AMPHIBIOUS NEUROPTERA _CONTINUED_—EPHEMERIDAE, MAY-FLIES
FAM. VII. EPHEMERIDAE—MAY-FLIES.
_Delicate Insects with atrophied mouth and small, short antennae; with
four membranous wings having much minute cross-veining; the hinder pair
very much smaller than the other pair, sometimes entirely absent: the
body terminated by three or two very elongate slender tails. The earlier
stages are passed through in water, and the individual then differs
greatly in appearance from the winged Insect; the passage between the two
forms is sudden; the creature in its first winged state is a subimago,
which by shedding a delicate skin reveals the final form of the
individual._
[Illustration: FIG. 273.—_Ephemera danica_, male, Britain.]
The may-flies are well known—in literature—as the types of a brief and
ineffective life. This supposed brevity relates solely to their existence
in the winged form. In the earlier stages the may-fly is so unlike its
subsequent self that it is not recognised as a may-fly by the uninitiated.
The total life of the individual is really quite as long as that of most
{430}other Insects. The earlier stages and life-histories of these Insects
are of great importance. The perfect Insects are so delicate and fragile
that they shrivel much in drying, and are very difficult to preserve in a
condition suitable for study.
The mouth of the imago is atrophied, the trophi scarcely existing as
separate parts. Packard says that in _Palingenia bilineata_ he could
discover no certain traces of any of the mouth-parts, but in _Leptophlebia
cupida_ he found, as he thought, the rudiments of the maxillae and labium,
though not of the mandibles. The antennae are always short, and consist of
one or two thick basal joints succeeded by a delicate needle-like segment,
which, though comparatively long, is not divided. The ocular organs are
remarkable for their large size and complex development; they are always
larger in the male than they are in the female. The compound eyes of the
former sex are in certain species, _e.g._ _Cloëon_ (Fig. 274), quite
divided, so that each eye becomes a pair of organs of a different
character; one part forms a pillar facetted at its summit, while the other
part remains as a true eye placed on the side of the head; in front of
these compound eyes there are three ocelli. Thus the Insect comes to have
three different kinds of eyes, together seven in number.
[Illustration: FIG. 274.—Front of head of _Cloëon_, male. _a_, Pillared
eye; _b_, sessile eye; _c_, ocellus.]
The prothorax is small, the pronotum being, however, quite distinct. The
mesothorax is very large; its notum forms by far the larger part of the
upper surface of the thoracic region, the metathorax being small and
different in structure, resembling in appearance a part of the abdomen, so
that the hind wings look as if they were attached to a first abdominal
segment. The mesosternum is also disproportionately large in comparison
with the homologous piece preceding it, and with that following it. The
pleural pieces are large, but their structure and disposition are only very
imperfectly understood. The coxae are small and are widely separated, the
anterior being, however, more elongate and approximate than the others. The
other parts of the legs are slender; the number of joints in the tarsi
varies from five to one. {431}The legs throughout the family exhibit a
considerable variety of structure, and the front pair in the males of some
species are remarkably long. The abdomen is usually slender, and consists
of ten segments; the terminal one bears three, or two, very long flexible
appendages. The first dorsal plate of the abdomen is either wanting or is
concealed to a considerable extent by the metanotum. The wings are
peculiar; the anterior pair vary a great deal in their width, but are never
very long in proportion to the width; the hind pair are always
disproportionately small, and sometimes are quite wanting. The venation
consists of a few, or of a moderate number, of delicate longitudinal veins
that do not pursue a tortuous course, but frequently are gracefully curved,
and form a system of approximately similar curves, most of the veins being
of considerable length; close to the anterior margin of the wing there are
two or three sub-parallel veins. Frequently there are very numerous fine,
short cross-veinlets, but these vary greatly and may be entirely wanting.
[Illustration: FIG. 275.—Wings of _Ephemera danica_. (After Eaton.)]
The earlier stages of the life of Ephemeridae are, it is believed, in the
case of all the species, aquatic. May-flies, indeed, during the period of
their post-embryonic development are more modified for an aquatic life than
any other Insects, and are provided with a complex apparatus of tracheal
gills. The eggs are committed to the waters without any care or foresight
on the part of the parent flies, thus the embryonic development is also
aquatic; little, however, is known of it. According to Joly[350] the
process in _Palingenia virgo_ is slow. The larva on emerging from the egg
has no respiratory system, neither could Joly detect any circulation or any
nervous system. The creature on emergence is very like _Campodea_ in form,
possessing long antennae and tails—caudal setae. Owing to the organisation
being inferior, the creature in its earlier stages is called a larvule; in
its later stages {432}it is usually spoken of as a nymph, but the term
larva is also frequently applied to it. Soon the gills begin to appear in
the form of small tubular caeca placed in the posterior and upper angles of
the abdominal rings; in fifteen days the gills begin to assume their
characteristic form, are penetrated by tracheae, and the circulation can be
seen. The amount of growth accomplished after hatching between March and
September is but small.
[Illustration: FIG. 276.—Nymph of _Cloëon dipterum_.[351] Wing-sheath of
left side, gills of right side, removed; _g_, tracheal gills. (After
Vayssière.)]
[Illustration: FIG. 277.—Larvule of _Cloëon dimidiatum_. (After Lubbock.)]
The metamorphosis of _Cloëon_ has been described by Sir John Lubbock; he
informs us that the young creature undergoes a constant and progressive
development, going through a series of more than twenty moults, each
accompanied by a slight change of form or structure. His observations were
made on captured {433}specimens, so that it is not certain that what he
calls[352] the first stage is really such. He found no tracheae in the
earliest stages; the small first rudiments of the gills became visible in
the third stage, when there were no tracheae; the fourth instar possessed
tracheae, and they could be seen in the gills. The wing rudiments could
first be detected in the ninth and tenth stages. The changes of skin during
the winter months are separated by longer intervals than those occurring at
other periods of the year.
[Illustration: FIG. 278.—Adult nymph of _Ephemera vulgata_. (After Eaton.)
Britain.]
The nymphs differ greatly in the structure and arrangement of their
tracheal gills, and display much variety in their general form and habits;
some of them are very curious creatures. Pictet[353] divides them in
accordance with their habits into four groups: (1) Fossorial larvae: these
live in the banks of streams and excavate burrows for shelter; they are of
cylindrical form, possess robust legs, abundant gills at the sides of the
body, and frequently processes projecting forwards from the head: examples,
_Ephemera_ (Fig. 278) and _Palingenia_. (2) Flat larvae: these live
attached to rocks, but run with rapidity when disturbed; they prefer rapid
streams, have the breathing organs attached to the sides of the body and
not reposing on the back; they are exclusively carnivorous, while the
fossorial forms are believed to obtain their nutriment by eating mud:
example, _Baëtis_. (3) Swimming larvae: elongate delicate creatures, with
feeble legs, and with strongly ciliated caudal setae: example, _Cloëon_
(Fig. 276). (4) Climbing larvae: these live in slowly-moving waters,
especially such as have much slimy mud in suspension, and they have a habit
of covering themselves with this mud sometimes to such an extent as to
become concealed by it: example, _Potamanthus_.
{434}[Illustration: FIG. 279.—Nymph of _Oligoneuria garumnica_, France.
_g_{2}_ and _g_{7}_, two of the dorsal tracheal gills. (After Vayssière.)]
The anatomy of the nymphs has been treated by Vayssière,[354] who arranges
them in five groups in accordance with the conditions of the tracheal
gills: (1) The gills are of large size, are exposed and furnished at the
sides with respiratory fringes: example, _Ephemera_ (Fig. 278). (2) The
branchiae are blade-like, not fringed, and are exposed at the sides of the
body: example, _Cloëon_ (Fig. 276). (3) The respiratory tubes are placed on
the under surface of plates whose upper surface is not respiratory:
example, _Oligoneuria garumnica_ (Fig. 279). (4) The anterior gill is
modified to form a plate that covers the others: example, _Tricorythus_
(Fig. 282, B). (5) The gills are concealed in a respiratory chamber:
example, _Prosopistoma_ (Fig. 280). The last of these nymphs is more
completely adapted for an aquatic life than any other Insect at present
known; it was for long supposed to be a Crustacean, but it has now been
shown to be the early stage of a may-fly, the sub-imago having been reared
from the nymph. The carapace by which the larger part of the body is
covered is formed by the union of the pro- and meso-thorax with the sheaths
of the anterior wings, which have an unusually extensive development; under
the carapace there is a respiratory chamber, the floor and sides of which
are formed by the posterior wing-sheaths, and by a large plate composed of
the united nota of the metathorax and the first six abdominal segments. In
this chamber there are placed five pairs of tracheal gills; entrance of
water to the chamber is effected by two laterally-placed orifices, and exit
by a single dorsal aperture. These nymphs use the body as a sucker, and so
adhere strongly to stones under water. When detached they swim rapidly by
means of their caudal setae; the form of these latter organs is different
from that {435}of other Ephemerid nymphs. This point and other details of
the anatomy of this creature have been described in detail by
Vayssière.[355] These nymphs have a very highly developed tracheal system;
they live in rapid watercourses attached to stones at a depth of three to
six inches or more under the water. Species of _Prosopistoma_ occur in
Europe, Madagascar, and West Africa.
