[Illustration: FIG. 63.—Diagram of arrangement of some of the internal

organs of an Insect: _a_, mouth; _b_, mandible; _c_, pharynx; _d_,

oesophagus; _e_, salivary glands (usually extending further backwards);

_f_, eye; _g_, supra-oesophageal ganglion; _h_, sub-oesophageal ganglion;

_i_, tentorium; _j_, aorta; _k_{1}_, _k_{2}_, _k_{3}_, entothorax;

_l_{1}-l_{8}_, ventral nervous chain; _m_, crop; _n_, proventriculus; _o_,

stomach; _p_, Malpighian tubes; _q_, small intestine; _r_, large intestine;

_s_, heart; _t_, pericardial septum; _u_, ovary composed of four egg-tubes;

_v_, oviduct; _w_, spermatheca (or an accessory gland); _x_, retractile

ovipositor; _y_, cercus; _z_, labrum.]

{115}Many of the anatomical structures have positions in the body that are

fairly constant throughout the class. Parts of the respiratory and muscular

systems and the fat-body occur in most of the districts of the body. The

heart is placed just below the dorsal surface; the alimentary canal extends

along the middle from the head to the end of the body. The chief parts of

the nervous system are below the alimentary canal, except that the brain is

placed above the beginning of the canal in the head. The reproductive

system extends in the abdomen obliquely from above downwards, commencing

anteriorly at the upper part and terminating posteriorly at the lower part

of the body cavity.

In Fig. 63 we show the arrangement of some of the chief organs of the body,

with the exception of the muscular and respiratory systems, and the

fat-body. It is scarcely necessary to point out that the figure is merely

diagrammatic, and does not show the shapes and sizes of the organs as they

will be found in any one Insect.

MUSCLES.

The muscular system of Insects is very extensive, Lyonnet[31] having found,

it is said, nearly 4000 muscles in the caterpillar of the goat-moth; a

large part of this number are segmental repetitions, nevertheless the

muscular system is really complex, as may be seen by referring to the study

of the flight of dragon-flies by von Lendenfeld.[32]

The minute structure of the muscles does not differ essentially from what

obtains in Vertebrate animals. The muscles are aggregations of minute

fibrils which are transversely striated, though in variable degree. Those

in the thorax are yellow or pale brown, but in other parts the colour is

more nearly white. The muscles of flight are described as being penetrated

by numerous tracheae, while those found elsewhere are merely surrounded by

these aerating tubules.

The force brought into play by the contractions of Insect muscles is very

great, and has been repeatedly stated to be much superior to that of

Vertebrate animals; very little reliance can, however, be {116}placed on

the assumptions and calculations that are supposed to prove this, and it is

not supported by Camerano's recent researches.[33]

Some of the tendons to which the muscles are attached are very elaborate

structures, and are as hard as the chitinous skeleton, so as to be like

small bones in their nature. A very elaborate tendon of this kind is

connected with the prothoracic trochantin in Coleoptera, and may be readily

examined in _Hydrophilus_. It has been suggested that the entothorax is

tendinous in its origin, but other morphologists treat it, with more

reason, as an elaborate fold inwards of the integument.

[Illustration: FIG. 64.—Cephalic and ventral chain of ganglia: A, larva of

_Chironomus_; B, imago of _Hippobosca_. (After Brandt.)]

NERVOUS SYSTEM.

Insects are provided with a very complex nervous system, which may be

treated as consisting of three divisions:—(1) The cephalic system; (2) the

ventral, or ganglionic chain; (3) an accessory sympathetic system, or

systems. All these divisions are intimately connected. We will consider

first the most extensive, viz. the ventral chain. This consists of a series

of small masses of nervous matter called ganglia which extend in the

longitudinal direction of the body along the median line of the lower

aspect, and are connected by longitudinal commissures, each ganglion being

joined to that following it by two threads of nervous matter. Each of the

ganglia of the ventral chain really consists of two ganglia placed side by

side and connected by commissures as well as cellular matter. In larvae

some of the ganglia may be contiguous, so that the commissures do not

exist. From the ganglia motor nerves proceed to the various parts of the

{117}body for the purpose of stimulating and co-ordinating the contractions

of the muscles. The number of the ganglia in the ventral chain differs

greatly in different Insects, and even in the different stages of

metamorphosis of the same species, but never exceeds thirteen. As this

number is that of the segments of the body, it has been considered that

each segment had primitively a single ganglion. Thirteen ganglia for the

ventral chain can, however, be only demonstrated in the embryonic state; in

the later stages of life eleven appears to be the largest number that can

be distinguished, and so many as this are found but rarely, and then

chiefly in the larval stage. The diminution in number takes place by the

amalgamation or coalescence of some of the ganglia, and hence those Insects

in which the ganglia are few are said to have a highly concentrated nervous

system. The modes in which these ganglia combine are very various; the most

usual is perhaps that of the combination of the three terminal ganglia into

one body. As a rule it may be said that concentration is the concomitant of

a more forward position of the ganglia. As a result of this it is found

that in some cases, as in Lamellicorn beetles, there are no ganglia situate

in the abdomen. In the perfect state of the higher Diptera, the thoracic

and abdominal ganglia are so completely concentrated in the thorax as to

form a sort of thoracic brain. In Fig. 64 we represent a very diffuse and a

very concentrated ganglionic chain; A being that of the larva of

_Chironomus_, B that of the imago of _Hippobosca_. In both these sketches

the cephalic ganglia as well as those of the ventral chain are shown.

Turning next to the cephalic masses, we find these in the perfect Insect to

be nearly always two in number: a very large and complex one placed above

the oesophagus, and therefore called the supra-oesophageal ganglion; and a

smaller one, the sub- or infra-oesophageal, placed below the oesophagus.

The latter ganglion is in many Insects so closely approximated to the

supra-oesophageal ganglion that it appears to be a part thereof, and is

sometimes spoken of as the lower brain. In other Insects these two ganglia

are more remote, and the infra-oesophageal one then appears part of the

ventral chain. In the embryo it is said that the mode of development of the

supra-oesophageal ganglion lends support to the idea that it may be the

equivalent of three ganglia; there being at one {118}time three lobes,

which afterwards coalesce, on each side of the mouth. This is in accordance

with the view formulated by Viallanes[34] to the effect that this great

nerve-centre, or brain, as it is frequently called, consists essentially of

three parts, viz. a Proto-, a Deuto-, and a Trito-cerebron. It is, however,

only proper to say that though the brain and the ventral chain of ganglia

may appear to be one system, and in the early embryonic condition to be

actually continuous, these points cannot be considered to be fully

established. Dr. L. Will has informed us[35] that in Aphididae the brain

has a separate origin, and is only subsequently united with the ganglionic

chain. Some authorities say that in the early condition the sub-oesophageal

ganglion is formed from two, and the supra-oesophageal from the same number

of ganglia; the division in that case being 2 and 2, not 3 and 1, as

Viallanes' views would suggest. The inquiries that are necessary to

establish such points involve very complex and delicate investigations, so

that it is not a matter of surprise that it cannot yet be said whether each

of these views may be in certain cases correct. The supra- and

sub-oesophageal ganglia are always intimately connected by a commissure on

each side of the oesophagus; when very closely approximated they look like

one mass through which passes the oesophagus (Fig. 66, A). The large

supra-oesophageal ganglion supplies the great nerves of the cephalic

sense-organs, while the smaller sub-oesophageal centre gives off the nerves

to the parts of the mouth. From the lower and anterior part of the

supra-oesophageal ganglion a nervous filament extends as a ring round the

anterior part of the oesophagus, and supplies a nerve to the upper lip.[36]

This structure is not very well known, and has been chiefly studied by

Liénard,[37] who considers that it will prove to be present in all Insects.

Whether the two cephalic ganglia be considered as really part of a single

great ganglionic chain, or the reverse, they are at any rate always

intimately connected with the ventral ganglia. We have already stated that

the two cephalic masses are themselves closely approximated in many

Insects, and may add that in some Hemiptera the first thoracic ganglion of

the ventral chain is amalgamated into one body with the sub-oesophageal

ganglion, {119}and further that there are a few Insects in which this

latter centre is wanting. If the cephalic ganglia and ventral chain be

looked on as part of one system, this may be considered as composed

originally of seventeen ganglia, which number has been demonstrated in some

embryos.

The anatomy of the supra-oesophageal ganglion is very complex; it has been

recently investigated by Viallanes[38] in the wasp (_Vespa_) and in a

grasshopper (_Caloptenus italicus_). The development and complication of

its inner structure and of some of its outer parts appear to be

proportional with the state of advancement of the instinct or intelligence

of the Insect, and Viallanes found the brain of the grasshopper to be of a

more simple nature than that of the wasp.

[Illustration: FIG. 65.—Brain of Worker Ant of _Formica rufa_. (After

Leydig, highly magnified.) Explanation in text.]

Brandt, to whom is due a large part of our knowledge of the anatomy of the

nervous system in Insects, says that the supra-oesophageal ganglion varies

greatly in size in various Insects, its mass being to a great extent

proportional with the development of the compound eyes; hence the absolute

size is not a criterion for the amount of intelligence, and we must rather

look to the complication of the structure and to the development of certain

parts for an index of this nature. The drone in the honey-bee has,

correlatively with the superior development of its eyes, a larger brain

than the worker, but the size of the hemispheres, and the development of

the gyri cerebrales is superior in the latter. In other words, the mass of

{120}those great lobes of the brain that are directly connected with the

faceted eyes must not be taken into account in a consideration of the

relation of the size and development of the brain to the intelligence of

the individual. The weight of the brain in Insects is said by Lowne to vary

from 1/150 to 1/2500 of the weight of the body.

Figure 65 gives a view of one side of the supra-oesophageal ganglion of the

worker of an ant,—_Formica rufa_,—and is taken from Leydig, who gives the

following elucidation of it: _A_, primary lobe, _a_, homogeneous granular

inner substance, _b_, cellular envelope; _B_, stalked bodies (gyri

cerebrales), _a_, _b_, as before; _C_, presumed olfactory lobes, _c_, inner

substance, _d_, ganglionic masses; _D_, ocular lobes, _e_, _f_, _g_, _h_,

various layers of the same; _E_, origin of lateral commissures; _F_, median

commissure in interior of brain; _G_, lower brain (sub-oesophageal

ganglion); _H_, ocelli; _J_, faceted eye.

[Illustration: FIG. 66.—Stomato-gastric nerves of Cockroach: A, with brain

_in situ_, after Koestler; B, with the brain removed, after Miall and

Denny: _s.g_, supra-oesophageal ganglion; _o_, optic nerve; _a_, antennary

nerve; _f.g_, frontal ganglion; _oe_, oesophagus; _c_, connective; _p.g_,

paired ganglia; _v.g_, crop or ventricular ganglion; _r_, recurrent nerve.]

Besides the brain and the great chain of ganglia there exists an accessory

system, or systems, sometimes called the sympathetic, vagus, or visceral

system. Although complex, these parts are delicate and difficult of

dissection, and are consequently not so well known as is the ganglionic

chain. There is a connecting or median nerve cord, communicating with the

longitudinal commissures of each segment, and itself dilating into ganglia

at intervals; this is sometimes called the unpaired system. There is

another group of nerves having paired ganglia, {121}starting from a small

ganglion in the forehead, then connecting with the brain, and afterwards

extending along the oesophagus to the crop and proventriculus (Fig. 66).

This is usually called the stomatogastric system. The oesophageal ring we

have already spoken of.

By means of these accessory nervous systems all the organs of the body are

brought into more or less direct relation with the brain and the ganglionic

chain.

Our knowledge of these subsidiary nervous systems is by no means extensive,

and their nomenclature is very unsettled; little is actually known as to

their functions.

ORGANS OF SENSE.

Insects have most delicate powers of perception, indeed they are perhaps

superior in this respect to the other classes of animals. Their senses,

though probably on the whole analogous to those of the Vertebrata, are

certainly far from corresponding therewith, and their sense organs seem to

be even more different from those of what we call the higher animals than

the functions themselves are. We have already briefly sketched the

structure of the optical organs, which are invariably situate on the head.

This is not the case with the ears, which certainly exist in one Order,—the

Orthoptera,—and are placed either on the front legs below the knee, or at

the base of the abdomen. Notwithstanding their strange situation, the

structures alluded to are undoubtedly auditory, and somewhat approximate in

nature to the ear of Vertebrates, being placed in proximity to the inner

face of a tense membrane; we shall refer to them when considering the

Orthoptera. Sir John Lubbock considers—no doubt with reason—that some ants

have auditory organs in the tibia. Many Insects possess rod-like or

bristle-like structures in various parts of the body, called chordotonal

organs; they are considered by Graber[39] and others to have auditory

functions, though they are not to be compared with the definite ears of the

Orthoptera.

