto by means of the caprifig. Whether the

_Blastophaga_ be really of use, as has been for so long supposed, is,

however, a matter for doubt. The reasons for this are (1) that those who

think caprification beneficial do not agree as to the mode in which they

suppose it to be so; (2) that there is but little reason for believing that

when introduced amongst the cultivated figs the _Blastophaga_ occupies

itself to any great extent therewith; and (3) that in some parts of the

world caprification is not performed, but the cultivated fig nevertheless

ripens its fruit there. Hence many writers on the subject—

Solms-Laubach,[460] Mayer,[461] and Saunders[462]—entertain considerable

doubt as to whether caprification is at present anything {549}more than an

old custom destitute of practical utility. On the other hand, Riley

states[463] "that the perfect Smyrna fig, the most esteemed of the edible

species, can be produced only by the intervention of the _Blastophaga

psenes_ [_grossorum_]."

Although the questions connected with the effect the _Blastophaga_ is

supposed to produce on the fruit are of a botanical rather than a

entomological nature, we may briefly say that two views have been held: (1)

that, as in the fruit of the cultivated fig, only female flowers are

produced, the _Blastophaga_ is necessary for their fertilisation and the

subsequent development of the fruit; (2) that the Insects stimulate the fig

by biting parts thereof or by burrowing in it, and so give rise to the

processes that have as their result the edible fruit. There seems to be

little doubt that the Insect agency is necessary to the fertilisation of

some species of figs. Cunningham, who has recently carried out an elaborate

investigation as to the fertilisation of _Ficus roxburghii_,[464] concludes

that in this fig, and probably also in other kinds, the perfect development

is dependent on the access of the fig-Insects to the interior of the

receptacular cavity. Should access fail to occur, both male and female

flowers abort, without the formation of pollen grains by the former or

seeds by the latter. The access of the _Blastophaga_ is thus as necessary

for the perfect evolution of the normal male and female flowers as it is

for that of the modified ♀ or gall-flowers, with their contained ova and

Insect-embryos. Whether the successful fertilisation of the flowers is

really essential to the production of the edible fig is not a question for

our discussion.

Fig-Insects are apparently more numerous in South America than they are in

any other part of the world; and Fritz-Müller has discovered[465] a number

of species there of a very extraordinary character, several of them

possessing two forms of the male, one winged like the female, the other

wingless and so different in character that they were considered to belong

to a different genus. The wingless male of a species found in Madagascar,

_Kradibia_ _cowani_, has the peculiarity of possessing only four legs, the

middle pair being represented merely by minute two-jointed rudiments. Some

of these Insects live in galls on the figs. The fig-Insects {550}were

formerly considered to belong to the Proctotrypidae or to the Cynipidae

(gall-makers), but there can be no doubt, notwithstanding they differ so

much in their habits from the parasitic Chalcididae, that they probably

belong to the same family. If treated as different from Chalcididae, they

should be separated as a distinct family rather than united with the

Cynipidae.[466]

It is impossible for us to do more than allude to the extraordinary shapes

exhibited by some Chalcididae. The genus _Thoracantha_ is specially

remarkable in this respect. _T. latreillei_ is said to resemble a beetle of

the family Mordellidae, and has the wings concealed by false

wing-cases—really projections from the thorax—so that from above the Insect

bears no resemblance to the other Insects of the Order it really belongs

to.

[Illustration: FIG. 359.—_Thoracantha latreillei._ Bahia. A, Upper, B,

lateral aspect. (After Waterhouse.)]

Howard has called attention to some peculiarities in the pupation of

Chalcididae.[467] Like the Cynipidae, they do not make a silken cocoon, but

some of them that change to pupae inside the victims on which they were

nourished have the power of forming oval cells in which to undergo their

transformation, and they thus cause a peculiar inflation of the skin of

their deceased victim, which after death still continues to serve as a

protection to the destroyers. The statement made by Haliday, and repeated

subsequently in various works, to the effect that Coryna spins a cocoon

under the _Aphis_ in which it has lived, is an error, the cocoon being

really formed by _Praon_, a Braconid that is a parasite of the _Aphis_, and

on which the Chalcid _Pachycrepis (Coryna)_ lives as a hyperparasite. The

pupae of some species differ from those of other Hymenoptera, in that the

integument is hard, and the limbs are soldered to the body as in

Lepidoptera. These forms pupate external to the victim.