[Illustration: FIG. 280.—_Prosopistoma punctifrons_, nymph. France. (After
Vayssière.) _o_, Orifice of exit from respiratory chamber.]
According to Eaton,[356] in the nymphs of some Ephemeridae the rectum
serves, to a certain extent, as a respiratory agent; he considers that
water is admitted to it and expelled after the manner we have described in
Odonata, p. 421.
[Illustration: FIG. 281.—A, Last three abdominal segments and bases of the
three caudal processes of _Cloëon dipterum_: _r_, dorsal vessel; _kl_,
ostia thereof; _k_, special terminal chamber of the dorsal vessel with its
entrance _a_; _b_, blood-vessel of the left caudal process; B, twenty-sixth
joint of the left caudal process from below; _b_, a portion of the
blood-vessel; _o_, orifice in the latter. (After Zimmermann.)]
The internal anatomy of the nymphs of Ephemeridae shows some points of
extreme interest. The long caudal setae are respiratory organs of a kind
that is almost if not quite without parallel in the other divisions of
Insecta. The dorsal vessel for the circulation of the blood is elongate,
and its chambers are arranged one to each segment of the body. It drives
the blood forwards in the usual manner, but the posterior chamber possesses
three blood-vessels, one of which is prolonged into each caudal seta. This
terminal chamber is so arranged as to drive the blood backwards into the
vessels of the setae; on the under surface of the vessels there are oval
orifices by which the blood escapes into the cavity of the seta so as to be
submitted to the action of the surrounding medium for some of the purposes
of respiration. This structure has been described by {436}Zimmermann,[357]
who agrees with Creutzberg[358] that the organ by which the blood is
propelled into the setae is a terminal chamber of the dorsal vessel;
Verlooren,[359] who first observed this accessory system of circulation,
thought the contractile chamber was quite separate from the heart. The
nature of the connexion between this terminal chamber that drives the blood
backwards and the other chambers that propel the fluid forwards appears
still to want elucidation.
[Illustration: FIG. 282.—A, Nymph of _Ephemerella ignita_ with gills of
left side removed; _g_, gills: B, nymph of _Tricorythus_ sp. with gill
cover of right side removed; _g.c_, gill cover; _g_, _g′_, gills. (After
Vayssière.)]
The nymphs of the Ephemeridae being creatures adapted for existence in
water, the details of their transformation into creatures having an
entirely aerial existence cannot but be of much interest. In the nymphs the
tracheal system is well developed, but differs from that of air-breathing
Insects in the total absence of any spiracles. Palmén has investigated this
subject,[360] and finds that the main longitudinal tracheal trunks of the
body of the nymph are not connected with the skin of the body by tracheae,
but are attached thereto by ten pairs of slender strings extending between
the chitinous integument and the tracheal trunks. When the skin is shed
these strings—or rather a chitinous axis in each one—are drawn out of the
body, and bring with them the chitinous linings of the tracheae. Thus
notwithstanding the absence of spiracles, the body wall is at each moult
pierced by openings that extend to the tracheae. After the ordinary moults
these orifices close immediately, but at the change to the winged state
they remain open and form the spiracles. At the same time the tracheal
gills are {437}completely shed, and the creature is thus transformed from a
water-breather to an Insect breathing air as usual. In addition to this
change there are others of great importance, such as the development of the
great eyes and the complete atrophy of the mouth-parts. The precise manner
of these changes is not known; they occur, however, within the nymph skin.
The sudden emergence of the winged Insect from the nymph is one of the most
remarkable facts in the life-history of the may-fly; it has been observed
by Sir John Lubbock,[361] who describes it as almost instantaneous. The
nymph floats on the water, the skin of the back opens, and the winged
Insect flies out, upwards and away; "from the moment when the skin first
cracks not ten seconds are over before the Insect has flown away." The
creature that thus escapes has not, however, quite completed its
transformation. It is still enveloped in a skin that compresses and
embarrasses it; this it therefore rapidly gets rid of, and thus becomes the
imago, or final instar of the life-cycle. The instar in which the creature
exists winged and active, though covered with a skin, is called the
sub-imago. The parts of the body in the sub-imago are as a whole smaller
than they are in the imago, and the colour is more dingy; the
appendages—wings, legs, and caudal setae—are generally considerably shorter
than they are in the imago, but attain their full length during the process
of extraction. The creatures being, according to Riley, very impatient and
eager to take to the wing, the completion of the shedding of the skin of
the sub-imago is sometimes performed while the Insect is flying in the air.
[Illustration: FIG. 283.—Lingua of _Heptagenia longicauda_, × 16. _m_,
Central; _l_, lateral pieces. (After Vayssière.)]
The food of young Ephemeridae is apparently of a varied and mixed nature.
Eaton says[362] that though sometimes the stronger larvae devour the
weaker, yet the diet is even in these cases partly vegetable. The
alimentary canal frequently contains much mud; very small organisms, such
as diatoms and confervae, are thought to form a large part of the bill of
fare of Ephemerid nymphs. Although {438}the mouth is atrophied in the
imago, yet it is highly developed in the nymphs. This is especially notable
in the case of the lingua or hypopharynx (Fig. 283); indeed Vayssière[363]
seems to incline to the opinion that this part of the mouth may be looked
on in these Insects as a pair of appendages of a head-segment (see p. 96
_ante_), like the labium or maxillae.
The life-history has not been fully ascertained in the case of any species
of may-fly; it is known, however, that the development of the nymph
sometimes occupies a considerable period, and it is thought that in the
case of some species this extends to as much as three years. It is rare to
find the post-embryonic development of an Insect occupying so long a
period, so that we are justified in saying that brief as may be the life of
the may-fly itself, the period of preparation for it is longer than usual.
Réaumur says, speaking of the winged fly, that its life is so short that
some species never see the sun. Their emergence from the nymph-skin taking
place at sunset, the duties of the generation have been, so far as these
individuals are concerned, completed before the morning, and they die
before sunrise. He thinks, indeed, that individuals living thus long are to
be looked on as Methuselahs among their fellows, most of whom, he says,
live only an hour or half an hour.[364] It is by no means clear to which
species these remarks of Réaumur refer; they are doubtless correct in
certain cases, but in others the life of the adult is not so very short,
and in some species may, in all probability, extend over three or four
days; indeed, if the weather undergo an unfavourable change so as to keep
them motionless, the life of the flies may be prolonged for a fortnight.
The life of the imago of the may-fly is as remarkable as it is brief; in
order to comprehend it we must refer to certain peculiarities of the
anatomy with which the vital phenomena are connected. The more important of
these are the large eyes of the males, the structure of the alimentary
canal, and that of the reproductive organs. We have already remarked that
the parts of the mouth in the imago are atrophied, yet the canal itself not
only exists but is even of greater capacity than usual; it appears to have
much the same general arrangement of parts as it had in the nymph. Its
coats are, however, of great tenuity, and according {439}to Palmén[365] the
divisions of the canal are separated by changes in the direction of certain
portions anterior to, and of others posterior to, its central and greater
part—the stomach—in such a manner that the portions with diverted positions
act as valves. The stomach, in fact, forms in the interior of the body a
delicate capacious sac; when movement tends to increase the capacity of the
body cavity then air enters into the stomachic sac by the mouth orifice,
but when muscular contractions result in pressure on the sac they close the
orifices of its extremities by the valve-like structures we have mentioned
above; the result is, that as complex movements of the body are made the
stomach becomes more and more distended by air. It was known even to the
old naturalists that the dancing may-fly is a sort of balloon, but they
were not acquainted with the exact mode of inflation. Palmén says that in
addition to the valve-like arrangements we have described, the entry to the
canal is controlled by a circular muscle, with which are connected
radiating muscles attached to the walls of the head. Palmén's views are
adopted, and to a certain extent confirmed, by Fritze,[366] who has
examined the alimentary canal of the may-fly, and considers that though the
normal parts of the canal exist, the function is changed in the imago, in
which the canal serves as a sort of balloon, and aids the function of the
reproductive organs. The change in the canal takes place in an anticipatory
manner during the nymph and sub-imago stages.
The sexual organs of Ephemeridae are remarkable for their simplicity; they
are destitute of the accessory glands and diverticula that, in some form or
other, are present in most other Insects. Still more remarkable is the fact
that the ducts by which they communicate with the exterior continue as a
pair to the extremity of the body, and do not, as in other Insects, unite
into a common duct. Thus in the female there is neither bursa copulatrix,
receptaculum seminis, nor uterine portion of oviduct, and there is no trace
of an ovipositor; the terminations of the ducts are placed at the hind
margin of the seventh ventral plate, just in front of which they are
connected by a fold of the integument. The ovary consists of a very large
number of small egg-tubes seated on one side of a sac, which forms their
calyx, and one of whose extremities is continued backwards as one of the
{440}pair of oviducts. The male has neither vesiculae seminales, accessory
glands, nor ductus ejaculatorius. The testes are elongate sacs, whose
extremities are prolonged backwards forming the vasa deferentia; these open
separately at the extremity of the body, each on a separate intromittent
projection of more or less complex character, the two organs being,
however, connected by means of the ninth ventral plate, of which they are,
according to Palmén, appendages. We should remark that this authority
considers _Heptagenia_ to form, to some extent, an exception as regards the
structures of the female; while _Polymitarcys_ is in the male sex strongly
aberrant, as the two vasa deferentia, instead of being approximately
straight, are bent inwards at right angles near their extremities so as to
meet, and form in the middle a common cavity, which then again becomes
double to pass into the pair of intromittent organs.