The other senses and sense organs of Insects are even less known, and have

given rise to much perplexity; for though many structures have been

detected that may with more or less probability be looked on as sense

organs, it is difficult to assign a {122}particular function to any of

them, except it be to the sensory hairs. These are seated on various parts

of the body. The chitinous covering, being a dead, hard substance, has no

nerves distributed in it, but it is pierced with orifices, and in some of

these there is implanted a hair which at its base is in connexion with a

nerve; such a structure may possibly be sensitive not only to contact with

solid bodies, but even to various kinds of vibration. We give a figure

(Fig. 67) of some of these hairs on the caudal appendage of a cricket,

after Vom Rath. The small hairs on the outer surface of the chitin in this

figure have no sensory function, but each of the others probably has; and

these latter, being each accompanied by a different structure, must, though

so closely approximated, be supposed to have a different function; but in

what way those that have no direct connexion with a nerve may act it is

difficult to guess.

The antennae of Insects are the seats of a great variety of sense organs,

many of which are modifications of the hair, pit and nerve structure we

have described above, but others cannot be brought within this category.

Amongst these we may mention the pits covered with membrane (figured by

various writers), perforations of the chitin without any hair, and

membranous bodies either concealed in cavities or partially protruding

therefrom.

[Illustration: FIG. 67.—Longitudinal section of portion of caudal appendage

of _Acheta domestica_ (after Vom Rath): _ch_, chitin; _hyp_, hypodermis;

_n_, nerve; _h^1_, integumental hairs, not sensitive; _h^2_, ordinary hair;

_h^3_, sensory hair; _h^4_, bladder-like hair; _sz_, sense-cell.]

[Illustration: FIG. 68.—Longitudinal section of apex of palpus of _Pieris

brassicae_: _sch_, scales; _ch_, chitin; _hyp_, hypodermis; _n_, nerve;

_sz_, sense cells; _sh_, sense hairs. (After Vom Rath.)]

Various parts of the mouth are also the seats of sense organs of different

kinds, some of them of a compound character; in such cases there may be a

considerable number of hairs seated on branches of a common nerve as

figured {123}by Vom Rath[40] on the apex of the maxillary palp of _Locusta

viridissima_, or a compound organ such as we represent in Fig. 68 may be

located in the interior of the apical portion of the palp.

The functions of the various structures that have been detected are, as

already remarked, very difficult to discover. Vom Rath thinks the cones he

describes on the antennae and palpi are organs of smell, while he assigns

to those on the maxillae, lower lip, epipharynx, and hypopharynx the _rôle_

of taste organs, but admits he cannot draw any absolute line of distinction

between the two forms. The opinions of Kraepelin, Hauser, and Will, as well

as those of various earlier writers, are considered in Sir John Lubbock's

book on this subject.[41]

ALIMENTARY AND NUTRITIVE SYSTEM.

The alimentary canal occupies the median longitudinal axis of the body,

being situated below the dorsal vessel, and above the ventral nervous

chain; it extends from the mouth to the opposite extremity of the body. It

varies greatly in the different kinds of Insects, but in all its forms it

is recognised as consisting essentially of three divisions: anterior,

middle, and posterior. The first and last of these divisions are considered

to be of quite different morphological nature from the middle part, or true

stomach, and to be, as it were, invaginations of the extremities of a

closed bag; it is ascertained that in the embryo these invaginations have

really blind extremities (see Fig. 82, p. 151), and only subsequently

become connected with the middle part of the canal. There are even some

larvae of Insects in which the posterior portion of the canal is not opened

till near the close of the larval life; this is the case with many

Hymenoptera, and it is probable, though not as frequently stated certain,

that the occlusion marks the point of junction of the proctodaeum with the

stomach. The anterior and posterior parts of the canal are formed by the

ectoderm of the embryo, and in embryological and morphological language are

called respectively the stomodaeum and proctodaeum; the true stomach is

formed from the endoderm, {124}and the muscular layer of the whole canal

from the mesoderm.

[Illustration: FIG. 69.—Digestive system of _Xyphidria camelus_ (after

Dufour): _a_, head capsule; _b_, salivary glands; _c_, oesophagus; _d_,

crop; _e_, proventriculus; _f_, chyle, or true stomach; _g_, small

intestine; _h_, large intestine; _i_, Malpighian tubes; _k_, termination of

body.]

The alimentary canal is more complex anatomically than it is

morphologically, and various parts are distinguished, viz. the canal and

its appendicula; the former consisting of oesophagus, crop, gizzard, true

stomach, and an intestine divided into two or more parts. It should be

remarked that though it is probable that the morphological distinctions

correspond to a great extent with the anatomical lines of demarcation, yet

this has not been sufficiently ascertained: the origin of the proctodaeum

in _Musca_ is indeed a point of special difficulty, and one on which there

is considerable diversity of opinion. In some Hemiptera the division of the

canal into three parts is very obscure, so that it would be more correct,

as Dufour says, to define it as consisting in these Insects of two main

divisions—one anterior to, the other posterior to, the insertion of the

Malpighian tubes.

It should be borne in mind that the alimentary canal is very different in

different Insects, so that the brief general description we must confine

ourselves to will not be found to apply satisfactorily to any one Insect.

The oesophagus is the part behind the mouth, and is usually narrow, as it

has to pass through the most important nervous centres; extremely variable

in length, it dilates behind to form the crop. It may, too, have a

dilatation immediately behind the mouth, and in such case a pharynx is

considered to exist. The crop is broader than the oesophagus, and must be

looked on as a mere dilatation of the latter, as no line of

{125}demarcation can be pointed out between the two, and the crop may be

totally absent.

In some of the sucking Insects there is a lateral diverticulum, having a

stalk of greater or less length, called the sucking-stomach; it is by no

means certain that the function this name implies is correctly assigned to

the organ.

The gizzard or proventriculus (French, _gésier_; German, _Kaumagen_) is a

small body interposed in some Insects between the true stomach and the crop

or oesophagus. It is frequently remarkable for the development of its

chitinous lining into strong toothed or ridged processes that look as if

they were well adapted for the comminution of food. The function of the

proventriculus in some Insects is obscure; its structure is used by

systematists in the classification of ants. The extremity of the

proventriculus not infrequently projects into the cavity of the stomach.

The true stomach, or chylific ventricle (_Magen_ or _Mitteldarm_ of the

Germans), is present in all the post-embryonic stages of the Insect's life,

existing even in the imagines of those who live only for a few hours, and

do not use the stomach for any alimentary purpose. It is so variable in

shape and capacity that no general description of it can be given.

Sometimes it is very elongate, so that it is coiled and like an intestine

in shape; it very frequently bears diverticula or pouches, which are placed

on the anterior part, and vary greatly in size, sometimes they are only two

in number, while in other cases they are so numerous that a portion of the

outside of the stomach looks as if it were covered with villi. A division

of the stomach into two parts is in some cases very marked, and the

posterior portion may, in certain cases, be mistaken for the intestine; but

the position of the Malpighian tubes serves as a mark for the distinction

of the two structures, the tubes being inserted just at the junction of the

stomach with the intestine.

The intestine is very variable in length: the anterior part is the smaller,

and is frequently spoken of as the colon; at the extremity of the body the

gut becomes much larger, so as to form a rectum. There is occasionally a

diverticulum or "caecum" connected with the rectum, and in some Insects

stink-glands. In some Hemiptera there is no small intestine, the Malpighian

tubes being inserted at the junction of the stomach with the {126}rectum.

The total length of the alimentary canal is extremely variable; it is

necessarily at least as long as the distance between the mouth and anal

orifice, but sometimes it is five or six times as long as this, and some of

its parts then form coils in the abdominal cavity.

The alimentary canal has two coats of muscles: a longitudinal and a

transverse or annular. Both coexist in most of its parts. Internal to these

coats there exists in the anterior and posterior parts of the canal a

chitinous layer, which in the stomach is replaced by a remarkable

epithelium, the cells of which are renewed, new ones growing while the old

are still in activity. We figure a portion of this structure after Miall

and Denny, and may remark that Oudemans[42] has verified the correctness of

their representation. The layers below represent the longitudinal and

transverse muscles.

[Illustration: FIG. 70.—Epithelium of stomach of Cockroach (after Miall and

Denny): the lower parts indicate the transverse and longitudinal muscular

layers.]

In addition to the various diverticula we have mentioned, there are two

important sets of organs connected with the alimentary canal, viz. the

salivary glands and the Malpighian tubes.

The salivary glands are present in many Insects, but are absent in others.

They are situate in the anterior portion of the body, and are very variable

in their development, being sometimes very extensive, in other cases

inconspicuous. They consist either of simple tubes lined with cells, or of

branched tubes, or of tubes dilated laterally into little acini or groups

of bags, the arrangement then somewhat resembling that of a bunch of

grapes. There are sometimes large sacs or reservoirs connected with the

efferent tubes proceeding from the secreting portions of the glands. The

salivary glands ultimately discharge into the mouth, so that the fluid

secreted by them has to be {127}swallowed in the same manner as the food,

not improbably along with it. The silk so copiously produced by some larvae

comes from very long tubes similar in form and situation to the simple

tubes of the salivary glands.

The Malpighian tubules are present in most Insects, though they are

considered on good authority to be absent in many Collembola and in some

Thysanura. They are placed near the posterior part of the body, usually

opening into the alimentary canal just at the junction of the stomach and

the intestine, at a spot called the pylorus. They vary excessively in

length and in number,[43] being sometimes only two, while in other cases

there may be a hundred or even more of them. In some cases they are budded

off from the hind-gut of the embryo when this is still very small; in other

cases they appear later; frequently their number is greater in the adult

than it is in the young. In _Gryllotalpa_ there is one tube or duct with a

considerable number of finer tubes at the end of it. There is no muscular

layer in the Malpighian tubes, they being lined with cells which leave a

free canal in the centre. The tubes are now thought, on considerable

evidence, to be organs for the excretion of uric acid or urates, but it is

not known how they are emptied. Marchal has stated[44] that he has seen the

Malpighian tubes, on extraction from the body, undergo worm-like movements;

he suggests that their contents may be expelled by similar movements when

they are in the body.

The functions of the different portions of the alimentary canal, and the

extent to which the ingested food is acted on by their mechanical

structures or their products is very obscure, and different opinions

prevail on important points. It would appear that the saliva exercises a

preparatory action on the food, and that the absorption of the nutritive

matter into the body cavity takes place chiefly from the true stomach,

while the Malpighian tubes perform an excretory function. Beyond these

elementary, though but vaguely ascertained facts, little is known, though

Plateau's[45] and Jousset's researches on the digestion of Insects throw

some light on the subject.

{128}RESPIRATORY ORGANS.

The respiration of Insects is carried on by means of a system of vessels

for the conveyance of air to all parts of the body; this system is most

remarkably developed and elaborate, and contrasts strongly with the

mechanism for the circulation of the blood, which is as much reduced as the

air system is highly developed, as well as with the arrangement that exists

in the Vertebrates. There are in Insects no lungs, but air is carried to

every part of the body directly by means of tracheae. These tracheae

connect with the spiracles—the orifices at the sides of the body we have

already mentioned when describing the external structures—and the air thus

finds its way into the most remote recesses of the Insect's body. The

tracheae are all intimately connected. Large tubes connect the spiracles

longitudinally, others pass from side to side of the body, and a set of

tracheae for the lower part of the body is connected with another set on

the upper surface by means of several descending tubes. From these main

channels smaller branches extend in all directions, forking and giving off

twigs, so that all the organs inside the body can be supplied with air in

the most liberal manner. On opening a freshly deceased Insect the abundance

of the tracheae is one of the peculiarities that most attracts the

attention; and as these tubes have a peculiar white glistening appearance,

they are recognised without difficulty. In Insects of active flight,

possibly in some that are more passive, though never in larvae, there are

air-sacs, of more than one kind, connected with the tracheae, and these are

sufficiently capacious to have a considerable effect in diminishing the

specific gravity of the Insect. The most usual situation for these sacs is

the basal portion of the abdominal cavity, on the great lateral tracheal

conduits. In speaking of the external structure we have remarked that the

stigmata, or spiracles, by which the air is admitted are very various in

their size and in the manner in which they open and close. Some spiracles

have no power of opening; while others are provided with a muscular and

valvular apparatus for the purpose of opening and closing effectually.

The structure of the tracheae is remarkable: they are elastic and consist

of an outer cellular, and an inner chitinous layer; this latter is

strengthened by a peculiar spiral fibre, which gives {129}to the tubes,

when examined with the microscope, a transversely, closely striated

appearance. Packard considers[46] that in some tracheae this fibre is not

really spiral, but consists of a large number of closely placed rings. Such

a condition has not, however, been recorded by any other observer. The

spiral fibre is absent in the fine capillary twigs of the tracheal system,

as well as from the expanded sacs. The mode of termination of the capillary

branches is not clear. Some have supposed that the finest twigs anastomose

with others; on the other hand it has been said that they terminate by

penetrating cells, or that they simply come to an end with either open or

closed extremities. Wistinghausen[47] states that in the silk-glands the

tracheal twigs anastomose, and he is of opinion that the fine terminal

portions contain fluid. However this may be, it is certain that all the

organs are abundantly supplied with a capillary tracheal network, or

arboreal ramification, and that in some cases the tubes enter the substance

of tissues. Near their terminations they are said to be 1/30 to 1/60

millimetre in diameter.