Fritz Müller has recorded that the pupa of an unnamed species of Chalcid

that attacks a Brazilian ant (_Azteca instabilis_ Forel) is suspended on

the wall of the cell the ant lives in by its posterior extremity, just like

the chrysalis of a butterfly.

{551}Notwithstanding the small size of Chalcididae, their remains have been

detected in the tertiary strata of both Europe and North America.

FAM. IV. ICHNEUMONIDAE (ICHNEUMON-FLIES).

_Wings with a well-developed series of nervures and cells; the space on

the front wing separating the second posterior cell from the cubital

cells is divided into two cells by a transverse veinlet. The abdomen is

attached to the lower or posterior part of the median segment. Larvae

parasitic in habits._

[Illustration: FIG. 360.—_Lissonota setosa_, ♀. Britain. Parasite of the

goat-moth, etc. (After Ratzeburg.)]

The Ichneumonidae form a family of enormous extent, containing nearly 6000

described species. The study of the family is but little advanced, owing to

their parasitic habits and to this bewildering multiplicity in their

specific forms. Most of the species, in the larval state, live inside the

larvae of Lepidoptera, and they thus keep the myriads of caterpillars

within bounds, the number of these destroyed by Ichneumons being

prodigious. Some of the family are, however, external parasites, and some

are known to attack Spiders and Insects of other Orders than Lepidoptera.

Their antennae are not elbowed and are many-jointed, the joints being

closely compacted, especially towards the extremity. This character readily

distinguishes Ichneumonidae from the families we have previously

considered. The ocelli are well developed even in the apterous forms, and

are placed in a triangular position on the vertex. The pronotum is small in

front; and extends backwards at the sides to the points of insertion of the

front wings; it is fixed to the mesonotum. The wings (Fig. 367, A) have a

more complex neuration than those of most of the other parasitic

Hymenoptera, but are occasionally absent in one or both sexes of a species.

The metathorax is very small, and the middle and hind legs are placed close

together. The propodeum is very large, and is frequently covered with a

highly-developed sculpture.

{552}[Illustration: FIG. 361.—_Anomalon circumflexum_, larval development.

(After Ratzeburg.) A, First instar; B, second instar; C, the larva in the

third or encysted stage extracted from its cyst; D, the mature larva; E,

pupa.]

The hind body springs from the lower part of the propodeum; it is usually

of slender form, and its segmentation is very conspicuous. The females bear

an ovipositor, which differs greatly in length according to the species,

and is known in the case of one species to attain a length six times that

of the whole of the rest of the body.[468] The egg is deposited by some

species on the skin, by others within the body of the victim; it varies

much in form and colour, some eggs being stalked and of peculiar shape. The

larvae issuing from the eggs are legless maggots with a delicate integument

of pallid white or creamy colour. If the eggs are laid on the surface of

the body, the resulting larvae (except in the cases of the external

parasites) soon bore into the interior of their victim, and disappear

therein. The changes that take place in the lifetime of the larvae have

been studied in only a few cases; but if we can judge from Ratzeburg's

history[469] of the changes that take place in _Anomalon_, they are of

great interest. From observation of the differences existing amongst a

great number of larvae of _A. circumflexum_ he distinguished four stages.

It is of course impossible to follow directly the growth of one individual,

because it is concealed in the interior of the caterpillar in which it

lives, and to open this involves the death of both caterpillar and

Ichneumon-larva. The life history must therefore be constructed from a

great number of separate observations; and it is not ascertained that the

four instars described by Ratzeburg represent the number of moults of the

larva that actually take place. He, however, entertained no doubt that all

the forms he observed {553}were stages in the development of one species.