According to the views of Exner and others, the compound eyes of Insects
are chiefly organs for the perception of movement; if this view be correct,
movements such as those made during the dances of may-flies may, by the
number of the separate eyes, by their curved surfaces and innumerable
facets, be multiplied and correlated in a manner of which our own sense of
sight allows us to form no conception. We can see on a summer's evening how
beautifully and gracefully a crowd of may-flies dance, and we may well
believe that to the marvellous ocular organs of the flies themselves (Fig.
274) these movements form a veritable ballet. We have pointed out that by
this dancing the peculiarly formed alimentary canal becomes distended, and
may now add that Palmén and Fritze believe that the unique structure of the
reproductive organs is also correlated with the other anatomical
peculiarities, the contents of the sexual glands being driven along the
simple and direct ducts by the expansion of the balloon-like stomach.
During these dances the momentary conjugation of the sexes occurs, and
immediately thereafter the female, according to Eaton, resorts to the
waters appropriate for the deposition of her eggs. As regards this, Eaton
says:[367] "Some short-lived species discharge the contents of their
ovaries completely _en masse_, and the pair of fusiform or subcylindrical
egg-clusters laid upon the water rapidly disintegrate, so as to let the
eggs sink broadcast upon the river-bed. The less perishable species extrude
their eggs {441}gradually, part at a time, and deposit them in one or other
of the following manners: either the mother alights upon the water at
intervals to wash off the eggs that have issued from the mouths of the
oviducts during her flight, or else she creeps down into the water to lay
her eggs upon the under-side of stones, disposing them in rounded patches,
in a single layer evenly spread, and in mutual contiguity." The eggs are
very numerous, and it is thought may sometimes remain in the water as much
as six or seven months before they hatch.
The number of individuals produced by some kinds of may-flies is
remarkable. Swarms consisting of millions of individuals are occasionally
witnessed. D'Albertis observed _Palingenia papuana_ in countless myriads on
the Fly River in New Guinea: "For miles the surface of the river, from side
to side, was white with them as they hung over it on gauzy wings; at
certain moments, obeying some mysterious signal, they would rise in the
air, and then sink down anew like a fall of snow." He further states that
the two sexes were in very disproportionate numbers, and estimates that
there was but a single female to every five or six thousand males.
Ephemeridae in the perfect state are a favourite food of fishes, and it is
said that on some waters it is useless for the fly-fisher to try any other
lure when these flies are swarming. Most of the "duns" and "spinners" of
the angler are Ephemeridae; so are several of the "drakes," our large _E.
danica_ and _E. vulgata_ being known as the green drake and the gray drake.
Ronalds says[368] that the term "dun" refers to the pseud-imago condition,
"spinner" to the perfect Insect. _E. danica_ and _E. vulgata_ are perhaps
not distinguished by fishers; Eaton says that the former is abundant in
rapid, cool streams, while _E. vulgata_ prefers warmer and more tranquil
rivers.
These sensitive creatures are unable to resist the attractions of
artificial lights. Réaumur noticed this fact many years ago, and since the
introduction of the electric light, notes may frequently be seen in
journals recording that myriads of these Insects have been lured by it to
destruction. Their dances may frequently be observed to take place in
peculiar states of light and shade, in twilight, or where the sinking sun
has its light rendered broken by bushes or trees; possibly the broken
lights {442}are enhanced in effect by the ocular structures of the Insects.
It has recently been ascertained that a species of _Teleganodes_ is itself
luminous. Mr. Lewis,[369] who observed this Insect in Ceylon, states that
in life the whole of the abdomen was luminous, not brightly so, but
sufficient to serve as a guide for capturing the Insect on a dark night. It
has also been recorded that the male of _Caenis dimidiata_ gives a faint
blue light at night.
Nearly 300 species of Ephemeridae are known, but this may be only a
fragment of what actually exist, very little being known of may-flies of
other parts of the world than Europe and North America. One of the more
curious forms of the family is _Oniscigaster wakefieldi_; the body of the
imago is unusually rotund and furnished with lateral processes. In Britain
we have about forty species of may-fly. The family is treated as a distinct
Order by Brauer and Packard, and is called Plectoptera by the latter.
[Illustration: FIG. 284.—_Oniscigaster wakefieldi._ New Zealand. (After
M‘Lachlan.)]
That Insects so fragile, so highly organised, with a host of powerful
enemies, but themselves destitute of means of attack or defence, should
contrive to exist at all is remarkable; and it appears still more unlikely
that such delicate Insects as Ephemeridae should leave implanted in the
rocks their traces in such a manner that they can be recognised;
nevertheless, such is the case,—indeed, the may-fly palaeontological record
is both rich and remarkable. Several forms are preserved in amber. In the
Tertiary bed of the old lake at Florissant, Scudder has been able to
distinguish the remains of no less than six species; while in the Jurassic
layers of the Secondary epoch, in more than one locality, the remains of
several other species have been detected and described. Still more
remarkable is the fact that in the Devonian and Carboniferous layers of the
{443}Palaeozoic period, remains are found that appear to be akin to our
existing Ephemeridae. _Palingenia feistmantelii_ from the Carboniferous of
Bohemia is actually referred to a still existing genus; it is said to have
been of gigantic size for a may-fly.
The families Megasecopterides, Platypterides, and Stenodictyopterides of
the Carboniferous epoch (see p. 343) are all more or less closely allied to
the Ephemeridae, and in addition to these Brongniart has established the
family Protephemerides for some Insects that he considers to have been the
precursors in the Carboniferous epoch of our existing may-flies. These
ancient Insects differed in having the wings of another form from those of
existing Ephemeridae, and in having the hind wings equal in size to the
front pair. Besides this, these Insects had, as shown in Fig. 285,
prothoracic dorsal appendages; some had also projections from the abdominal
segments, considered by Brongniart to be of the nature of gills. Some doubt
must exist as to this point, for we find in the imago of one of our
existing Ephemeridae, _Oniscigaster wakefieldi_, Fig. 284, abdominal
processes that are not gills.
[Illustration: FIG. 285.—_Homaloneura bonnieri_; Carboniferous of
Commentry. (After Brongniart.)]
It is remarkable that may-flies, which now form a comparatively unimportant
part of the Insect tribe, should in far distant times have been represented
by so great a variety of allied forms. Our fragile, short-lived may-flies
appear to be, as Scudder says, the lingering fragments of an expiring
group.
{444}CHAPTER XX
NEUROPTERA PLANIPENNIA—SIALIDAE, ALDER-FLIES, SNAKE-FLIES—PANORPIDAE,
SCORPION-FLIES—HEMEROBIIDAE, ANT-LIONS, LACEWINGS, ETC.
FAM. VIII. SIALIDAE—ALDER-FLIES AND SNAKE-FLIES.
_Four wings of moderate size, meeting in repose over the back at an
angle; the hinder of the two pairs slightly the smaller; the anal area
small or nearly absent, not plicate. Nervures moderately numerous,
transverse veinlets moderately numerous, forming irregularly disposed
cells. The metamorphosis is great; there is a quiescent pupa. The larva
has the mandibles formed for biting, armed with strong teeth._
The Sialidae, though but a small family of only some six or eight genera,
comprise a considerable variety of forms and two sub-families—Sialides and
Raphidiides. The former group has larvae with aquatic habits possessed of
branchiae but no spiracles.
[Illustration: FIG. 286.—The alder-fly, _Sialis lutaria_. Britain. A, With
wings expanded; B, in profile.]
_Sialis lutaria_ is one of the commoner British Insects frequenting the
vegetation about the banks of tranquil streams; it is well known to
anglers, being used by them for a bait. According to Ronalds it is called
the alder or orl-fly, and in Wales the humpback.
{445}[Illustration: FIG. 287.—Portion of a row of eggs of _Sialis lutaria_.
(After Evans.)]
[Illustration: FIG. 288.—_Sialis lutaria_, larva.]
It is very unattractive in appearance, being of a blackish colour, with
wings of a yellow-brown tinge, and makes but a poor show when flying. The
female deposits patches of elongate eggs, placed on end and packed together
in a very clever manner (Fig. 287). These patches of eggs, of a stone-gray
colour, are common objects on rushes or stems of grass near water, and it
is stated that there may be no less than 2000 or 3000 eggs in one of them.
Our figure gives some idea of the mode in which the eggs are arranged, and
the curious narrow process that exists at the end of each. The eggs are
said to be sometimes placed at a considerable distance from water, so that
when the tiny larvae are hatched they must begin their lives by finding the
way to a suitable pool or stream. The larvae (Fig. 288) are objects of very
great interest owing to each of segments 1 to 7 of the hind body being
furnished on each side with a jointed filament, while the last segment ends
in a still longer, but unjointed process. These filaments are branchiae by
means of which the Insect obtains air, being, as we have said, destitute of
spiracles. It is an active creature and waves its filaments in a very
graceful manner; this process no doubt aids the branchiae in their
respiratory work. These larvae are well able to exist out of water if they
have a sufficiently damp environment. They live on animal matter, but their
life-history has not been followed in much detail and it is not known
{446}how many moults they make. The young larva has the head
disproportionately large and the branchial filaments longer. When the
growth is completed the larva returns to land, seeks a suitable situation
in the soil, and after an interval changes to a pupa, in which the
characters of the perfect Insect are plainly visible. Subsequently, without
becoming again active, it changes to the perfect Insect, and enjoys, for a
few days only, an aerial life.