[Illustration: FIG. 71.—Portion of the abdominal part of tracheal system of

a Locust (_Oedipoda_): _a_, spiracular orifices; _b_, tracheal tubes; _c_,

vesicular dilatations; _d_, tracheal twigs or capillaries. (After Dufour.)]

We must repeat that such a system as we have just sketched forms a striking

contrast to the imperfect blood-vascular system, and that Insects differ

profoundly in these respects from Vertebrate animals. In the latter the

blood-vessels penetrate to all {130}the tissues and form capillaries, while

the aerating apparatus is confined to one part of the body; in Insects the

blood-circulating system is very limited, and air is carried directly by

complex vessels to all parts; thus the tracheal system is universally

recognised as one of the most remarkable of the characters of Insects. Many

Insects have a very active respiratory system, as is shown by the rapidity

with which they are affected by agents like chloroform; but the exact

manner in which the breathing is carried on is unknown. In living Insects

rapid movements of contraction and expansion of parts of the body, chiefly

the abdomen, may be observed, and these body contractions are sometimes

accompanied by opening and shutting the spiracular orifices: it has been

inferred that these phenomena are respiratory. Although such movements are

not always present, it is possible that when they occur they may force the

air onwards to the tissues, though this is by no means certain. It is clear

that the tracheal system is the usual means of supplying the organisation

with oxygen, but it appears to be improbable that it can also act as the

agent for removing the carbonaceous products of tissue-changes. It has been

thought possible that carbonic acid might reach the spiracles from the

remote capillaries by a process of diffusion,[48] but it should be

recollected that as some Insects have no tracheal system, there must exist

some other mode of eliminating carbonic acid, and it is possible that this

mode may continue to operate as an important agent of purification, even

when the tracheal system is, as a bearer of air to the tissues, highly

developed. Eisig[49] has suggested that the formation of chitin is an act

of excretion; if so this is capable of relieving the system of carbonic

acid to some extent. Others have maintained that transpiration takes place

through the delicate portions of the integument. Lubbock[50] has shown that

Melolontha larvae breathe "partly by means of their skin." The mode in

which the carbon of tissue-change, and the nitrogen of inspiration are

removed, is still obscure; but it appears probable that the views expressed

by Réaumur, Lyonnet, and Lowne[51] as to inspiration and expiration may

prove to be nearer the truth than those which are more widely current. In

{131}connexion with this it should be recollected that the outer integument

consists of chitin, and is cast and renewed several times during the life

of the individual. Now as chitin consists largely of carbon and nitrogen,

it is evident that the moulting must itself serve as a carbonaceous and

nitrogenous excretion. If, as is suggested by Bataillon's researches,[52]

the condition accompanying metamorphosis be that of asphyxia, it is

probable that the secretion of the new coat of chitin may figure as an act

of excretion of considerable importance. If there be any truth in this

suggestion it may prove the means of enabling us to comprehend some points

in the development of Insects that have hitherto proved very perplexing.

Peyrou has shown[53] that the atmosphere extracted from the bodies of

Insects (_Melolontha_) is much less rich in oxygen than the surrounding

atmosphere is, and at ordinary temperatures always contains a much larger

proportion of carbonic acid: he finds, too, that as in the leaves with

which he makes a comparison, the proportion of oxygen augments as the

protoplasmic activity diminishes. Were such an observation carried out so

as to distinguish between the air in the tracheal system and the gas in

other parts of the body the result would be still more interesting.

We know very little as to the animal heat produced by insects, but it is

clear from various observations[54] that the amount evolved in repose is

very small. In different conditions of activity the temperature of the

insect may rise to be several degrees above that of the surrounding medium,

but there seems to be at present no information as to the physiological

mode of its production, and as to the channel by which the products—whether

carbonic acid or other matters—may be disposed of.

In the order Aptera (Thysanura and Collembola) the tracheal system is

highly peculiar. In some Collembola it apparently does not exist, and in

this case we may presume with greater certainty that transpiration of gases

occurs through the integument: in other members of this Order tracheae are

present in a more or less imperfect state of development, but the tracheae

of different segments do not communicate with one another, {132}thus

forming a remarkable contrast to the amalgamated tracheal system of the

other Orders of Insects, where, even when the tracheal system is much

reduced in extent (as in Coccidae), it is nevertheless completely unified.

_Gryllotalpa_ is, however, said by Dohrn[55] to be exceptional in this

respect; the tracheae connected with each spiracle remaining unconnected.

Water Insects have usually peculiarities in their respiratory systems,

though these are not so great as might _à priori_ have been anticipated.

Some breathe by coming to the surface and taking in a supply of air in

various manners, but some apparently obtain from the water itself the air

necessary for their physiological processes. Aquatic Insects are frequently

provided with gills, which may be either wing-like expansions of the

integument containing some tracheae (Ephemeridae larvae), or bunches of

tubes, or single tubes (Trichoptera larvae). Such Insects may either

possess stigmata in addition to the gills, or be destitute of them. In

other cases air is obtained by taking water into the posterior part of the

alimentary canal (many dragon-flies), which part is then provided with

special tracheae. Some water-larvae appear to possess neither stigmata nor

gills (certain Perlidae and Diptera), and it is supposed that these obtain

air through the integument; in such Insects tracheal twigs may frequently

be seen on the interior of the skin. In the imago state it is the rule that

Water Insects breathe by means of stigmata, and that they carry about with

them a supply of air sufficient for a longer or shorter period. A great

many Insects that live in water in their earlier stages and breathe there

by peculiar means, in their perfect imago state live in the air and breathe

in the usual manner. There are, in both terrestrial and aquatic Insects, a

few cases of exsertile sacs without tracheae, but filled with blood

(_Pelobius_ larva, _Machilis_, etc.); and such organs are supposed to be of

a respiratory nature, though there does not appear to be any positive

evidence to that effect.

BLOOD AND BLOOD-CIRCULATION.

Owing to the great complexity of the tracheal system, and to its general

diffusion in the body, the blood and its circulation are very different in

Insects from what they are in Vertebrates, so {133}that it is scarcely

conducive to the progress of physiological knowledge to call two fluids

with such different functions by one name. The blood of Insects varies

according to the species, and in all probability even in conformity with

the stage of the life of the individual. Its primary office is that of

feeding the tissues it bathes, and it cannot be considered as having any

aerating function. It is frequently crowded with fatty substances. Graber

says: "The richness of Insect blood in unsaponified or unelaborated fat

shows in the plainest manner that it is more properly a mixture of blood

and chyle; or indeed we might say with greater accuracy, leaving out of

consideration certain matters to be eliminated from it, that it is a

refined or distilled chyle." Connected in the most intimate manner with the

blood there is a large quantity of material called vaguely the fat-body;

the blood and its adjuncts of this kind being called by Wielowiejski[56]

the blood-tissue. We shall return to the consideration of this tissue after

sketching the apparatus for distributing the refined chyle, or blood as we

must, using the ordinary term, call it.

There is in Insects no complete system of blood-vessels, though there is a

pulsating vessel to ensure distribution of the nutritive fluid. This dorsal

vessel, or heart as it is frequently called, may be distinguished and its

pulsations watched, in transparent Insects when alive. It is situate at the

upper part of the body, extending from the posterior extremity, or near it,

to the head or thorax, and is an elongate tube, consisting as it were of a

number of united chambers; it is closed behind, except in some larvae, but

is open in front, and has several orifices at the sides; these orifices, or

ostia, are frequently absent from the front part of the tube, which portion

is also narrower, being called the aorta—by no means a suitable term. Near

the lateral orifices there are delicate folds, which act to some extent as

valves, facilitating, in conjunction with the mode of contraction of the

vessel, a forward movement of the blood. The composition of the tube, or

series of chambers, is that of a muscular layer, with internal and external

membranous coverings, the intima and adventitia. Olga Poletajewa states[57]

that in _Bombus_ the dorsal vessel consists of five chambers placed in

longitudinal succession, and not very intimately connected, and that there

is but little {134}valvular structure. In _Cimbex_ she finds a similar

arrangement, but there are ten chambers, and no aorta.

The dorsal vessel is connected with the roof of the body by some short

muscles, and is usually much surrounded by fat-body into which tracheae

penetrate; by these various means it is kept in position, though only

loosely attached; beneath it there is a delicate, incomplete or fenestrate,

membrane, delimiting a sort of space called the pericardial chamber or

sinus; connected with this membrane are some very delicate muscles, the

alary muscles, extending inwards from the body wall (_b_, Fig. 72): the

curtain formed by these muscles and the fenestrate membrane is called the

pericardial diaphragm or septum. The alary muscles are not directly

connected with the heart.

[Illustration: FIG. 72.—Dorsal vessel (_c_), and alary muscles (_b_), of

_Gryllotalpa_ (after Graber); _a_, aorta. _N.B._—The ventral aspect is here

dorsal, and nearly the whole of the body is removed to show these parts.]

[Illustration: FIG. 73.—Diagram of transverse section of pericardial sinus

of _Oedipoda coerulescens_. (After Graber, _Arch. Mikr. Anat._ ix.) H,

heart; _s_, septum; _m_, muscles—the upper suspensory, the lower alary.]

It has been thought by some that delicate vessels exist beyond the aorta

through which the fluid is distributed in definite channels, but this does

not appear to be really the case, although the fluid may frequently be seen

to move in definite lines at some distance from the heart.

There is still much uncertainty as to some of the details of the action of

the heart, and more especially as to the influence of the alary muscles.

The effect of the contraction of these must be to increase the area of the

pericardial chamber by rendering {135}its floor or septum less arched, as

shown in our diagram (Fig. 73), representing a transverse section through

the pericardial chamber, H being the dorsal vessel with _m_ its suspensory

muscles, and _s_ its septum, with _m_ the alary muscles. The contraction of

these latter would draw the septum into the position of the dotted line,

thus increasing the area of the sinus above; but as this floor or septum is

a fenestrated structure, its contraction allows fluid to pass through it to

the chamber above; thus this arrangement may be looked on as a means of

keeping up a supply of fluid to the dorsal vessel, the perforated septum,

when it contracts, exerting pressure on the tissues below; these are

saturated with fluid, which passes through the apertures to the enlarged

pericardial chamber.

Some misconception has prevailed, too, as to the function of the

pericardial chamber. This space frequently contains a large quantity of

fat-body—pericardial tissue—together with tracheae, and this has given rise

to the idea that it might be lung-like in function; but, as Miall and

Denny[58] have pointed out, this is erroneous; the tissues in Insects have

their own ample supplies of air. It has also been supposed that the alary

muscles cause the contraction of the heart, but this is not directly the

case, for they are not attached to it, and it pulsates after they have been

severed. It has been suggested that the contractions of this vessel are

regulated by small ganglia placed on, or in, its substance. However this

may be, these contractions vary enormously according to the condition of

the Insect; they may be as many, it is said, as 100 or more in a minute, or

they may be very slow and feeble, if not altogether absent, without the

death of the Insect ensuing.

The expulsion of the blood from the front of the dorsal vessel seems to be

due to the rhythm of the contraction of the vessel as well as to its

mechanical structure. Bataillon says,[59] confirming an observation of

Réaumur, that at the period when the silkworm is about to change to the

chrysalis condition, the circulation undergoes periodical changes, the

fluid moving during some intervals of about ten minutes' duration in a

reversed direction, while at other times the blood is expelled in front and

backwards simultaneously, owing apparently to a rhythmical change in the

mode of contraction of the dorsal vessel.

{136}As the dorsal vessel consists of a number of distinct chambers, it has

been suggested that there is normally one of these for each segment of the

body; and it appears that the total number is sometimes thirteen, which is

frequently that of the segments of the body without the head. The number of

chambers differs, however, greatly, as we have previously stated, and

cannot be considered to support the idea of an original segmental

arrangement of the chambers. The dorsal vessel, though in the adult a

single organ, arises in the embryo from two lateral, widely separated parts

which only in a subsequent stage of the embryonic development coalesce in

the median line.

FAT-BODY.

In discussing the tracheae we remarked on the importance of their function

and on their abundant presence in the body. Equally conspicuous, and

perhaps scarcely less important in function, is the fat-body, which on

opening some Insects, especially such as are in the larval stage, at once

attracts attention. It consists of masses of various size and indefinite

form distributed throughout the body, loosely connected together, and more

or less surrounding and concealing the different organs. The colour varies

according to the species of Insect. This fat-body is much connected with

fine tracheal twigs, so that an organisation extending throughout the body

is thus formed. It may be looked on as a store of nutritious matter which

may be added to or drawn on with great rapidity; and it is no doubt on this

that many of the internal parasites, so common in the earlier stages of

Insects' lives, subsist before attacking the more permanent tissues of

their hosts. There is some reason to suppose that the fat-body may have

some potency in determining the hunger of the Insect, for some parasitised

larvae eat incessantly.