In the earliest stage, when only one millimetre in length and about as

thick as a horse-hair, the larva is free in the interior of the

caterpillar's body, and has a small head armed only with a pair of

mandibles. There are, in addition to the head, thirteen segments, and the

last of these is an elongate tail forming nearly one-half the length of the

creature. No trace of tracheae can be discovered. In the second stage the

larva is still free, an elongate tracheal tube exists, the tail has

diminished to half the length, the head has become much larger, and

rudimentary antennae of one joint are visible; possibly stigmata are

present at this stage, though they cannot afterwards be detected. In the

third stage (Fig. 361, C) the larva is encysted, the head is large, the

parts of the mouth are all developed, the tracheal system is extensive, and

the caudal termination of the body is quite short; notwithstanding the

extensive development of the tracheal system, no stigmata can be found. In

the fourth stage the larva is still encysted, the tail has disappeared, the

head and mouth parts are reduced in size and development, and the creature

has now the appearance of a normal larva. The changes to pupa and perfect

Insect take place within the body of the victim, in some cases, if not

usually, after it has undergone its metamorphosis into a chrysalis. Very

little information is extant as to the duration of the various stages, but

it appears to be the rule that only one generation appears annually, though

in some cases there are pretty certainly two.

It is very difficult to observe the act of oviposition; the Ichneumon-flies

usually decline to notice caterpillars with which they are placed in

confinement. Ratzeburg thinks they will only attack caterpillars that are

in a deficient state of health or vitality. Occasionally we may by a happy

chance observe the act in Insects at large, and from the records of

observers it may be deduced with tolerable certainty that the sense of

sight takes no part in the operation. Ratzeburg relates that he saw a

_Pimpla_ alight on a leaf of _Rhus_ and thrust its ovipositor through the

leaf. On looking to the under-side of the leaf he found that a cocoon of

_Bombyx neustria_ was concealed there in such a position that it could not

have been seen by the Ichneumon.

{554}[Illustration: FIG. 362.—_Thalessa lunator._ Oviposition. (After

Riley.)]

Among the most remarkable of the Ichneumon-flies are the Insects of the

genera _Rhyssa_ and _Thalessa_. These fine Insects have an ovipositor three

or four inches in length, and are parasitic on species of the family

Siricidae, which, as we have previously described, live in solid wood. In

order therefore to deposit the egg in a suitable place, the wood must be

pierced by the Ichneumon. The ovipositor is not only of extreme length, but

is also furnished with serrations on its apical part, so that it forms a

very effective boring apparatus. It is brought into use by being bent on

itself over the back of the Insect (Fig. 362), so as to bring the tip

vertically down on to the wood, through which it is then forced by a series

of efforts; the sheaths do not enter the wood. The egg is laid anywhere in

the burrow of the _Sirex_; the young larva seeks its prey, and lives on it

as an external parasite (Fig. 342, D). Erne, however, states[470] that the

young larva of _Rhyssa persuasoria_ enters its victim, and remains within

the latter till its death occurs. This happens when the young _Rhyssa_ is

two or three lines in length, and it then makes its exit from the interior

of the body and gradually eats it up. Should the larva it has attacked be

of large size, it of itself affords sufficient food for the completion of

the growth of the _Rhyssa_. Should the _Rhyssa_, however, have attacked a

small larva, this does not furnish it with sufficient food, and it

consequently dies without seeking another larva. Erne says, indeed, that it

will not eat another if offered to it, so that in order to rear the

_Rhyssa_ in captivity, the victim it has first attacked must always be

given to it. The same observer states that the _Rhyssa_ larva is sometimes

transported by the _Sirex_ deep into the wood, so that when it has

completed its metamorphoses the Ichneumon-fly may find itself buried in

solid wood to a depth of about two inches. In that case it excavates the

wood with its mandibles, and should it fail to gain the exterior after

{555}three days of work, it dies. In the case of _Thalessa_ it is stated

that it sometimes bores into wood where there are no larvae, but Riley

thinks this erroneous; it is, on the other hand, certain that the Insect

after penetrating the wood is frequently unable to withdraw the ovipositor,

and consequently dies.

Packard has recorded,[471] without mentioning the species, the oviposition

of an Ichneumon of which the egg is deposited externally. It was placed on

the head of the caterpillar, and speedily hatched; the young larva at once

bored through the prothoracic segment of the victim, the head of the latter

then became swollen, and covered the opening into the prothorax, made by

the parasite.

[Illustration: FIG. 363.—Young larva of _Paniscus_ in position of feeding

on the skin of _Mamestra_. (After Newport.) _a_, The egg-shell.]