The anatomy of the larva has been treated by Dufour.[370] The
supra-oesophageal ganglion is remarkably small; nothing is said as to the
existence of an infra-oesophageal ganglion; there are three thoracic and
eight abdominal ganglia; the first pair of these latter are nearer together
than the others, and this is also the case with the last three. The
alimentary canal in the adult is provided with a large paunch attached to
the crop by a narrow neck,[371] but Dufour could find no trace of this in
the larva. The structure of the branchiae has also been described by the
indefatigable French entomotomist. A tracheal tube sends a branch into one
of the appendages (Fig. 289); the branch gives off numerous smaller
tracheae, which at their extremities break up into branchlets close to the
integument. The tracheal tube that receives each main branchial trachea,
sends off from near the point of entry of the latter another trachea, that
distributes its branchlets on the alimentary canal. The margins of each
appendage are set with swimming hairs, so that the branchiae act as organs
of locomotion as well as of respiration, and by their activity in the
former capacity increase the efficiency of their primary function.
[Illustration: FIG. 289.—Structure of tracheal gill of _Sialis lutaria_.
(After Dufour.) _a_, Base of the gill; _b_, tracheal trunk with which it is
connected; _c_, trachea given off to alimentary canal.]
The genus _Sialis_ occurs in a few species only, throughout the {447}whole
of the Palaearctic and Nearctic regions, and reappears in Chili,[372]
though absent in all the intervening area. Several other genera of Insects
exhibit the same peculiarity of distribution.
[Illustration: FIG. 290.—_Corydalis crassicornis_, male, with greater
portions of the wings removed. Texas. (After M‘Lachlan.)]
[Illustration: FIG. 291.—_Raphidia notata_, female. Britain. (After
Curtis.)]
The genera _Corydalis_ and _Chauliodes_ form a group distinct from
_Sialis_, and are totally different in appearance, being gigantic Insects,
sometimes with the mandibles of the male enormously elongated (Fig. 290).
The species of _Corydalis_ are called in North America Hellgrammites;
Riley has described and figured the metamorphosis of _C. cornutus_,[373]
the life-history being very similar to that of our little _Sialis_. A mass
consisting of two or three thousand eggs is formed by the female, and the
young larva has long filaments at the sides of the body like _Sialis_.
These in the later larval life are comparatively shorter, but the Insect is
then provided with another set of gills in the form of spongy masses on the
under-side of the body. Riley, however, considers that these organs serve
the purpose of attachment rather than of respiration. The larvae are known
to the Mississippi fishermen as crawlers, and are greatly esteemed as bait.
The Raphidiides or snake-flies form the second tribe of Sialidae. There are
only two genera, _Raphidia_ and _Inocellia_, peculiar to the Palaearctic
and Nearctic regions. The perfect Insects are chiefly remarkable for the
elongation of the prothorax and back of the head to form a long neck, and
for the existence in the female of an elongate exserted ovipositor.
{448}The species are rather numerous, and have been recently monographed by
Albarda.[374] The three or four British species of the genus are all rare
Insects, and occur only in wooded regions.
The Raphidiides, like the Sialides, have a carnivorous larva, which,
however, is terrestrial in habits, feeding, it would appear, chiefly on
Insects that harbour in old timber. The snake-fly larvae (Fig. 292) are
very ingenious in their manner of escaping, which is done by an extremely
rapid wriggling backwards. They are capable of undergoing very prolonged
fasts, and then alter in form a good deal, becoming shorter and more
shrivelled; Fig. 292 is taken from a specimen that had been fasting for
several weeks. They are excessively voracious, and hunt after the fashion
of beasts of prey; their habits have been described by Stein,[375] who
states that he kept a larva from August to the end of May of the following
year without food; it then died in a shrivelled-up state. The larva of the
snake-fly changes to a pupa that is remarkably intermediate in form between
the perfect Insect and the larva; the eyes, legs, wing-pads, and ovipositor
being but little different from those of the imago, while the general form
is that of the larva, and the peculiar elongation of the neck of the imago
is absent. This pupa differs from that of _Sialis_ in the important
particular that before undergoing its final ecdysis it regains its activity
and is able to run about.
[Illustration: FIG. 292.—_Raphidia notata_, larva. New Forest.]
The internal anatomy of _Raphidia_ has been treated by Loew,[376] and is of
a very remarkable character; we can here only mention that the salivary
glands consist of a pair of extremely elongate tubes, that there is a very
definite paunch attached as an appendage to one side of the crop, and that
the most peculiar character consists of the fact that, according to Loew,
four of the six Malpighian tubes have not a free extremity, being attached
{449}at each end so as to form elongate loops; the mesenteron is very
complex in character.
A considerable number of fossil remains from both Tertiary and Mesozoic
strata are referred to Sialidae; and a larval form from the red sandstone
of Connecticut has been considered by Scudder to be a Sialid, and named
_Mormolucoides articulatus_, but the correctness of this determination is
very doubtful (Fig. 293). These fossils are, however, of special interest
as being the most ancient Insect larvae yet brought to light. A still older
fossil, from the Carboniferous strata of Illinois called _Miamia bronsoni_,
is considered by Scudder to have several points of resemblance to Sialidae.
[Illustration: FIG. 293.—_Mormolucoides articulatus_, larva. Trias of
Connecticut. (After Scudder.)]
FAM. IX. PANORPIDAE—SCORPION-FLIES.
_Head prolonged to form a deflexed beak, provided with palpi near its apex;
wings elongate and narrow, shining and destitute of hair, with numerous,
slightly divergent veins and moderately numerous transverse veinlets (in
one genus the wings are absent). Larvae provided with legs, and usually
with numerous prolegs like the saw-flies: habits carnivorous._
[Illustration: Fig. 294.—_Panorpa communis_, male. Cambridge.]
The majority of the members of this family are very readily distinguished
by the beak-like front of the head, this being chiefly due to enlargement
of parts of the head itself, and in a less degree to prolongation of the
mouth-parts. The upper (or front) face of the beak is formed entirely by
the clypeus, the labrum being scarcely {450}visible, though it may be
detected at the sides of the tip of the beak; the sutures between the
various parts of the head are nearly or quite obliterated, but it is
probable that the sides of the beak are formed by the genae and by the
stipites of the maxillae, and its under-surface chiefly by the submentum:
the mentum itself is but small, the ligula is small, bifid at the
extremity, and each branch bears a two-jointed palpus, the basal article
being of very peculiar structure in _Panorpa_. The mandibles are but small,
and are placed at the apex of the beak; they have each the form of an
oblong plate armed with two very sharp teeth, and they cross freely. The
maxillae are the only parts of the mouth-pieces that are very elongated;
each cardo is articulated at the base of the head, and the stipes extends
all the length of the side of the beak; each maxilla bears a five-jointed
palpus and two small but very densely ciliated lobes. The antennae are
long, very slender, and flexible, and are many-jointed; they are inserted
between the eyes in large foramina; there are three ocelli, or none, and
the compound eyes are moderately large. The prothorax is small, its notum
is quite small or moderate in size, and the prothoracic stigma is placed
behind it; the side-pieces are small, and there is no chitinous prosternum
except a small longitudinal strip placed in the membrane between the coxae;
these latter are of only moderate size, and are free and dependent. The
meso- and meta-thorax are large, their side-pieces are of considerable
dimensions and bear large, dependent coxae and supporting-pieces (Fig. 58);
there is a stigma placed between the meso- and meta-thorax at the hind
margin of the upper part of the meso-trochantin; both meso- and meta-notum
are transversely divided. The abdomen is elongate, slender,
conico-cylindrical, consisting of nine segments; the basal segment is
membranous and concealed; the terminal appendages are of variable nature
according to the species and sex. The legs are elongate and slender, the
tarsi five-jointed. The internal anatomy of _Panorpa communis_ has been
examined by Dufour[377] and Loew.[378] They agree in describing the
alimentary canal as being of peculiar structure: there is a short, slender
oesophagus leading to an organ in which there is seated a remarkable
arrangement of elongate hairs; this structure might be looked on as the
proventriculus, but Loew considers it to be rather a {451}division of the
true stomach. The particulars given by these two anatomists as to some
other parts of the internal anatomy are very discrepant.
The Panorpidae form a small family of only nine or ten genera, two or three
of these being exotic and only imperfectly known; the three genera found in
Europe are composed of very curious Insects. The scorpion-flies—_Panorpa_
proper—are very common Insects, and have received their vernacular name
from the fact that the males have the terminal segments elongate and
slender and very mobile, and carry them curved up somewhat after the
fashion of the scorpions (Fig. 294). It is said that Aristotle was
acquainted with these Insects, and considered them to be really winged
scorpions.