The matter extracted from the food taken into the stomach of the Insect,

after undergoing some elaboration—on which point very little is known—finds

its way into the body-cavity of the creature, and as it is not confined in

any special vessels the fat-body has as unlimited a supply of the nutritive

fluid as the other organs: if nutriment be present in much greater quantity

than is required for the purposes of immediate activity, metamorphosis or

reproduction, it is no doubt taken up by the {137}fat-body which thus

maintains, as it were, an independent feeble life, subject to the demands

of the higher parts of the organisation. It undoubtedly is very important

in metamorphosis, indeed it is possible that one of the advantages of the

larval state may be found in the fact that it facilitates, by means of the

fat-body, the storage in the organisation of large quantities of material

in a comparatively short period of time.

A considerable quantity of fat tissue is found in the pericardial sinus,

where it is frequently of somewhat peculiar form, and is spoken of as

pericardial cells, or pericardial tissue. Some large cells, frequently of

pale yellow colour, and containing no fat, are called oenocytes by

Wielowiejski. They are connected with the general fat-body, but are not

entirely mingled with it; several kinds have been already distinguished,

and they are probably generally present. The phagocytes, or leucocytes, the

cells that institute the process of histolysis in the metamorphosis of

_Muscidae_, are a form of blood cell; though these cells are amoeboid some

writers derive them from the fat-body. The cells in the blood have no doubt

generally an intimate relation with the fat-body, but very little accurate

information has been obtained as to these important physiological points,

though Graber has inaugurated their study.[60]

ORGANS OF SEX.

The continuation of the species is effected in Insects by means of two

sexes, each endowed with special reproductive organs. It has been stated

that there are three sexes in some Insects—male, female, and neuter; but

this is not correct, as the so-called neuters are truly sexed

individuals,—generally females,—though, as a rule, they are not occupied

with the direct physiological processes for continuing the species.

The offspring is usually produced in the shape of eggs, which are formed in

ovaries. These organs consist of egg-tubes, a cluster of which is placed on

each side of the body, and is suspended, according to Leydig[61] and

others, to the tissue connected with the heart by means of the thread-like

terminations of the tubes.

{138}[Illustration: FIG. 74.—Sex organs of female of _Scolia interrupta_

(after Dufour); _a_, egg-tubes; _b_, oviducts; _c_, poison glands; _d_,

duct of accessory gland (or spermatheca); _e_, external terminal parts of

body.]

The number of egg-tubes varies greatly in different Insects; there may be

only one to each ovary (_Campodea_), but usually the number is greater, and

in the queen-bee it is increased to about 180. In the Queens of the

Termitidae, or white ants, the ovaries take on an extraordinary

development; they fill the whole of the greatly distended hind-body. Three

thousand egg-tubes, each containing many hundred eggs, may be found in a

Queen Termite, so that, as has been said by Hagen,[62] an offspring of

millions in number is probable. There is considerable variety in the

arrangements for the growth of the eggs in the egg-tubes. Speaking

concisely, the tubes may be considered to be centres of attraction for

nutritive material, of which they frequently contain considerable stores.

Next to the terminal thread, of which we have already spoken, there is a

greater or smaller enlargement of the tube, called the terminal chamber;

and there may also be nutriment chambers, in addition to the dilatations

which form the egg-chambers proper. Korschelt[63] distinguishes three

principal forms of egg-tubes, viz. (1) there are no special nutriment

chambers, a condition shown in Figure 74; (2) nutriment chambers alternate

with the egg-chambers, as shown in our Figure of an egg-tube of _Dytiscus

marginalis_; (3) the terminal chamber takes on an unusual development,

acting as a large nutriment chamber, there being no other special nutriment

chambers. This condition is found in _Rhizotrogus solstitialis_. The

arrangements as to successive or simultaneous production of the eggs in the

tubes seem to differ in different Insects. In some forms, such as the white

ants, the process of egg-formation (oogenesis) attains a rapidity that is

almost incredible, and is continued, it is said, for periods of many

months. There is no point in which Insects differ more than in that of the

number of eggs produced by one {139}female. The egg-tubes are connected

with a duct for the conveyance of the eggs to the exterior, and the

arrangements of the tubes with regard to the oviduct also vary much. An

interesting condition is found in _Machilis_ (see Fig. 94, p. 188), where

the seven egg-tubes are not arranged in a bunch, but open at a distance

from one another into the elongated duct. The two oviducts usually unite

into one chamber, called the azygos portion or the uterus, near their

termination. There are a few Insects (Ephemeridae) in which the two

oviducts do not unite, but have a pair of orifices at the extremity of the

body. Hatchett-Jackson has recently shown[64] that in _Vanessa io_ of the

Order Lepidoptera, the paired larval oviducts are solid, and are fixed

ventrally so as to represent an Ephemeridean stage; that the azygos system

of ducts and appended structures develop separately from the original

oviducts, and that they pass through stages represented in other Orders of

Insects to the stage peculiar to the Lepidoptera. _Machilis_, according to

Oudemans, is a complete connecting link between the Insects with single and

those with paired orifices.

There are in different Insects more than one kind of diverticula and

accessory glands in connexion with the oviducts or uterus; a receptaculum

seminis, also called spermatheca, is common. In the Lepidoptera there is

added a remarkable structure, the bursa copulatrix, which is a pouch

connected by a tubular isthmus with the common portion of the oviduct, but

having at the same time a separate external orifice, so that there are two

sexual orifices, the opening of the bursa copulatrix being the lower or

more anterior. The organ called by Dufour in his various contributions

_glande sébifique_, is now considered to be, in some cases at any rate, a

spermatheca. The special functions of the accessory glands are still very

obscure.

[Illustration: FIG. 75.—Egg-tube of _Dytiscus marginalis_; _e.c_,

egg-chamber; _n.c_, nutriment chamber; _t.c_, terminal chamber; _t.t_,

terminal thread. (After Korschelt.)]

The ovaries of the female are replaced in the male by a pair {140}of

testes, organs exhibiting much variety of form. The structure may consist

of an extremely long and fine convoluted tube, packed into a small space

and covered with a capsule; or there may be several shorter tubes. As

another extreme may be mentioned the existence of a number of small

follicles opening into a common tube, several of these small bodies forming

together a testis. As a rule each testis has its own capsule, but cases

occur—very frequently in the Lepidoptera—in which the two testes are

enclosed in a common capsule; so that there then appears to be only one

testis. The secretion of each testis is conveyed outwards by means of a

slender tube, the vas deferens, and there are always two such tubes, even

when the two testes are placed in one capsule. The vasa deferentia differ

greatly in their length in different Insects, and are in some cases many

times the length of the body; they open into a common duct, the ductus

ejaculatorius. Usually at some part of the vas deferens there exists a

reservoir in the form of a sac or dilatation, called the vesicula

seminalis. There are in the male, as well as in the female, frequently

diverticula, or glands, in connexion with the sexual passages; these

sometimes exhibit very remarkable forms, as in the common cockroach, but

their functions are quite obscure. There is, as we have already remarked,

extreme variety in the details of the structure of the internal

reproductive apparatus in the male, and there are a few cases in which the

vasa deferentia do not unite behind, but terminate in a pair of separate

orifices. The genus _Machilis_ is as remarkable in the form of the sexual

glands and ducts of the male as we have already mentioned it to be in the

corresponding parts of the female.

[Illustration: FIG. 76.—_Tenthredo cincta._ _a_, _a_, testes; _b_, _b_,

vasa deferentia; _c_, _c_, vesiculæ seminales; _d_, extremity of body with

copulatory armature. (After Dufour.)]

Although the internal sexual organs are only fully developed in the imago

or terminal stage of the individual life, yet in reality their rudiments

appear very early, and may be detected from the embryo state onwards

through the other preparatory stages.

The spermatozoa of a considerable number of Insects, especially of

Coleoptera, have been examined by {141}Ballowitz;[65] they exhibit great

variety; usually they are of extremely elongate form, thread-like, with

curious sagittate or simply pointed heads, and are of a fibrillar

structure, breaking up at various parts into finer threads.

EXTERNAL SEXUAL ORGANS.—The terminal segments of the body are usually very

highly modified in connexion with the external sexual organs, and this

modification occurs in such a great variety of forms as to render it

impossible to give any general account thereof, or of the organs

themselves. Some of these segments—or parts of the segments, for it may be

dorsal plates or ventral plates, or both—may be withdrawn into the

interior, and changed in shape, or may be doubled over, so that the true

termination of the body may be concealed. The comparative anatomy of all

these parts is especially complex in the males, and has been as yet but

little elucidated, and as the various terms made use of by descriptive

entomologists are of an unsatisfactory nature we may be excused from

enumerating them. We may, however, mention that when a terminal chamber is

found, with which both the alimentary canal and the sexual organs are

connected, it is called a cloaca, as in other animals.

PARTHENOGENESIS.

There are undoubted cases in Insects of the occurrence of parthenogenesis,

that is, the production of young by a female without concurrence of a male.

This phenomenon is usually limited to a small number of generations, as in

the case of the Aphididae, or even to a single generation, as occurs in the

alternation of generations of many Cynipidae, a parthenogenetic alternating

with a sexual generation. There are, however, a few species of Insects of

which no male is known (in Tenthredinidae, Cynipidae, Coccidae), and these

must be looked on as perpetually parthenogenetic. It is a curious fact that

the result of parthenogenesis in some species is the production of only one

sex, which in some Insects is female, in others male; the phenomenon in the

former case is called by Taschenberg[66] Thelyotoky, in the latter case

Arrhenotoky; Deuterotoky being applied to the cases in which two sexes are

produced. In some forms of {142}parthenogenesis the young are produced

alive instead of in the form of eggs. A very rare kind of parthenogenesis,

called paedogenesis, has been found to exist in two or three species of

Diptera, young being produced by the immature Insect, either larva or pupa.

GLANDS.

Insects are provided with a variety of glands, some of which we have

alluded to in describing the alimentary canal and the organs of sex; but in

addition to these there are others in connexion with the outer integument;

they may be either single cells, as described by Miall in _Dicranota_

larva,[67] or groups of cells, isolated in tubes, or pouches. The minute

structure of Insect glands has been to some extent described by Leydig;[68]

they appear to be essentially of a simple nature, but their special

functions are very problematic, it being difficult to obtain sufficient of

their products for satisfactory examination.

{143}CHAPTER V

DEVELOPMENT

EMBRYOLOGY–EGGS–MICROPYLES–FORMATION OF EMBRYO–VENTRAL PLATE–ECTODERM AND

ENDODERM–SEGMENTATION–LATER STAGES–DIRECT OBSERVATION OF EMBRYO–

METAMORPHOSIS–COMPLETE AND INCOMPLETE–INSTAR–HYPERMETAMORPHOSIS–

METAMORPHOSIS OF INTERNAL ORGANS–INTEGUMENT–METAMORPHOSIS OF BLOWFLY–

HISTOLYSIS–IMAGINAL DISCS–PHYSIOLOGY OF METAMORPHOSIS–ECDYSIS.

The processes for the maintenance of the life of the individual are in

Insects of less proportional importance in comparison with those for the

maintenance of the species than they are in Vertebrates. The generations of

Insects are numerous, and the individuals produced in each generation are

still more profuse. The individuals have as a rule only a short life;

several successive generations may indeed make their appearances and

disappear in the course of a single year.

Although eggs are laid by the great majority of Insects, a few species

nevertheless increase their numbers by the production of living young, in a

shape more or less closely similar to that of the parent. This is well

known to take place in the Aphididae or green-fly Insects, whose rapid

increase in numbers is such a plague to the farmer and gardener. These and

some other cases are, however, exceptional, and only emphasise the fact

that Insects are pre-eminently oviparous. Leydig, indeed, has found in the

same _Aphis_, and even in the same ovary, an egg-tube producing eggs while

a neighbouring tube is producing viviparous individuals.[69] In the Diptera

pupipara the young are {144}produced one at a time, and are born in the

pupal stage of their development, the earlier larval state being undergone

in the body of the parent: thus a single large egg is laid, which is really

a pupa.

The eggs are usually of rather large size in comparison with the parent,

and are produced in numbers varying according to the species from a few—15

or even less in some fossorial Hymenoptera—to many thousands in the social

Insects: somewhere between 50 and 100 may perhaps be taken as an average

number for one female to produce. The whole number is frequently deposited

with rapidity, and the parent then dies at once. Some of the migratory

locusts are known to deposit batches of eggs after considerable intervals

of time and change of locality. The social Insects present extraordinary

anomalies as to the production of the eggs and the prolongation of the life

of the female parent, who is in such cases called a queen.

The living matter contained in the egg of an Insect is protected by three

external coats: (1) a delicate interior oolemm; (2) a stronger, usually

shell-like, covering called the chorion; (3) a layer of material added to

the exterior of the egg from glands, at or near the time when it is

deposited, and of very various character, sometimes forming a coat on each

egg and sometimes a common covering or capsule for a number of eggs. The

egg-shell proper, or chorion, is frequently covered in whole or part with a

complex minute sculpture, of a symmetrical character, and in some cases

this is very highly developed, forming an ornamentation of much delicacy;

hence some Insects' eggs are objects of admirable appearance, though the

microscope is of course necessary to reveal their charms. One of the

families of butterflies, the Lycaenidae, is remarkable for the complex

forms displayed by the ornamentation of the chorion (see Fig. 78, B).