A second European genus, _Boreus_, is still more peculiar; it is destitute
of wings, and has the appearance of a minute wingless grasshopper; it is
found from late autumn to early spring in moss and under stones, and is
said to be sometimes found disporting itself on the surface of the snow:
the female of this Insect has an exserted ovipositor. The writer has found
this little creature in Scotland among moss in November, and under stones
early in March (Fig. 295). The third European genus, _Bittacus_, does not
occur in our islands, but is common on many parts of the Continent; the
perfect Insect has a great resemblance to a _Tipula_, or "daddy-long-legs"
fly, and attaches itself to the stems of grasses, and preys on flies;
according to Brauer it has the peculiar habit of using the hind pair of
legs as hands (Fig. 296), instead of the front pair, as is usual in
Insects. This remarkable genus is widely distributed, and species of it are
found even in the Antipodes. A species inhabiting caves has been mentioned
by M‘Lachlan.[379]
[Illustration: FIG. 295.—_Boreus hiemalis_, female. Dumfriesshire.]
The early stages of the Panorpidae were for long unknown, but have recently
been discovered by Brauer: he obtained eggs of _Panorpa_ by confining a
number of the perfect flies in a vessel containing some damp earth on which
was placed a piece of meat; when the young larvae were hatched they buried
themselves in the earth and nourished themselves with the meat or its
juices.
{452}[Illustration: FIG. 296.—_Bittacus tipularius_ holding a fly in its
hind legs. Austria. (After Brauer.)]
[Illustration: FIG. 297.—Young larva of _Panorpa communis_. (After
Brauer.)]
These larvae (Fig. 297) bear a great resemblance to those of the
Hymenopterous family Tenthredinidae; they have biting mandibles and
palp-bearing maxillae, and show no approach to the peculiar mouth structure
found in the Hemerobiidae; there are three pairs of feet placed on the
three thoracic segments, and there is also a pair of less perfect feet on
each of the first eight abdominal segments, those behind being the larger.
The upper surface of the body bears spines, which, however, disappear after
the first change of skin, with the exception of the larger processes on the
posterior segment, which persist throughout the life of the larva. The
larvae are active for about one month; after this they become quiescent,
but do not change to the pupa state for several weeks; when this happens
they change in form and cannot creep, although their limbs are not enclosed
in any pupa case. Brauer also discovered larvae of _Panorpa communis_ at
large in numbers in an old tree stump that was quite covered with moss, and
contained many ants in the mouldering wood. The ants appeared to be on
friendly terms with the _Panorpa_ larvae. The earlier stages of
{453}_Boreus_ and _Bittacus_ were also observed by Brauer; they are
essentially similar to those of _Panorpa_, but the larva in _Boreus_ is not
provided with abdominal prolegs. The Panorpidae have been separated from
the other Neuroptera by certain naturalists as a distinct Order, called
Panorpatae by Brauer, Mecaptera by Packard; but in their structure as well
as in their metamorphoses they are not so distinct from the Phryganeidae
and the Hemerobiidae as to justify this step.
Fossil forms of _Bittacus_ and of _Panorpa_ have been found in amber and in
the Tertiary strata, and Scudder has described some forms from Florissant
in which there are no cross-veinlets in the wings. Some remains from the
English Lias have been referred to Panorpidae by Westwood under the name
_Orthophlebia_, but it is by no means certain that they really belong to
the family.
FAM. X. HEMEROBIIDAE—ANT-LIONS, LACEWING-FLIES, ETC.
_Head vertical; maxillae free, with five-jointed palpi; labial palpi
three-jointed. Wings subequal in size, with much reticulation, without
anal area. Tarsi five-jointed. Metamorphosis great; the larvae with
mandibles and maxillae coadapted to form spear-like organs that are
suctorial in function. Pupa, similar in general form to the imago,
enclosed in a cocoon._
[Illustration: FIG. 298.—_Drepanepteryx phalaenoides._ Scotland.]
The Hemerobiidae are an extremely varied assemblage of Neuroptera; the
perfect Insects of the various sub-families are very different in
appearance, but the family as a whole is naturally defined by the very
peculiar structure of the mouth-organs of the larvae. These Insects have,
in fact, a suctorial {454}mouth in their early life, and one of the
ordinary biting type in adult life.
This is a very unusual condition, being the reverse of what we find in
Lepidoptera and some other of the large Orders, where the mouth is
mandibulate in the young and suctorial in the adult. The suctorial
condition is in Hemerobiidae chiefly due to modification of the mandibles;
but this is never the case in the Insects that have a suctorial mouth in
the imaginal instar. Nearly all the Hemerobiidae are terrestrial Insects in
all their stages; a small number of them are, to a certain extent,
amphibious in the larval life, while one or two genera possess truly
aquatic larvae. The metamorphosis is, so far as the changes of external
form are concerned, quite complete. There are no wingless forms in the
adult stage.
The classification given by Hagen[380] and generally adopted recognises
seven sub-families. These we shall mention seriatim.
SUB-FAM. 1. MYRMELEONIDES OR ANT-LIONS.—_Antennae short, clubbed, the
apical space of the wing with regular, oblong cellules._
[Illustration: FIG. 299.—_Tomateres citrinus._ S. E. Africa. (After
Hagen.)]
The ant-lions in their perfect state are usually unattractive Insects, and
many are nocturnal in their habits; the species of the genus _Palpares_ and
allies (Fig. 299) are, however, of more handsome appearance, and attain a
large expanse of wing. No member of the sub-family is an inhabitant of
Britain, though species of the typical genus _Myrmeleon_ are common in
Central and Northern Europe. The {455}remarkable habits of their larvae
attracted the attention of naturalists so long ago as two hundred years. We
owe to Réaumur an accurate and interesting account of _M. formicarius_, the
species found in the neighbourhood of Paris. The larvae are predaceous, and
secure their prey by means of pitfalls they excavate in the earth, and at
the bottom of which they bury themselves, leaving only their elongate jaws
projecting out of the sand at the bottom of the pit. They move only
backwards, and in forming their pit use their broad body as a plough, and
throw out the sand by placing it on the head and then sending it to a
distance with a sudden jerk. When about to construct its trap the larva
does not commence at the centre, but makes first a circular groove of the
full circumference of the future pit. Burying its abdomen in the surface of
the earth, the Insect collects on to its head, by means of the front leg,
the sand from the side which is nearest to the centre, and then jerks the
sand to a distance. By making a second circuit within the first one, and
then another, the soil is gradually removed, and a conical pit is formed,
at the bottom of which the ant-lion lurks, burying its body but leaving its
formidable mandibles widely extended and projecting from the sand. In this
position the young ant-lion waits patiently till some wandering Insect
trespasses on its domains. An ant or fly coming over the edge of the
pitfall finds the sand of the sloping sides yielding beneath its body, and
in its effort to secure itself probably dislodges some more of the sand,
which, descending to the bottom of the pit, brings the lurking lion into
activity. Availing himself of his power of throwing sand with his head, the
ant-lion jerks some in the neighbourhood of the trespasser, and continues
to do so until the victim is brought to the bottom of the pit and into the
very jaws of its destroyer; then there is no further hope of escape; the
mandibles close, empale their prey, and do not relax their hold till the
body of the victim is exhausted of its juices. The position chosen is in a
place that will keep dry, as the larva cannot carry on its operations when
the sand is wet or damp, hence the soil at the base of a high wall or a
rock frequently harbours these Insects.
{456}[Illustration: FIG. 300.—Larva of _Myrmeleon pallidipennis_. (After
Meinert.)]
The parts of the mouth of the _Myrmeleon_ are perfectly adapted for
enabling it to empty the victim without for a moment relaxing its hold.
There is no mouth-orifice of the usual character, and the contents of the
victim are brought into the buccal cavity by means of a groove extending
along the under side of each mandible; in this groove the elongate and
slender lobe that replaces the maxilla—there being no maxillary palpi—plays
backwards and forwards, probably raking or dragging backwards to the buccal
cavity at each movement a small quantity of the contents of the empaled
victim. The small lower lip is peculiar, consisting in greater part of the
two lobes that support the labial palpi. The pharynx is provided with a
complex set of muscles, and, together with the buccal cavity, functions as
an instrument of suction. After the prey has been sucked dry the carcass is
jerked away to a distance. When the ant-lion larva is full grown it forms a
globular cocoon by fastening together grains of sand with fine silk from a
slender spinneret placed at the posterior extremity of the body; in this
cocoon it changes to an imago of very elongate form, and does not emerge
until its metamorphosis is quite completed, the skin of the pupa being,
when the Insect emerges, left behind in the cocoon. The names by which the
European ant-lion has been known are very numerous. It was called Formicajo
and Formicario by Vallisneri about two hundred years ago; Réaumur called it
Formica-leo, and this was adopted by some modern authors as a generic name
for some other of the ant-lions. The French people call these Insects
Fourmilions, of which ant-lion is our English equivalent. The Latinised
form of the term ant-lion, Formicaleo, is not now applied to the common
ant-lion as a generic term, it having been proposed to replace it by
_Myrmecoleon_, _Myrmeleo_, or _Myrmeleon_; this latter name at present
seems likely to become generally adopted. There are several species of the
genus found in Europe, and their trivial names have been confounded by
various authors in such a way as to make it quite uncertain, without
reference to a synonymic list, what species is intended by any particular
writer. The species found in the neighbourhood of Paris, and to which it
may be presumed Réaumur's history refers, is now called {457}_Myrmeleon
formicarium_ by Hagen and others; M‘Lachlan renamed it _M. europaeus_, but
now considers it to be the _M. nostras_ of Fourcroy. The popular name
appears to be due to the fact that ants—Formica in Latin, Fourmi in
French—form a large part of the victims; while lion—the other part of the
name—is doubtless due to its prowess as a destroyer of animal life, though,
as Réaumur long ago remarked, it is a mistake to apply the term lion to an
Insect that captures its prey by strategy and by snares rather than by
rapidity and strength. The imago of _Myrmeleon_ is of shy disposition, and
is rarely seen even in localities where the larva is abundant. It is of
nocturnal habits, and is considered by Dufour to be carnivorous.