[Illustration: FIG. 77.—Upper or micropylar aspect of egg of _Vanessa

cardui_. (After Scudder.)]

The egg-shell at one pole of the egg is perforated by one or more minute

orifices for the admission to the interior of the spermatozoon, and it is

the rule that the shell hereabouts is symmetrically sculptured (see Fig.

77), even when it is {145}unornamented elsewhere: the apertures in question

are called micropyles. They are sometimes protected by a micropyle

apparatus, consisting of raised processes, or porches: these are developed

to an extraordinary extent in some eggs, especially in those of

Hemiptera-Heteroptera (see Fig. 78, C). Some of these peculiar structures

have been described and figured by Leuckart.[70] The purpose they serve is

quite obscure.

[Illustration: FIG. 78.—Eggs of Insects: A, blowfly (after Henking); B,

butterfly, _Thecla_ (after Scudder); C, Hemipteron (Reduviid).]

FORMATION OF EMBRYO.

The mature, but unfertilised, egg is filled with matter that should

ultimately become the future individual, and in the process of attaining

this end is the seat of a most remarkable series of changes, which in some

Insects are passed through with extreme rapidity. The egg-contents consist

of a comparatively structureless matrix of a protoplasmic nature and of

yolk, both of which are distributed throughout the egg in an approximately

even manner. The yolk, however, is by no means of a simple nature, but

consists, even in a single egg, of two or three kinds of spherular or

granular constituents; and these vary much in their appearance and

arrangement in the early stages of the development of an egg, the yolk of

the same egg being either of a homogeneously granular nature, or consisting

of granules and larger masses, as well as of particles of fatty matter;

these latter when seen through the microscope looking sometimes like

shining, nearly colourless, globules.

{146}[Illustration: FIG. 79.—Showing the two extruded polar bodies P_{1},

P_{2} now nearly fused and reincluded, and the formation of the spindle by

junction of the male and female pronuclei. (After Henking.)]

The nature of the matrix—which term we may apply to both the protoplasm and

yolk as distinguished from the minute formative portions of the egg—and the

changes that take place in it have been to some extent studied, and

Kowalewsky, Dohrn,[71] Woodworth,[72] and others have given some

particulars about them. The early changes in the formative parts of the

mature egg have been observed by Henking in several Insects, and

particularly in _Pyrrhocoris_, his observations being of considerable

interest. When the egg is in the ovary and before it is quite mature,—at

the time, in fact, when it is receiving nutriment from ovarian cells,—it

contains a germinal vesicle including a germinal spot, but when the egg is

mature the germinal vesicle has disappeared, and there exists in its place

at one portion of the periphery of the egg-contents a cluster of minute

bodies called chromosomes by Henking, whom we shall follow in briefly

describing their changes. The group divides into two, each of which is

arranged in a rod or spindle-like manner, and may then be called a

directive rod or spindle. The outer of these two groups travels quite to

the periphery of the egg, and there with some adjacent matter is extruded

quite outside the egg-contents (not outside the egg-coverings), being in

its augmented form called a polar or directive body. While this is going on

the second directive spindle itself divides into two groups, the outer of

which is then extruded in the manner we have already described in the case

of the first polar body, thus completing the extrusion of two directive

bodies. The essential parts of the bodies that are successively formed

during these processes are the aggregates, called chromosomes; the number

of these chromosomes appears to be constant in each species; their

movements and dispositions are of a very interesting character, the systems

they form in {147}the course of their development having polar and

equatorial arrangements. These we cannot further allude to, but may mention

that the extrusion of the directive bodies is only temporary, they being

again included within the periphery of the egg by the growth and extension

of adjacent parts which meet over and thus enclose the bodies.

The arrangements and movements we have briefly alluded to have been limited

to the unfertilised condition of the egg (we should rather say, the

fertilising element has taken no part in them), and have as their result

the union of the chromosomes existing after the extrusion of the two polar

bodies, into a small body called the female pronucleus or egg-nucleus

(Eikern), while the position of the movements has been an extremely minute

portion of the egg near to its outer surface or periphery. The introduction

of a sperm, or male, element to the egg through the micropyle gives rise to

the formation of another minute body placed more in the interior of the

egg, and called the sperm-nucleus. The egg-nucleus, travelling more into

the interior of the egg, meets the sperm-nucleus; the two amalgamate,

forming a nucleus or body that goes through a series of changes resulting

in its division into two daughter-bodies. These two again divide, and by

repetitions of such division a large number of nuclei are formed which

become arranged in a continuous manner so as to form an envelope enclosing

a considerable part (if not quite the whole) of the egg-mass. This envelope

is called the blastoderm, and together with its contents will form the

embryo. We must merely allude to the fact that it has been considered that

some of the nuclei forming the blastoderm arise directly from the egg-mass

by a process of amalgamation, and if this prove to be correct it may be

admitted that some portions of the embryo are not entirely the result of

division or segmentation of combined germ and sperm-nuclei. Wheeler

states[73] that some of the nuclei formed by the first differentiation go

to form the vitellophags scattered throughout the yolk. We should also

remark that, according to Henking, the blastoderm when completed shows at

one part a thickening, immediately under which (_i.e._ included in the area

the blastoderm encloses) are the two polar bodies, which, as we have seen,

were formed by the germinating body at an earlier stage of its activity.

Fig. 79 {148}represents a stage in the development of _Pyrrhocoris_,

showing the interior of the egg after a body has been formed by the union

of the sperm and egg-nuclei; this body is about to undergo division or

segmentation, and the equatorial arrangement where this will take place is

seen. The two polar bodies P_{1}, P_{2}, after having been excluded, are

nearly reincluded in the egg.

THE VENTRAL PLATE.

The next important change after the formation of the blastoderm is the

partial detachment of a part of its periphery to become placed in the

interior of the other and larger portion. The way in which this takes place

will be gathered from the accompanying diagrammatic figures taken from

Graber: a thickened portion (_a_ _b_) of the blastoderm becomes indrawn so

as to leave a fold (_c_ _d_) at each point of its withdrawal, and these

folds afterwards grow and meet so as to enclose the thickened portion. The

outer envelope, formed in part by the original blastoderm and in part by

the new growth, is called the serosa (_e_ _f_), the inner layer (_g_) of

the conjoined new folds being termed the amnion: the part withdrawn to the

interior and covered by the serosa and amnion is called the ventral plate,

or germinal band (_Keimstreif_), and becomes developed into the future

animal. The details of the withdrawal of the ventral plate to the interior

are very different in the various Insects that have been investigated.

[Illustration: FIG. 80.—Stages of the enclosure of the ventral plate: A,

_a_, _b_, ventral plate; B, _c_, _d_, folds of the blastoderm that form the

commencement of the amnion and serosa; C, _e_, _f_, part of the serosa;

_g_, amnion.]

One of the earliest stages in the development is a differentiation of a

portion of the ventral plate into layers from which the future parts of the

organisation will be derived. This separation of endoderm from ectoderm

takes place by a sort of invagination, analogous with that by which the

ventral plate itself is formed. A longitudinal depression running along the

middle of the ventral plate appears, and forms a groove or channel, which

becomes obliterated as to its outer face by the meeting together of the two

margins of the groove (except on the {149}anterior part, which remains

open). The more internal layer of the periphery of this closed canal is the

origin of the endoderm and its derivatives. Subsequently the ventral plate

and its derivatives grow so as to form the ventral part and the internal

organs of the Insect, the dorsal part being completed much later by growths

that differ much in different Insects; Graber, who has specially

investigated this matter, informing us[74] that an astonishing

multifariousness is displayed. It would appear that the various modes of

this development do not coincide with the divisions into Orders and

Families adopted by any systematists.

We should observe that the terms ectoderm, mesoderm, and endoderm will

probably be no longer applied to the layers of the embryo when

embryologists shall have decided as to the nature of the derived layers,

and shall have agreed as to names for them. According to the nomenclature

of Graber[75] the blastoderm differentiates into Ectoblast and Endoblast;

this latter undergoing a further differentiation into Coeloblast and

Myoblast. This talented embryologist gives the following table of the

relations of the embryonic layers and their nomenclature, the first term of

each group being the one he proposed to use:—

Periblast-------Ectoblast Part of yolk cells.

(Epiblast, \ (Ectoderm, outer /

/ blastoderm).\ layer). /

/ \ /

Protoblast. \ / Coeloblast

\ Centroblast Endoblast or--------(Endoderm in narrower

\(Yolk-cells, Hypoblast, \ sense).

hypoblast, inner layer. \ _Darmdrüsenblatt_.

endoderm (Mesoblast of \

in part of Balfour.) \

Balfour.) Mesoderm and Myoblast (Mesoderm

endoderm. of most authors).

_Darmmuskelblatt_

Nussbaum considers[76] that "there are four layers in the cockroach-embryo,

viz. (1) _epiblast_, from which the integument and nervous system are

developed; (2) _somatic layer of mesoblast_, mainly converted into the

muscles of the body-wall; (3) _splanchnic layer of mesoblast_, yielding the

muscular coat of the alimentary canal; and (4) _hypoblast_, yielding the

epithelium of the mesenteron."

{150}[Illustration: FIG. 81.—Early stages of the segmentation of a beetle

(_Lina_): A, segmentation not visible, 1 day; B, segmentation of head

visible; C, segmentation still more advanced, 2¼ days; PC, procephalic

lobes; _g^1_, _g^2_, _g^3_, segments bearing appendages of the head; _th_,

thorax; _th^1_, _th^2_, _th^3_, segments of the thorax; _a^1_, _a^2_,

anterior abdominal.]

Turning our attention to the origin of the segmentation, that is so marked

a feature of Insect structure, we find that evidence of division or

arrangement of the body into segments appears very early, as shown in our

Figure of some of the early stages of development of _Lina_ (a beetle),

Fig. 81. In A the segmentation of the ectoderm has not commenced, but the

procephalic lobes (_P C_) are seen; in B the three head segments are

distinct, while in C the thoracic segmentation has occurred, and that of

the abdomen has commenced. Graber considers that in this species the

abdomen consists of ten segmental lobes, and a terminal piece or telson.

According to Graber[77] this is not a primitive condition, but is preceded

by a division into three or four parts, corresponding with the divisions

that will afterwards be head, thorax, and abdomen. This primary

segmentation, he says, takes place in the Hypoblast (Endoderm) layer of the

ventral plate; this layer being, in an early stage of the development of a

common grasshopper (_Stenobothrus variabilis_), divided into four sections,

two of which go to form the head, while the others become thorax and

abdomen respectively. In _Lina_ the primary segmentation is, Graber says,

into three instead of four parts. Graber's opinion on the primary

segmentation does not appear to be generally accepted, and Wheeler, who has

studied[78] the {151}embryology of another Orthopteron, considers it will

prove to be incorrect. When the secondary segmentation occurs the anterior

of the two cephalic divisions remains intact, while the second divides into

the three parts that afterwards bear the mouth parts as appendages. The

thoracic mass subsequently segments into three parts, and still later the

hind part of the ventral plate undergoes a similar differentiation so as to

form the abdominal segments; what the exact number of these may be is,

however, by no means easy to decide, the division being but vague,

especially posteriorly, and not occurring all at once, but progressing from

before backwards.

The investigations that have been made in reference to the segmentation of

the ventral plate do not at present justify us in asserting that all

Insects are formed from the same number of embryonic segments. The matter

is summarised by Lowne, to the effect that posterior to the procephalic

lobes there are three head segments and three thoracic segments, and a

number of abdominal segments, "rarely less than nine or more than eleven."

It will be seen by referring to Figure 81 that the segmentation appears,

not simultaneously, but progressively from the head backwards; this of

course greatly increases the difficulty of determining by means of a

section the real number of segments.

[Illustration: FIG. 82.—Embryo of a moth (_Zygaena_) at the fifth day

(after Graber): _am_, amnion; _s_, serosa; _p_, procephalic lobes; _st_,

stomodaeum; _pr_, proctodaeum; _g^1_, _g^2_, _g^3_, the mouth parts or head

appendages; _th^1_, _th^2_, _th^3_, appendages of the thoracic segments;

_a^1_-_a^{10}_, abdominal segments; _s.g_, salivary gland.]

The later stages in the development of Insects are already proved to be so

various that it would be impossible to attempt to follow them in detail;

but in Fig. 82 we represent a median section of the embryo of _Zygaena

filipendula_ at the fifth day. It shows well some of the more important of

the general features of the development at a stage subsequent to those

represented in Fig. 81, A, B, C. The very distinct stomodaeum (_st_) and

proctodaeum (_pr_) are seen as inflexions of the external wall of the body;

the segmentation and the development of the {152}ventral parts of the

embryo are well advanced, while the dorsal part of the embryo is still

quite incomplete.

The method of investigation by which embryologists chiefly carry on their

researches is that of dividing the egg after proper preparation, into a

large number of thin sections, which are afterwards examined in detail, so

as to allow the arrangement to be completely inferred and described.

Valuable as this method is, it is nevertheless clear that it should, if

possible, be supplemented by direct observation of the processes as they

take place in the living egg: this method was formerly used, and by its aid

we may still hope to obtain exact knowledge as to the arrangements and

rearrangements of particles by which the structures develop. Such questions

as whether the whole formative power in the egg is absolutely confined to

one or two small centres to which the whole of the other egg contents are

merely, as it were, passive accessories, or whether an egg is a combination

in which some portion of the powers of rearrangement is possessed by other

particles, as well as the chromosomes, in virtue of their own nature or of

their position at an early period in the whole, can scarcely be settled

without the aid of direct observation of the processes during life.

The importance of the yolk is recognised by most of the recent writers.

Nussbaum states (_loc. cit._) that "scattered yolk-cells associate

themselves with the mesoblast cells, so that the constituents of the

mesoblast have a twofold origin." Wheeler finds[79] that amoeboid cells—he

styles them vitellophags—traverse the yolk and assist in its rearrangement;

he insists on the importance both as regards quantity and quality of the

yolk.

The eggs of some insects are fairly transparent, and the process of

development in them can, to a certain extent, be observed by simple

inspection with the microscope; a method that was used by Weismann in his

observations on the embryology of _Chironomus_. There is a moth (_Limacodes

testudo_), that has no objection to depositing its eggs on glass

microscope-slides. These eggs are about a millimetre long, somewhat more

than half that width, are very flat, and the egg-shell or chorion is very

thin and perfectly transparent. When first laid the contents of this egg

appear nearly homogeneous and evenly distributed, a finely granular

appearance being presented throughout; but in {153}twenty-four hours a

great change is found to have taken place. The whole superficial contents

of the egg are at that time arranged in groups, having the appearance of

separate rounded or oval masses, pressed together so as to destroy much of

their globular symmetry. The egg contents are also divided into very

distinct forms, a granular matter, and a large number of transparent

globules, these latter being the fatty portion of the yolk; these are

present everywhere, though in the centre there is a space where they are

very scanty, and they also do not extend quite to the circumference. But

the most remarkable change that has taken place is the appearance in the

middle of the field of an area different from the rest in several

particulars; it occupies about one-third of the width and one-third of the

length; it has a whiter and more opaque appearance, and the fat globules in

it are fewer in number and more indistinct. This area is afterwards seen to

be occupied by the developing embryo, the outlines of which become

gradually more distinct. Fig. 83 gives an idea of the appearance of the egg

about the middle period of the development. In warm weather the larva

emerges from this egg ten or eleven days after it has been deposited.

[Illustration: FIG. 83.—A, Egg of _Limacodes testudo_ about the middle of

the development of the embryo; B, micropyles and surrounding sculpture of

chorion.]

The period occupied by the development of the embryo is very different in

the various kinds of Insects; the blowfly embryo is fully developed in less

than twenty-four hours, while in some of the Orthoptera the embryonic stage

may be prolonged through several months. According to Woodworth the

blastoderm in _Vanessa antiopa_ is complete in twenty-four hours after the

deposition of the egg, and the involution of the ventral plate is

accomplished within three days of deposition.

METAMORPHOSIS.

The ontogeny, or life history of the individual, of Insects is peculiar,

inasmuch as a very large part of the development takes {154}place only late

in life and after growth has been completed. Insects leave the egg in a

certain form, and in that condition they continue—with, however, a greater

or less amount of change according to kind—till growth is completed, when,

in many cases, a very great change of form takes place. Post-embryonic

development, or change of form of this kind, is called metamorphosis. It is

not a phenomenon peculiar to Insects, but exists to a greater or less

extent in other groups of the Metazoa; while simpler post-embryonic

development occurs in nearly all, as in scarcely any complex animals are

all the organs completely formed at the time the individual becomes

possessed of a separate existence. In many animals other than Insects the

post-embryonic development assumes most remarkable and complex forms,

though there are perhaps none in which the phenomenon is very similar to

the metamorphosis of Insects. The essential features of metamorphosis, as

exhibited in the great class we are writing of, appear to be the separation

in time of growth and development, and the limitation of the reproductive

processes to a short period at the end of the individual life. The peculiar

phenomena of the post-embryonic development of the white ants show that

there exists some remarkable correlation between the condition of the

reproductive organs and the development of the other parts of the

organisation. If we take it that the post-embryonic physiological processes

of any individual Insect are of three kinds,—growth, development, and

reproduction,—then we may say that in the higher Insects these three

processes are almost completely separated, and go on consecutively, the

order being,—first, growth; second, development; third, reproduction.

While, if we complete the view by including the processes comprised in the

formation of the egg and the development therein, the series will be—(1)

oogenesis, or egg-growth; (2) development (embryonic); (3) growth

(post-embryonic); (4) development (post-embryonic); (5) reproduction.

The metamorphosis of Insects is one of the most interesting parts of

entomology. It is, however, as yet very little known from a scientific

point of view, although the simpler of its external characters have for

many ages past attracted the attention and elicited the admiration of

lovers of nature. It may seem incorrect to say that little is yet known

scientifically of a phenomenon concerning which references almost

{155}innumerable are to be found in literature: nevertheless the

observations that have been made as to metamorphosis, and the analysis that

has been commenced of the facts are at present little more than sufficient

to show us how vast and complex is the subject, and how great are the

difficulties it presents.

There are three great fields of inquiry in regard to metamorphosis, viz.

(1) the external form at the different stages; (2) the internal organs and

their changes; (3) the physiological processes. Of these only the first has

yet received any extensive attention, though it is the third that precedes

or underlies the other two, and is the most important. We will say a few

words about each of these departments of the inquiry. Taking first the

external form—the instar. But before turning to this we must point out that

in limiting the inquiry to the post-embryonic development, we are making

one of those limitations that give rise to much misconception, though they

are necessary for the acquisition of knowledge as to any complex set of

phenomena. If we assume five well-marked stages as constituting the life of

an Insect with extreme metamorphosis, viz. (1) the formation and growth of

the egg; (2) the changes in the egg culminating in its hatching after

fertilisation; (3) the period of growth; (4) the pupal changes; (5) the

life of the perfect Insect; and if we limit our inquiry about development

to the latter three, we are then shutting out of view a great preliminary

question, viz. whether some Insects leave the egg in a different stage of

development to others, and we are consequently exposing ourselves to the

risk of forgetting that some of the distinctions we observe in the

subsequent metamorphosis may be consequential on differences in the

embryonic development.

INSTAR AND STADIUM.

Figs. 84 and 85 represent corresponding stages in the life of two different

Insects, Fig. 84 showing a locust (_Acridium_), and Fig. 85 a white

butterfly. In each A represents the newly-hatched individual; B, the insect

just before its perfect state; C, the perfect or imago stage. On comparing

the two sets of figures we see that the C stages correspond pretty well as

regards the most important features (the position of the wings being

unimportant), that the A stages are moderately different, {156}while the B

states are not to be recognised as equivalent conditions.

[Illustration: FIG. 84.—Locust (_Acridium peregrinum_): A, newly hatched;

B, just antecedent to last ecdysis; C, perfect Insect.]

[Illustration: FIG. 85.—Butterfly (_Pieris_): A, the newly hatched young,

or larva magnified; B, pupa (natural size) just antecedent to last ecdysis;

C, perfect Insect.]

Every Insect after leaving the egg undergoes during the process of growth

castings of the skin, each of which is called a moult or ecdysis. Taking

for our present purpose five as the number of ecdyses undergone by both the

locust and butterfly, we may express the differences in the successions of

change we portray in Figs. 84 and 85 by saying that previous to the first

ecdysis the two Insects are moderately dissimilar, that the locust

undergoes a moderate change before reaching the fifth ecdysis, and

undergoes another moderate change at this moult, thus reaching its perfect

condition by a slight, rather gradual series of {157}alterations of form.

On the other hand, the butterfly undergoes but little modification,

remaining much in the condition shown by A, Fig. 85, till the fourth, or

penultimate, ecdysis, but then suffers a complete change of form and

condition, which apparently is only inferior to another astonishing change

that takes place at the fifth or final moult. The chief, though by no means

the only, difference between the two series consists in the fact that the

butterfly has interposed between the penultimate and the final ecdyses a

completely quiescent helpless condition, in which it is deprived of

external organs of sense, locomotion, and nutrition; while in the locust

there is no loss of these organs, and such quiescent period as exists is

confined to a short period just at the fifth ecdysis. The changes exhibited

by the butterfly are called "complete metamorphosis," while this phenomenon

in the locust is said to be "incomplete." The Insect with complete

metamorphosis is in its early stage called a larva, and in the quiescent

state a pupa. The adult state in both butterfly and locust is known as

imago or perfect Insect.

The most conspicuous of the differences between Insects with complete and

those with incomplete metamorphosis is, as we have remarked, the existence

in the former of a pupa. The pupal state is by no means similar in all the

Insects that possess it. The most anomalous conditions in regard to it

occur in the Order Neuroptera. In some members of that Order—the

Caddis-flies for instance—the pupa is at first quiescent, but becomes

active before the last ecdysis; while in another division—the May-flies—the

last ecdysis is not preceded by a formed pupa, nor is there even a distinct

pupal period, but the penultimate ecdysis is accompanied by a change of

form to the winged condition, the final ecdysis being merely a casting of

the skin after the winged state has been assumed. In the _Odonata_ or

Dragon-flies there is no pupal stage, but the change of form occurring at

the last ecdysis is very great. In those Insects where the interval between

the last two moults is not accompanied by the creature's passing into a

definite, quiescent pupa, the individual is frequently called then a nymph;

but the term nymph has merely a distinctive meaning, and is not capable of

accurate definition, owing to the variety of different conditions covered

by the word. Eaton, in describing this term as it is used for

_Ephemeridae_, says, "Nymphs are young which lead an {158}active life,

quitting the egg at a tolerably advanced stage of morphological

development, and having the mouth-parts formed after the same main type of

construction as those of the adult insect."[80]

The intervals between the ecdyses are called stadia, the first stadium

being the period between hatching and the first ecdysis. Unfortunately no

term is in general use to express the form of the Insect at the various

stadia; entomologists say, "the form assumed at the first moult," and so

on. To avoid this circumlocution it may be well to adopt a term suggested

by Fischer,[81] and call the Insect as it appears at hatching the first

instar, what it is as it emerges from the first ecdysis the second instar,

and so on; in that case the pupa of a Lepidopteron that assumed that

condition at the fifth ecdysis would be the sixth instar, and the butterfly

itself would be the seventh instar.

Various terms are used to express the differences that exist in the

metamorphoses of Insects, and as these terms refer chiefly to the changes

in the outer form, we will here mention them. As already stated, the locust

is, in our own language, said to have an incomplete metamorphosis, the

butterfly a complete one. The term Holometabola has been proposed for

Insects with complete metamorphosis, while the appellations Ametabola,

Hemimetabola, Heterometabola, and Paurometabola have been invented for the

various forms of incomplete, or rather less complex, metamorphosis. Some

writers use the term Ametabola for Insects that are supposed to exhibit no

change of external form after quitting the egg, the contrasted series of

all other Insects being then called Metabola. Westwood and others use the

word Homomorpha for Insects in which the condition on hatching more or less

resembles that attained at the close of the development, and Heteromorpha

for those in which the form on emergence from the egg differs much from

what it ultimately becomes.

HYPERMETAMORPHOSIS.

There are certain unusual changes to which the term hypermetamorphosis has

been applied; these we can here only briefly allude to.

{159}Insects that have complete metamorphoses, and are not supplied with

food by their parents or guardians, are provided during their larval life

with special modifications of extremely various kinds to fit them for the

period of life during which they are obtaining food and growing. Thus

caterpillars possess numerous adaptations to fit them for the period during

which they live on leaves, while maggots have modifications enabling them

to live amongst decomposing flesh. Some larvae are greatly modified in this

adaptive way, and when the adaptations change greatly during the life of

the larva, hypermetamorphosis is said to exist. As an instance we may

mention some beetle larvae that are born with legs by whose aid they can

cling to a bee, and so get carried to its nest, where they will in future

live on the stores of food the bee provides for its own young. In order

that they may be accommodated to their totally different second

circumstances, they change their first form, losing their legs, and

becoming almost bladder-like creatures, fitted for floating on the honey

without being injured by it. Such an occurrence has been described by

Fabre[82] in the case of _Sitaris humeralis_, and his figures have been

reproduced in Sir John Lubbock's book on the metamorphoses of Insects,[83]

as well as in other works, yet they are of so much interest that we give

them again, especially as the subject is still only in its infancy; we at

present see no sufficient reason for the later of these larval states.

Little is, we believe, known as to the internal anatomy of the various

instars in these curious cases.

[Illustration: FIG. 86.—Preparatory stages of _Sitaris humeralis_: 9, 10,

11, 12, first, second, third, and fourth larval instars; 13, pupa. (After

Lubbock and Fabre.)]

{160}There are certain minute Hymenoptera that deposit their eggs inside

the eggs of other Insects, where the beings hatched from the parasitic eggs

subsequently undergo their development and growth, finding their sustenance

in the yolk or embryo contained in the host-egg. It is evident that such a

life is very anomalous as regards both food and the conditions for

respiration, and we consequently find that these tiny egg-parasites go

through a series of changes of form of a most remarkable character.[84] It

would appear that in these cases the embryonic and post-embryonic

developments are not separated in the same way as they are in other

Insects. We are not aware that any term has yet been proposed for this very

curious kind of Insect development, which, as pointed out by Brauer,[85] is

doubtless of a different nature from the hypermetamorphosis of _Sitaris_.