Considerable difference of opinion has existed as to the structure of the
mouth and of the alimentary canal in these larvae. Réaumur was of opinion
that there exists no posterior orifice to the alimentary canal, but Dufour
ridiculed this idea, and stated positively that such an orifice undoubtedly
exists. It is also usually said that the mouth is closed by a membrane.
Meinert has recently examined these points,[381] and he states that the
mouth is not closed by any membrane, but is merely compressed. He finds
that there is no posterior exit from the stomach; that there is a compact
mass without any cavity between the stomach and the point where the
Malpighian tubes connect with the small intestine. The portions of aliment
that are not assimilated by the larva collect in the stomach and are
expelled as a mass, but only after the Insect has become an imago. This
peculiar excrementitious mass consists externally of uric acid, and from
its form and appearance has been mistaken for an egg by several
naturalists. The posterior portions of the alimentary canal are, according
to Meinert, of a remarkable nature. The small intestine is elongate,
slender, and is coiled. There are eight very long and slender Malpighian
tubes; a pair of these have free extremities, but the other six in the
posterior part of their course are surrounded by a common membrane, and,
following the course of the intestine, form ultimately a dilated body
seated on a coecum. These six Malpighian tubes are considered to be
partially, if not entirely, organs for the secretion of silk for forming
the cocoon, the coecum being a reservoir. The canal terminates as a slender
tube, which acts as a spinneret and is surrounded by a sheath. A complex
set of muscles {458}completes this remarkable spinning apparatus. The
alimentary canal of the imago has been described and figured by
Dufour[382]; it is very different from that of the larva.
The ant-lion is capable of sustaining prolonged fasts. Dufour kept
specimens for six months without any food. These Insects are said to give
off a peculiar ant-like odour, due, it is thought, to their ant-eating
habits. Although no species inhabits Great Britain, yet one is found in
Southern Sweden. Introduced specimens get on very well in confinement in
our country,[383] and would probably flourish at large for some years if
they were liberated.
Although the number of known species and genera of Myrmeleonides is
considerable—that of the species being now upwards of 300—the members of
the small genus _Myrmeleon_ are the only forms that are known to make pits
of the kind we have described. Other larvae[384] are known similar in
general form to the common ant-lion, but they walk forwards in the normal
manner, and apparently hunt their prey by lurking in a hidden place and,
when a chance occurs, rushing on the victim with rapidity. Brauer has
observed this habit in the case of _Dendroleon pantherinus_ in the Prater
at Vienna.
[Illustration: FIG. 301.—Upper aspect of head and alimentary canal of
_Myrmeleon_: _a_, crop; _b_, stomach; _c_, free extremities of two
Malpighian tubes; _c′_, terminal common portion of other six tubes; _d_,
coecum; _e_, spinneret; _f_, _f_, muscles for protruding its sheath; _g_,
_g_, maxillary glands. (After Meinert.)]
The most remarkable forms of Myrmeleonides are contained in the genus
_Palpares_. We figure _Tomateres citrinus_ (Fig. 299), an allied genus
found in Eastern Africa as far south as Natal. These Insects have
conspicuous blotches and marks on their wings. The species of _Myrmeleon_
are similar in form, but are smaller, more feeble, and less ornate in
appearance.
{459}Pitfalls, formed in all probability by ant-lions, have been noticed in
the Galapagos islands and in Patagonia, though none of the Insects forming
them have been found.
SUB-FAM. 2. ASCALAPHIDES.—_Antennae elongate, with a knob at the tip; the
apical area of the wing with irregular cellules._
[Illustration: FIG. 302.—_Ascalaphus coccajus._ East Pyrenees.]
The sub-family Ascalaphides is not represented by any species in Britain,
though _Ascalaphus longicornis_ occurs as far north as Paris. In the
mountainous regions of Central and Southern Europe some species of the
group form a conspicuous part of the Insect fauna, owing to their bold and
active flight; they are predaceous in their habits, and fly about in a
hawking fashion somewhat like that of dragon-flies. Some of the larger of
the numerous exotic species of the group are very like dragon-flies, but
can be distinguished by a glance at the elongate antennae with a knob at
the end. The sub-family consists of two groups—Holophthalmi and
Schizophthalmi. M‘Lachlan says[385]: "The eyes in the Schizophthalmous
division are really double, the upper portion overlapping the under; if the
upper portion be separated the lower division looks like a small spherical
ordinary eye." There appears, however, to be considerable differences in
the genera in this respect.
{460}[Illustration: FIG. 303.—A, Eggs of _Ascalaphus macaronius_. B, Sketch
of position of the young larvae of _Helicomitus insimulans_ (?); C, outline
of natural size. (After Westwood.)]
When the weather is wet or cold the Ascalaphi repose on the stems of grass,
with their wings placed in a roof-like manner, with the head downwards, and
are then very successful in concealing themselves by the positions they
assume, and by sidling round the stems to escape from enemies. Some
information as to their metamorphosis has been obtained, though knowledge
of this point is far from complete even as regards our European species of
the typical genus _Ascalaphus_. For a long time it was supposed that a
larva mentioned by Bonnet in his writings was that of _Ascalaphus_, but
Brauer[386] is of opinion that such is not the case, and as he has
described the metamorphoses of _A. macaronius_ he is no doubt correct. The
eggs (Fig. 303, A), forty or fifty in number, are laid in two parallel rows
on the stems of grass. The larvae (Fig. 304, larva of _Helicomitus_ ?) are
in general appearance somewhat like those of _Myrmeleon_; they are
carnivorous in their habits, like the ant-lions, and have similar
extraordinarily developed mandibles. Efforts to rear the young larvae
failed, but they were kept alive for some time by supplying them with
Aphidides found on _Centaurea jacea_. The cocoon is globular, and the
change from the nymph state to the imago is made in the cocoon, the
structure of the mandibles of the pupa being peculiar, and specially
adapted to the purpose of opening the cocoon.[387] The larvae of
Ascalaphides, although so like the ant-lions in appearance, do not form
pitfalls for the capture of their prey, but lurk under leaves on the
ground, or under stones; they do not move backwards, but progress forwards
in an ordinary manner; the habit of backward movement that we noticed in
_Myrmeleon_ being probably correlative with the habit of forming pitfalls.
Hagen states[388] that the larvae of Ascalaphides and Myrmeleonides, in
addition to their peculiarities of form and mandibular structure, are
distinguished from those of other Hemerobiidae by the hind legs having the
tibia and tarsus united {461}without articulation. Westwood[389] has
recently given an account of the young larvae of a Ceylonese Ascalaphid of
doubtful species, but possibly _Helicomitus insimulans_; these were
observed by Mr. Staniforth Green to have the very peculiar habit of sitting
together in a long row on the stem of a plant, with the jaws widely
extended and the body of each one covered by the head of the individual
next it (Fig. 303, B). The little creatures waited patiently in this
position until a fly walked between the mandibles of one of them, then
these formidable weapons immediately closed, and did not relax their hold
until the fly was sucked dry. If Westwood is correct, the young larva of
this species differs much from the adult one, the back of the head being
broad and the setigerous processes of the body very much more developed.
Nearly thirty genera of Ascalaphides are known.[390] In the genus
_Haplogenius_ we find an exception to the usual rule that the wings in
repose are held in a roof-like manner, it having been noticed by Bates that
in the species in question the wings are held expanded as in the
dragon-flies.
[Illustration: FIG. 304.—Larva of _Helicomitus insimulans_ (?). (After
Westwood.)]
Guilding has described[391] a very peculiar mode of oviposition on the part
of _Ulula macleayana_ in the island of St. Vincent; the eggs are said to be
deposited by the female in circles on the extremity of a twig, and nearer
the base of this there is placed a kind of barrier to repel intruders. "The
female may be seen expelling from her ovary these natural barriers with as
much care as her real eggs." Guilding's description was accompanied by
drawings of the eggs, barriers and larvae, but unfortunately these were
never published, and no further information has been obtained on the
subject. Hagen[392] suggests that the barriers may {462}be somewhat similar
to the long stalks on which the eggs of _Chrysopa_ (Fig. 314) are placed.
SUB-FAM. 3. NEMOPTERIDES.—_Head more or less produced and beak-like. Hind
wings of peculiar form, being elongate and somewhat strap-like._
[Illustration: FIG. 305.—_Nemoptera ledereri._ Asia Minor. (After Selys.)