CHANGES IN INTERNAL ORGANS.

In relation to the post-embryonic development of the internal organs of the

body there is but little exact generalisation to be made, the anatomical

condition of these organs at the time of emergence from the egg having been

ascertained in but few Insects. We know that in Holometabolous Insects the

internal anatomy differs profoundly in the larval and imaginal instars. As

to Insects with more imperfect metamorphosis very little information

exists, but it appears probable that in many no extensive distinctions

exist between the newly-hatched and the adult forms, except in the

condition of the reproductive organs. Differences of minor importance

doubtless exist, but there is almost no information as to their extent, or

as to the periods at which the changes occur; so that we do not know to

what extent they may be concentrated at the final ecdysis. In Insects with

perfect metamorphosis the structures of the internal organs are, as we have

said, in many cases totally different in the larval and imaginal periods of

the life; but these changes are far from being uniform in all Holometabola.

The nervous system in some cases undergoes a great concentration of the

ganglia, in others does not, and important distinctions exist in this

respect even within the limits of a single Order, such as the Coleoptera.

{161}Some Insects take the same kind of food throughout their lives, but

many others change totally in this respect, and their organs for the

prehension and digestion of food undergo a corresponding change.

Butterflies suck food in the form of liquid juices from flowers by means of

a delicate and long proboscis, while the young butterfly—the

caterpillar—disdains sweets, and consumes, by the assistance of powerful

mandibles, a great bulk of leaves. Other Holometabola undergo no such total

change of habits; the tiger-beetle, for instance, is as ferocious a

consumer of the juices of Insects in its young stage as it is in the adult

condition. Hence Brauer[86] divides Insects, as regards this point, into

three categories. The forms in which both the young and adult take food by

suction he calls Menorhyncha; those in which both the imago and immature

forms feed by mandibles he calls Menognatha; while his Metagnatha consists

of those insects that take food by jaws when young, but by suction with

tubular mouths when mature. Besides these main divisions there are some

exceptional cases to which we need not here allude, our present object

being to indicate that in the Metagnatha the digestive organs are of a very

different nature in the young and in the adult states of existence.

The internal organs for the continuance of the species are known to be

present in a rudimentary stage in the embryo, and it is a rule that they do

not attain their full development until growth has been completed; to this

rule there may possibly be an exception in the case of the Aptera. But

little information of a comparative character exists as to the dorsal

vessel and the changes it undergoes during metamorphosis. There is

considerable difficulty in connexion with the examination of this

structure, but it appears probable that it is one of the organs that

changes the least during the process of metamorphosis.

The exact nature of the internal changes that occur during metamorphosis is

almost a modern subject. It is of course a matter of great difficulty to

observe and record changes that go on in the interior of such small

creatures as Insects, and when the phenomena occur with great rapidity, as

is frequently the case in Insect metamorphosis, the difficulty is much

increased. Nevertheless the subject is of such great interest that it has

been investigated with a skill and perseverance that call for the

{162}highest admiration. The greater part of the information obtained

refers to a single Insect, the blowfly; and amongst those who have made

important contributions to it we may mention Weismann,[87] Viallanes,[88]

Ganin,[89] and Van Rees,[90] and it is at present under investigation by

Lowne. A good deal, too, is becoming known about the processes in the case

of the silkworm.

INTEGUMENT AND ECDYSIS.

The integument consists of a cellular layer, usually called the hypodermis,

situated on a basement membrane. The hypodermis, or layer of chitinogenous

cells, excretes a matter which remains attached to the body, forming the

hard outer layer of the skin. This layer consists of chitin and has no

vitality, but its presence no doubt exerts a very important influence on

the physiological processes of the Insect. The chitinous investment varies

much in thickness and in other properties; in some Insects it is hard, even

glassy, so as to be difficult to pierce with a pin, in others it is

pliable, and in some very delicate. Chitin is a substance very difficult to

investigate; according to the recent researches of Krawkow[91] it may prove

to be of somewhat variable chemical composition.

After a time the hypodermis excretes a fresh supply of chitin, and,

possibly by the commencement of this process, the older chitinous

investment becomes separated and is shed. The details have, however, not

been ascertained, though their importance has been suggested by Hatchett

Jackson.[92] The newly exposed layer of integument is pallid, but

afterwards becomes coloured in a manner varying according to the species,

the process being possibly due to some secondary exudation permeating the

freshly exposed chitin, or modifying some part of its exterior.

Lowne informs us that in the imago of the blowfly the great majority of the

hypodermic cells themselves enter into the composition of the chitinous

integument; and it is perhaps not a matter for surprise that the cells

should die on the completion of their functional activity, and should form

a part of the chitinous {163}investment. Some writers say that the

chitinous layer may be shown to be covered by a delicate extima or outer

coat.

The number of ecdyses varies greatly in Insects, but has been definitely

ascertained in only a few forms outside the Order Lepidoptera. In

_Campodea_ Grassi says there is a single fragmentary moult, and in many

Hymenoptera the skin that is cast is extremely delicate, and the process

perhaps only occurs twice or three times previous to the pupal stage. In

most Insects, however, ecdysis is a much more important affair, and the

whole of the chitinous integument is cast off entire, even the linings of

the tracheae, and of the alimentary canal and its adjuncts being parted

with. Sir John Lubbock observed twenty-three moults in a May-fly of the

genus _Cloëon_,[93] this being the maximum yet recorded, though Sommer

states[94] that in _Macrotoma plumbea_ moulting goes on as long as life

lasts, even after the Insect has attained its full size.

Some Insects get quit of a considerable quantity of matter by their

ecdyses, while in others the amount is comparatively slight. It has been

thought that the moulting is effected in order to permit of increase of

size of the Insect, but there are facts which point to the conclusion that

this is only a factor of secondary importance in the matter. One of these

is that many Insects make their first ecdysis almost immediately after they

leave the egg; this is the case with the young larva of the blowfly, which,

according to Lowne, moults within two hours of its emergence from the egg.

We have already referred to the important suggestion made by Eisig[95]

that, since chitin is a nitrogenous substance, the ecdyses may be a means

of getting rid of waste nitrogenous matter; to which we have added that as

chitin also consists largely of carbon, its excretion may be of importance

in separating carbonaceous products from the blood.

METAMORPHOSIS OF BLOWFLY.

The phenomena of metamorphosis are displayed to their greatest extent in

the transformations and physiological processes of the _Muscid Diptera_, of

which the common blowfly is an {164}example. We will briefly consider the

information that has been obtained on this subject.

The development of the embryo in the egg of the blowfly is unusually rapid,

occupying only a period of twenty to twenty-four hours. After its first

moult the blowfly larva grows rapidly during a period of about ten to

fourteen days, during which it undergoes moults, the number of which

appears not to be definitely ascertained. After becoming full-fed the larva

loses its active state, and passes for a period into a condition of

comparative quiescence, being spoken of in this state as a resting larva.

This quiet period occurs in most full-grown larvae, and is remarkable for

the great variation that may occur in its duration, it being in many

Insects subject to prolongation for months, in some cases possibly even for

years, though in favourable circumstances it may be very short. Lowne

informs us that in the blowfly this period of the life is occupied by very

great changes in the internal organs, which are undergoing very extensive

processes of destruction and rebuilding. After some days the outer skin of

the resting larva shrivels, and is detached from the internal living

substances, round which it hardens and forms the sort of cocoon or capsule

that is so well known. This using of the cast larval skin as a cocoon is,

however, limited to certain of the two-winged flies, and perhaps a few

other Insects, and so must be considered an exceptional condition. The

capsule conceals from view a most remarkable state, known to the old

naturalist Réaumur as the "spheroidal condition," but called by more recent

writers the pronymph. The pronymphal state may be looked on as being to a

great extent a return of the animal to the condition of an egg, the

creature becoming an accumulation of soft creamy matter enclosed in a

delicate skin. This spheroidal condition, however, really begins in the

resting larva, and Van Rees and others think that the delicate membrane

enclosing the substance of the pronymph is really the hypodermis of the

integument of the larva. Although this seems probable, from the resemblance

this condition would in that case present to the phenomena usual in

ecdysis, it is not generally admitted, and there is much difficulty in

settling the point. Lowne is of a contrary opinion, looking on the limiting

membrane as a subsequent formation; he calls it the paraderm. The process

of forming the various organs goes on in the pronymph, till the

{165}"nymph" has completed its development, the creature having then again

taken on a definite form which apparently corresponds to the pupa of

Hymenoptera. Great doubt, however, exists as to this equivalence, and

indeed as to any exact correspondence between the metamorphic stadia of

different Insects, a view which long since was expressed by Sir John

Lubbock[96] and Packard. The term nymph is used in this case not because

there is any resemblance to the condition similarly named in Insects with

less complete metamorphosis, but because the term pupa is applied to the

outer case together with the contained nymph. The transformation of the

nymph into the perfect blowfly occupies a period very variable according to

the temperature.

HISTOLYSIS.—The processes by which the internal organs of the maggot are

converted into those of the fly are of two kinds,—histolysis or breaking

down, histogenesis or building up, of tissue. The intermediary agents in

histolysis are phagocytes, cells similar to the leucocytes or white

corpuscles of the blood: the intermediary agents in histogenesis are

portions of tissue existing in the larval state incorporated with the

different organs, or preserving a connexion therewith even when they are to

a great extent separated therefrom. In this latter case they are called

imaginal discs, though Professor Miall prefers to term them imaginal

folds.[97] The two processes of histolysis and histogenesis, though to some

extent mutually dependent (for the material to be built up has to be

largely obtained by previous destruction), do not go on _pari passu_,

though they are to a great extent contemporaneous. In the resting larva

histolysis is predominant, while in the nymph histogenesis is more

extensive. Microscopic observation shows that the phenomena connected with

the histolysis of the muscular tissue are scarcely distinguishable from

those of an inflammatory process, and Viallanes[98] dilates on this fact in

an instructive manner. The phagocytes attach themselves to, or enter, the

tissues which are to be disintegrated, and becoming distended, assume a

granular appearance. By this pseudo-inflammatory process the larval

structures are broken down into a creamy substance; the buds, or germs,

from which the new organs are to be developed being exempt from the

destruction. These buds, of which about sixty or upwards have already been

detected, undergo {166}growth as they are liberated, and so the new

creature is formed, the process of growth in certain parts going on while

destruction is being accomplished in others. Considerable discrepancy

prevails as to the extent to which the disintegration of some of the

tissues is carried.

[Illustration: FIG. 87.—Imaginal discs of _Muscidae_ in process of

development: A, Brain and ventral ganglion of a larva 7 mm. long of _M.

vomitoria_; _v_, ventral ganglion; _c_, cephalic ganglion; _h_, head

rudiment; _vc_, portion of ventral chain; _pd_, prothoracic rudiment;

_vc_{3}_, third nerve; _md_, mesothoracic rudiment: B, mesothoracic

rudiment, more advanced, in a pupa just formed of _Sarcophaga carnaria_,

showing the base of the sternum and folds of the forming leg, the central

part (_f_) representing the foot: C, the rudimentary leg of the same more

advanced; _f_, femur; _t_, tibia; _f_{1}_, _f_{5}_, tarsal joints: D, two

discs from a larva 20 mm. long of _Sarcophaga_, attached to tracheae;

_msw_, mesonotal and wing-rudiment; _mt_, metathoracic rudiment: E, _r_,

mesothoracic rudiment of a 7 mm. long larva attached to a tracheal twig.

(After Weismann and Graber.)]

According to Kowalevsky[99] it would appear that after the phagocytes have

become loaded with granules they serve as nutriment for the growing

tissues, and he thinks they become blood-cells in the imago. The process of

histolysis has been chiefly studied in the blowfly, and not much is known

of it in other Insects, yet it occurs to a considerable extent, according

to Bugnion[100] and others, in the metamorphosis of Lepidoptera. Indeed it

would almost seem that the processes of histolysis and histogenesis may be

looked on as exaggerated forms of the phenomena of the ordinary life of

tissues, due to greater rapidity and discontinuity of tissue nutrition.

{167}IMAGINAL DISCS.—The imaginal discs are portions of the larval

hypoderm, detached from continuity with the main body of the integument,

but connected therewith by strings or pedicels which may be looked on as

portions of the basement membrane. Whether these discs, or histoblasts as

they are called by Künckel d'Herculais,[101] are distinguished by any

important character from other buds or portions of regenerative tissue

that, according to Kowalevsky,[102] Korschelt and Heider,[103] and others,

exist in other parts of the body, does not appear to be at present

ascertained.

We give some figures, taken from Weismann and Graber, of the imaginal

rudiments existing in the larvae of _Muscidae_. Although by no means good,

they are the best for our purpose we can offer to the reader. Other figures

will be found in Lowne's work on the blowfly now in course of publication.

Weismann's paper[104] is now thirty years old, and, when it was written, he

was not aware of the intimate connexion the rudiments have with the

integument; this has, however, now been demonstrated by several observers.

Pratt states[105] that the formation of the imaginal discs in _Melophagus

ovinus_ takes place in the later stages of the embryonic development, and

after the manner formerly suggested by Balfour, viz. invagination of the

ectoderm.

[Illustration: FIG. 88.—Median longitudinal section through larva of

blowfly during the process of histolysis. (After Graber.) Explanation in

text.]

Both the regenerative buds and the rudimentary sexual glands are known to

be derived directly from the embryo; neither of them undergoes any

histolysis, so that we have in them embryonic structures which exist in a

quiescent condition during the period in which the larva is growing with

great rapidity, and which when the larva has attained its full growth and

is disintegrating, then {168}appropriate the products of the disintegration

so as to produce the perfect fly.

Our Fig. 88, taken from Graber, represents a longitudinal median section of

a full-grown larva of _Musca_, in which the processes of metamorphosis are

taking place. The position of some of the more important imaginal rudiments

is shown by it: _b^1_, _b^2_, _b^3_, rudiments of the three pairs of legs

of the imago; _an_, of antennae; between _an_ and _w_, rudiment of eye;

_w_, of wings; _h_, of halteres; _f_, fat-body; _d_, middle of alimentary

canal; _n_, ventral chain; _st_, stigma; 6, 7, sixth and seventh body

segments.

PHYSIOLOGY OF METAMORPHOSIS.

Many years ago, Harvey perceived the probable existence of a physiological

continuity between the earlier and later stages of the Insect's life.

Modern investigation has shown that in the blowfly a remarkable analogy

exists between the conditions of the pupa and the egg. The outer shell of

the pupa corresponds to the chorion or egg-shell, and the delicate outer

membrane of the pronymph to the oolemn or lining membrane of the egg; the

creamy matter corresponds with the yolk, and the regenerative buds are

analogous to the formative portions of the developing egg. The process of

histolysis as carried out by the phagocytes of the later life appears also

to find a parallel in the vitellophags of the embryonic life.[106] It

appears probable that the physiological processes of the post-embryonic

metamorphosis may be essentially a repetition—or an interrupted

continuation—of those of the embryonic period.

The inquiry as to what are the determining causes of the metamorphic

changes of the blowfly and other Insects has as yet but little advanced.

Why does the larva grow up to a certain period with great rapidity, then

cease its appropriating power and break up the parts that have been so

rapidly and recently formed? And why do the imaginal buds remain quiescent

till the other tissues are being disintegrated, and then, instead of

sharing the general condition of disintegration, commence a career of

development? To these questions no satisfactory answer has yet been given,

though the remarkable studies, already referred to, of Bataillon on the

later larval life {169}of the silkworm suggest the direction in which

knowledge may be found, for they show that the physiological conditions of

the later larval life are different from those of the earlier life,

possibly as the direct result of the mere aggregation of matter, and the

consequent different relations of the parts of the organism to atmospheric

and aqueous conditions.

If we wish to understand metamorphosis, we must supplement the old opinion

that ecdysis is merely an occurrence to facilitate expansion, by the more

modern conception that it is also an important physiological process. That

shedding the skin is done solely to permit of enlargement of size is a view

rendered untenable by many considerations. The integument can increase and

stretch to an enormous extent without the aid of moulting; witness the

queen-termite, and the honey-bearers of the _Myrmecocystus_ ants. Many

moults are made when increase of size does not demand them, and the

shedding of the skin at the time of pupation is accompanied by a decrease

in size. And if moulting be merely connected with increase of size, it is

impossible to see why _Cloëon_ should require two dozen moults, while

_Campodea_ can do with one, or why a collembolon should go on moulting

during the period of life subsequent to the cessation of growth.

The attention of entomologists has been chiefly directed to the ecdyses

connected with the disclosure of the pupal and imaginal instars. Various

important transformations may, however, occur previous to this, and when

they do so it is always in connexion with ecdyses. Caterpillars frequently

assume a different appearance and change their habits or character at a

particular ecdysis; and in Orthoptera each ecdysis is accompanied by a

change of form of the thoracic segments; this change is very considerable

at one of the intermediate ecdyses.

The assumption of the pupa state is the concomitant of an ecdysis, and so

also is the appearance of the imago; but the commencement of each of these

two stages precedes the ecdysis, which is merely the outward mark of the

physiological processes. The ecdysis by which the pupa is revealed occurs

after the completion of growth and when great changes in the internal

organs have occurred and are still taking place; the ecdysis by which the

imago appears comes after development has been quite or nearly completed.

Although the existence of a pupa is to the eye the most {170}striking of

the differences between Insects with perfect and those with imperfect

metamorphosis, yet there is reason for supposing that the pupa and the

pupal period are really of less importance than they at first sight appear

to be. In Fig. 85 we showed how great is the difference in appearance

between the pupa and the imago. The condition that precedes the appearance

of the pupa is, however, really the period of the most important change. In

Fig. 89 we represent the larva and pupa of a bee; it will be seen that the

difference between the two forms is very great, while the further change

that will be required to complete the perfect Insect is but slight. When

the last skin of the larva of a bee or of a beetle is thrown off, it is, in

fact, the imago that is revealed; the form thus displayed, though

colourless and soft, is that of the perfect Insect; what remains to be done

is a little shrinking of some parts and expansion of others, the

development of the colour, the hardening of certain parts. The colour

appears quite gradually and in a regular course, the eyes being usually the

first parts to darken. After the coloration is more or less

perfected—according to the species—a delicate pellicle is shed or rubbed

off, and the bee or beetle assumes its final form, though usually it does

not become active till after a farther period of repose.

[Illustration: FIG. 89.—Larva and pupa of a bee, _Xylocopa violacea_: A,

larva; B, pupa, ventral aspect; C, pupa, dorsal aspect. (After Lucas.)]

{171}CHAPTER VI

CLASSIFICATION—THE NINE ORDERS OF INSECTS—THEIR CHARACTERS—PACKARD'S

ARRANGEMENT—BRAUER'S CLASSIFICATION—CLASSIFICATIONS BASED ON

METAMORPHOSIS—SUPER-ORDERS—THE SUBDIVISIONS OF ORDERS.

CLASSIFICATION.

We have already alluded to the fact that Insects are the most numerous in

species and individuals of all land animals: it is estimated that about

250,000 species have been already described and have had scientific names

given to them, and it is considered that this is probably only about

one-tenth of those that really exist. The classification in a

comprehensible manner of such an enormous number of forms is, it will be

readily understood, a matter of great difficulty. Several methods or

schemes have since the time of Linnaeus been devised for the purpose, but

we shall not trouble the reader to consider them, because most of them have

fallen into disuse and have only a historical interest. Even at present

there exists, however, considerable diversity of opinion on the question of

classification, due in part to the fact that some naturalists take the

structure of the perfect or adult Insect as the basis of their arrangement,

while others prefer to treat the steps or processes by which the structure

is attained, as being of primary importance. To consider the relative

values of these two methods would be beyond our scope, but as in practice a

knowledge of the structures themselves must precede an inquiry as to the

phases of development by which the structures are reached; and as this

latter kind of knowledge has been obtained in the case of a comparatively

small portion of the known forms,—the embryology and metamorphosis having

been investigated in but {172}few Insects,—it is clear that a

classification on the basis of structure is the only one that can be at

present of practical value. We shall therefore for the purposes of this

work make use of an old and simple system, taking as of primary importance

the nature of the organs of flight, and of the appendages for the

introduction of food to the body by the perfect Insect. We do not attempt

to disguise the fact that this method is open to most serious objections,

but we believe that it is nevertheless at present the most simple and

useful one, and is likely to remain such, at any rate as long as knowledge

of development is in process of attainment.

ORDERS.

The great groups of Insects are called Orders, and of these we recognise

nine, viz. (1) Aptera, (2) Orthoptera, (3) Neuroptera, (4) Hymenoptera, (5)

Coleoptera, (6) Lepidoptera, (7) Diptera, (8) Thysanoptera, (9) Hemiptera.

These names are framed to represent the nature of the wings; and there is

some advantage in having the Orders named in a uniform and descriptive

manner. The system we adopt differs but little from that proposed by

Linnaeus.[107] The great Swedish naturalist did not, however, recognise the

Orders Orthoptera and Thysanoptera; and his order Aptera was very different

from ours.

These Orders may be briefly defined as follows,—the reader being asked to

recall the fact that by a mandibulate mouth we understand one in which the

mandibles, or the maxillæ, or both, are fitted for biting, crushing, or

grasping food; while the term suctorial implies that some of the mouth

parts are of a tubular form or are protrusible as a proboscis, which

assists, or protects, a more minute and delicate sucking apparatus:—

1. _Aptera_ (ἀ without, πτερόν a wing). Wingless[108] Insects; mouth

mandibulate or very imperfectly suctorial. Metamorphosis very little.

2. _Orthoptera_ (ὀρθός straight, πτερόν a wing). Four wings are present,

the front pair being coriaceous (leather-like), usually smaller than the

other pair, which are of more delicate texture, and contract in repose

after the manner of a fan. Mouth mandibulate. Metamorphosis slight.

3. _Neuroptera_ (νεῦρον nerve, πτερόν a wing). Four wings of membranous

{173}consistency, frequently with much network; the front pair not much,

if at all, harder than the other pair, the latter with but little or no

fanlike action in closing. Mouth mandibulate. Metamorphosis variable, but

rarely slight.

4. _Hymenoptera_ (ὑμήν membrane, πτερόν a wing). Four wings of membranous

consistency; the front pair larger than the hind, which are always small

and do not fold up in repose. Mouth mandibulate, sometimes provided also

with a tubular proboscis. Metamorphosis very great.

5. _Coleoptera_ (κολεός sheath, πτερόν a wing). Four wings; the upper

pair shell-like in consistency, and forming cases which meet together

over the back in an accurate line of union, so as to entirely lose a

winglike appearance, and to conceal the delicate membranous hind pair.

Mouth mandibulate. Metamorphosis great.

6. _Lepidoptera_ (λεπίς scale, πτερόν a wing). Four large wings covered

with scales. Mouth suctorial. Metamorphosis great.

7. _Diptera_ (δίς double, πτερόν a wing). Two membranous wings. Mouth

suctorial, but varying greatly. Metamorphosis very great.

8. _Thysanoptera_ (θύσανος fringe, πτερόν a wing). Four very narrow

fringed wings. Mouth imperfectly suctorial. Metamorphosis slight.

9. _Hemiptera_ (ἡμι half, πτερόν a wing). Four wings; the front pair

either leather-like with more membranous apex, or entirely parchment-like

or membranous. Mouth perfectly suctorial. Metamorphosis usually slight.

We must again ask the reader to bear in mind that numerous exceptions exist

to these characters in most of the great Orders; for instance, wingless

forms are not by any means rare in several of the Orders.

Before remarking further on this system we will briefly sketch two other

arrangements of the Orders of Insects, for which we are indebted to Packard

and Brauer.

PACKARD'S CLASSIFICATION.

Packard has devoted much attention to the subject, and has published two or

three successive schemes, of which the following is the most recent:[109]

the definitions are those of the author himself, but the information in

brackets is given to institute a concordance with the system we adopt:—

1. _Thysanura._ Wingless; often with a spring (equivalent to our

_Aptera_).

2. _Dermaptera._ Front wings minute, elytra-like (= _Forficulidae_, a

part of our _Orthoptera_).

3. _Orthoptera._ Wings net-veined; fore wings narrow, hind wings folded

(= our _Orthoptera_ after subtraction of _Dermaptera_).

{174}4. _Platyptera._ Four net-veined wings; mouth parts adapted for

biting (= _Termitidae_ and _Mallophaga_, parts of our _Neuroptera_).

5. _Odonata._ Wings net-veined, equal (= _Odonata_, a division of our

_Neuroptera_).

6. _Plectoptera._ Wings net-veined, unequal (= _Ephemeridae_, a part of

our _Neuroptera_).

7. _Thysanoptera._ Mouth beaklike but with palpi (= our _Thysanoptera_).

8. _Hemiptera._ Mouth parts forming a beak for sucking. No palpi (= our

_Hemiptera_).

The above eight Orders form the group AMETABOLA, while the following eight

constitute the METABOLA:—

9. _Neuroptera._ Wings net-veined; metamorphosis complete (= a small part

of our _Neuroptera_).

10. _Mecaptera._ Wings long and narrow (for a small part of our

_Neuroptera_; the _Panorpatae_ of Brauer).

11. _Trichoptera._ Wings not net-veined (= our division of _Neuroptera_

with the same name).

12. _Coleoptera._ Fore wings sheathing the hinder ones (= our

_Coleoptera_).

13. _Siphonaptera._ Wingless, parasitic. Flea (= a division of

_Diptera_).

14. _Diptera._ One pair of wings (= our _Diptera_ after subtraction of

_Siphonaptera_).

15. _Lepidoptera._ Four wings (and body) scaled (= our _Lepidoptera_).

16. _Hymenoptera._ Four clear wings; hinder pair small; a tongue (= our

_Hymenoptera_).