A, The imago; B, its head seen from in front and magnified.]
[Illustration: FIG. 306.—Presumed larva of _Nemoptera_ (_Necrophilus
arenarius_). After Roux. Pyramids of Egypt.]
The Nemopterides are a small group of delicate, graceful Insects. About
thirty species are known. Knowledge of the group is still very imperfect. A
larva has been found of a most remarkable nature that probably belongs to
it; it was described under the name of _Necrophilus arenarius_, and
considered to be a fully-developed Insect. This larva occurs in the tombs
and pyramids of Egypt where sand has accumulated. The perfect Insects of
the genus _Nemoptera_ are, however, found in open places amongst bushes,
and flit about in a very graceful manner. Several species are found in
Southern Europe and the Mediterranean region {463}(Fig. 305, _N.
ledereri_), but none come so far north as Central Europe. Formerly the
genus _Nemoptera_ was considered to be allied to _Panorpa_ on account of
the beak-like front of the head. The parts of the mouth are, however,
different from those of _Panorpa_, and it seems more probable that if the
Nemopterides have to be merged in any of the divisions of Hemerobiidae,
they will be placed in Chrysopides or Osmylides. The species of the
sub-family were for a long time believed to be peculiar to the continental
regions of the Old World, but a species has recently been discovered in
Northern Chili.[393]
SUB-FAM. 4. MANTISPIDES.—_Prothorax elongate; the raptorial front legs
inserted at its anterior part._
The members of this small group are readily recognised by the peculiar
structure of the front legs; these organs resembling those of the
Orthopterous family Mantidae, so that the earlier systematic entomologists,
deceived by this resemblance, placed the Mantispides in the Order referred
to.
[Illustration: FIG. 307.—_Mantispa areolaris._ Brazil. (After Westwood.)]
The Mantispides possess four membranous wings, either sub-equal in size or
the posterior pair smaller than the front pair and not folded; the veins of
these wings are rather numerous, as are also the cells they form; there is
considerable difference amongst the species in this latter respect, owing
to the transverse veinlets differing in their abundance. The antennae are
short, not in the least thickened at the tip. The head is not produced into
a beak. The anterior legs, placed quite at the front part of the thorax,
have the coxae very long; the femur is somewhat incrassate, and is armed on
one side with spines; the tibia is shaped and articulated so as to fold
closely on to the spines, and to thus constitute a formidable and perfect
prehensile organ, the tarsus being merely a small appendage.
{464}[Illustration: FIG. 308.—_Mantispa styriaca._ A, Larva newly hatched,
or first form; B, mature larva. (After Brauer.)]
Only a few species of _Mantispa_ are found in Southern Europe; but the
group has representatives in most of the warmer regions of the world, and
will probably prove to be rather numerous in species. The front legs are
used for the capture of prey in the same way as the somewhat similar front
legs of the Mantidae. The transformations have been observed by Brauer[394]
in the case of one of the European species, _M. styriaca_. The eggs are
numerous but very small, and are deposited in such a manner that each is
borne by a long slender stalk, as in the lacewing flies. The larvae are
hatched in autumn; they then hibernate and go for about seven months before
they take any food. In the spring, when the spiders of the genus _Lycosa_
have formed their bags of eggs, the minute _Mantispa_ larvae (Fig. 308, A)
find them out, tear a hole in the bag, and enter among the eggs; here they
wait until the eggs have attained a fitting stage of development before
they commence to feed. Brauer found that they ate the spiders when these
were quite young, and then changed their skin for the second time, the
first moult having taken place when they were hatched from the egg. At this
second moult the larva undergoes a considerable change of form; it becomes
unfit for locomotion, and the head loses the comparatively large size and
high development it previously possessed. The _Mantispa_ larva—only one of
which flourishes in one egg-bag of a spider—undergoes this change in the
midst of a mass of dead young spiders it has gathered together in a
peculiar manner. It undergoes no further change of skin, and is full fed in
a few days; after which it spins a cocoon in the interior of the egg-bag of
the spider, and changes to a nymph inside its larva-skin. {465}Finally the
nymph breaks through the barriers—larva-skin, cocoon, and egg-bag of the
spider—by which it is enclosed, and after creeping about for a little,
appears in its final form as a perfect _Mantispa_. Thus in this Insect
hypermetamorphosis occurs; the larval life consisting of two different
instars, one of which is specially adapted for obtaining access to the
creature it is to prey on. It should be noticed that though this Insect is
so destructive to the young spiders, the mother spider shows no hostility
to it, but allows the destroying larva to enter her bag of eggs without any
opposition; she appears, indeed, to be so unconscious of the havoc that is
going on amongst her young that in one case she continued to watch over and
protect the egg-bag in which the destruction was taking place during the
whole of the period of the larval development and half the period of
pupation of the _Mantispa_.
The larval history of a second species of the Mantispides, _Symphrasis
varia_, is partially known;[395] this Insect lives parasitically in the
nests of a South American wasp, and each larva when full fed spins a cocoon
in one of the cells of the Hymenopteron.
SUB-FAM. 5. HEMEROBIIDES.—_Wings in repose forming an angular roof over
the body; the antennae moniliform or pectinate, not clavate._
The Hemerobiides consist of several minor groups about whose number and
characters systematists are not very well agreed, and about some of which
very little is known. We merely mention the latter, giving details as to
some of the better known only.
1. The Dilarina are a small group found chiefly in the Old World, where,
however, they have a wide distribution. North and South America have each
one species. They are distinguished by their antennae, which, in the male,
are pectinate somewhat like those of many Lepidoptera, this character being
of extremely rare occurrence in the Neuroptera; the abdomen of the female
terminates in a long ovipositor. The metamorphoses are not known.
2. Nymphidina: Australian Insects resembling Myrmeleonides, but having
antennae without club. Metamorphoses not known.
{466}3. Osmylina: a group of delicate and elegant Insects of small or
moderate size, distinguished by the possession of three simple eyes placed
on the middle of the head just above the antennae. A species of this group,
_Osmylus chrysops_ (_maculatus_ of some authors), is an inhabitant of
Britain (Fig. 212); its larva is to some extent amphibious. The
metamorphoses have been observed by Dufour, Brauer, and Hagen;[396] it
lurks under stones in or close to the water, or in moss, or on the stems of
aquatic plants, and pierces and empties small Insects with its
sucking-spears, which are very elongate. The young are hatched from the egg
in the autumn and hibernate before becoming full grown; when this moment
arrives the larva spins a round cocoon of silk mixed with sand. The pupa,
or nymph, in general appearance somewhat resembles the perfect Insect,
except that it is shorter and has the short wing-pads clinging close to the
body. Dufour denied the existence of abdominal spiracles in either larva or
imago, but, according to Hagen, they are certainly present in both. It
would appear that in the larva the alimentary canal is not open beyond the
chylific ventricle, and that its terminal section is modified to form a
spinning apparatus.
[Illustration: FIG. 309.—_Osmylus chrysops._ A, Larva; B, side view of head
of larva (after Brauer); C, pupa (after Hagen).]
_Osmylus_ and its allies, including _Sisyra_, are now frequently treated as
a separate sub-family, Osmylides, equivalent to the Chrysopides. In it is
placed a very anomalous Insect—_Psectra dispar_—of great rarity. The male
has only two wings, the posterior pair being the merest rudiments, though
the female has the four wings normally developed. Individuals of the male
have been found[397] in widely separated localities in the Palaearctic
region—Somersetshire being one of them—and also in North America.
{467}The genus _Sisyra_ forms for some Neuropterists the type of a separate
group called Sisyrina, though by others it is placed, as we have said, with
the Osmylina, though it is destitute of ocelli. The larvae of at least one
species of this genus are aquatic, and have been found in abundance living
in _Spongilla_ (_Ephydatia_) _fluviatilis_, a fresh-water sponge; when
discovered their nature was not at first recognised, as they possess on
each ventral segment a pair of articulated appendages, looking like legs,
but which are considered to be more of the nature of gills. The
sucking-spears of this Insect are so long and slender as to look like
hairs; whether the little animal draws its nutriment from the sponge, or
merely uses this latter as a place of shelter, is not ascertained.
[Illustration: FIG. 310.—A, Larva of _Sisyra fuscata_, ventral aspect; B,
an abdominal appendage. (After Westwood.)]
[Illustration: FIG. 311.—Larva of _Hemerobius_ sp. from Kent. A, The larva
bare; B, the same, partially concealed by the remains of its victims, etc.;
a portion of the covering has been removed in order to show the head.]
4. Hemerobiina: a somewhat numerous group of small or more rarely
moderate-sized Insects, with moniliform antennae, no ocelli, a complex and
comparatively regular system of wing-nervures; the veinlets are especially
numerous at the margins, owing to the mode of forking of the nervures there
(Fig. 298, _Drepanepteryx phalaenoides_). The larvae of most of the species
of which the habits are known {468}live on Aphides, which they suck dry,
and at least one species, in all probability several, has the habit of
covering itself with the skins of the victims it has sucked; to these
remains it adds other small debris, and the whole mass completely covers
and conceals the Insect (Fig. 311, B). The larva is furnished at the sides
with projections which serve as pedicels to elongate divergent hairs, and
these help to keep the mass in place on the back of the Insect; some fine
threads are distributed through this curious mantle and serve to keep it
from disintegration, but whether they are fragments of spiders' webs or are
spun by the Insect itself is not quite clear.
[Illustration: FIG. 312.—Portions of wings of _Drepanepteryx phalaenoides_.
A, Under-face of basal parts of the two wings; _a_, base of front wing;
_b_, of hind wing. B, Portion of front wing, showing the apparent
interruption of nervures.]
The genus _Drepanepteryx_ consists of several species, and appears to be
best represented in the Antipodes; we have, however, one species in
Britain—_D. phalaenoides_ (Fig. 298)—an extremely interesting member of our
fauna. This Insect has, like several of its congeners, a moth-like
appearance, and it has a peculiar structure for bringing the hind and fore
wings into correlation, the costa at the base of the hind wing being
interrupted and prominent, furnished with setae (Fig. 312, A), and playing
in a cavity on the under-surface of the front wing. This character is of
great interest in connexion with analogous structures of a more perfect
nature existing in various moths. M‘Lachlan has described and figured[398]
a more primitive, though analogous, condition of the wings in _Megalomus
hirtus_, also a species of British Hemerobiina. Another very curious
feature of _D. phalaenoides_ is shown in Fig. 312, B, there being a narrow
space on the hind part of the front wing from which the colour is absent,
while the nervures appear to be interrupted; they are, however, really
present, though transparent; the nature of this peculiar mark is quite
unknown, but is of considerable interest in connexion with the small
transparent spaces that exist on the wings of some butterflies.
{469}SUB-FAM. 6. CHRYSOPIDES, LACEWING-FLIES.—_Fragile Insects with
elongate, setaceous antennae._
[Illustration: FIG. 313.—_Chrysopa flava._ Cambridge.]
[Illustration: FIG. 314.—Eggs of _Chrysopa_. A, Five eggs on a leaf; B, one
egg, more magnified. (After Schneider.)]
[Illustration: FIG. 315.—Larva of _Chrysopa_ sp. Cambridge. A, The Insect
magnified; B, foot more magnified; C, terminal apparatus of the claws,
highly magnified.]
The lacewing-flies—also called stink-flies and golden-eyes—are excessively
delicate Insects, of which we possess about 15 species in Britain. Their
antennae are more slender and less distinctly jointed than they are in
Hemerobiides, and the Chrysopides are more elongate Insects. The peculiar
metallic colour of their eyes is frequently very conspicuous, the eyes
looking, indeed, as if they were composed of shining metal; this fades very
much after death. Although not very frequently noticed, the Chrysopides are
really common Insects, and are of considerable importance owing to their
keeping "greenfly" in check.
{470}[Illustration: FIG. 316.—_Chrysopa_ (_Hypochrysa_) _pallida_, larva.
(After Brauer.)]
The eggs are very remarkable objects (Fig. 314), each one being supported
at the top of a stalk many times as long as itself; in some species (_C.
aspersa_) the eggs are laid in groups, those of each group being supported
on a common stalk. The larvae (Fig. 315) are of a very voracious
disposition, and destroy large quantities of plant-lice by piercing them
with sucking-spears, the bodies of the victims being afterwards quickly
exhausted of their contents by the action of the apparatus connected with
the spears. The larvae of two or three species of _Chrysopa_ cover
themselves with the skins of their victims after the manner of the larvae
of _Hemerobius_; but most of the larvae of _Chrysopa_ are unclothed, and
hunt their victims after the fashion of the larvae of Coccinellidae, to
which these _Chrysopa_ larvae bear a considerable general resemblance.
These larvae have a remarkable structure at the extremity of their feet,
but its use is quite unknown (Fig. 315, B, C). Some larvae of the genus
make use of various substances as a means of disguise or protection. Dewitz
noticed[399] that some specimens he denuded of their clothing and placed in
a glass, seized small pieces of paper with their mandibles and, bending the
head, placed the morsels on their backs; here the pieces remained in
consequence of the existence of hooked hairs on the skin. Green algae or
cryptogams are much used for clothing, and Dewitz supposes that the Insect
spins them together with webs to facilitate their retention. According to
Constant and Lucas[400] the larvae of _Chrysopa_ attack and kill the larvae
{471}of Lepidoptera and Phytophagous Hymenoptera. The curious form we
figure (Fig. 316) has been hatched from eggs found by Brauer on _Pinus
abies_ in Austria. The eggs were of the stalked kind we have described; the
young escaped from them in the autumn, twelve days after deposition, but
did not take any food till the following spring.
The Chrysopides are widely distributed over the earth's surface. They form
an important part of the fauna of the Hawaiian islands.
SUB-FAM. 7. CONIOPTERYGIDES.—_Minute Insects with very few transverse
nervules in the wings; having the body and wings covered by a powdery
efflorescence._
These little Insects are the smallest of the Order Neuroptera, and have the
appearance of winged Coccidae; their claim to be considered members of the
Neuroptera was formerly doubted, but their natural history is quite
concordant with that of the Hemerobiid groups, near which they are now
always placed. Löw has made us acquainted with the habits and structure of
an Austrian species, _Coniopteryx lutea_ Wallg., but for which he has
proposed the new generic name _Aleuropteryx_; the larvae are found on
_Pinus mughus_ at Vienna feeding on _Aspidiotus abietis_, which they pierce
with sucking-spears, after the fashion of the Hemerobiides; when full fed
they spin a cocoon formed of a double layer of silk, in which metamorphosis
takes place in a manner similar to that of other Hemerobiidae. The
better-known genus _Coniopteryx_ differs from _Aleuropteryx_ in having the
sucking-spears short and nearly concealed by the front of the head, which
is somewhat prolonged.
[Illustration: FIG. 317.—_Coniopteryx psociformis._ Cambridge. (After
Curtis.) A, The insect with wings expanded, magnified; B, with wings
closed, natural size.]
{472}We may conclude this sketch of the Hemerobiid groups by remarking that
fossil remains of specimens of most of them have been detected in the
Tertiary strata, and that in the Secondary strata these groups are
represented by only a small number of fossils, which are referred specially
to Hemerobiina, Nymphidina, and Chrysopides.
[Illustration: FIG. 318.—A, Larva of _Coniopteryx tineiformis_ (?). (After
Curtis.) B, Head and prothorax of larva of _Coniopteryx_ sp.; C, upper
surface of head of larva of _Coniopteryx_ (after Löw), much magnified.]
{473}CHAPTER XXI
NEUROPTERA _CONTINUED_—TRICHOPTERA, THE PHRYGANEIDAE OR CADDIS-FLIES
FAM. XI. PHRYGANEIDAE—CADDIS-FLIES.
(TRICHOPTERA OF MANY AUTHORS)
_Wings more or less clothed with hair, nervures dividing at very acute
angles, very few transverse nervules; hind pair larger than the front,
with an anal area which is frequently large and in repose plicately
folded. Antennae thread-like, porrect, of many indistinct joints.
Mandibles absent or obsolete. Coxae elongate and free but contiguous.
Metamorphosis great; larvae caterpillar-like, usually inhabiting cases of
their own construction. Pupa resembling the perfect Insect in general
form, becoming active previous to the last ecdysis. Wingless forms of the
imago excessively rare._
[Illustration: FIG. 319.—_Halesus guttatipennis._ Britain. (After
M‘Lachlan.)]
The caddis-flies are Insects of moth-like appearance, found in the
neighbourhood of water; their larvae live in this element, where they may
sometimes be found in abundance. Phryganeidae are not very attractive
Insects, and there are few of large size; Hence they have been much
neglected by entomologists, and very little is known about the exotic forms
of the family. The habitations constructed by the larvae are, many of them,
of a {474}curious nature, and usually attract more attention than do the
creatures they serve to protect.
The Phryganeidae form the division or series Trichoptera; the two terms are
therefore synonymous; those entomologists who consider these Insects to
form a distinct Order use the latter appellation for it.
[Illustration: FIG. 320.—_Hydroptila angustella_ ♀. Britain. (After
M‘Lachlan.)]
The perfect Insect, though the wings are usually ample, has but feeble
powers of flight, and rarely ventures far from the water it was reared in;
it has a moth-like appearance, and the wings in repose meet, at an angle,
in a roof-like manner over the back (Fig. 326, E). The head is small, with
the front inflexed; it has two large compound eyes, and usually three
ocelli; the antennae are slender, thread-like, and occasionally attain a
great length. The parts of the mouth are very peculiar, the labrum and the
palpi—especially the maxillary palps—being well developed, while the lobes
of the maxillae and labium are amalgamated and therefore indistinct. The
labrum is more or less elongate, and is more mobile than is usual in
mandibulate Insects; it is held closely applied to the maxillae. These
latter are small, have usually only a single small free lobe; they are
united to one another and to the labium by membrane in such a manner as to
form a channel along the middle of the mouth, the labrum forming the roof
of this channel. The palpi are in some cases (Sericostomatides) of a
remarkable nature; their joints vary in number from three to five, and
differ sometimes in the sexes of the same species. The lower lip appears as
a plate supporting the labial palpi, which are three-jointed and do not
exhibit any peculiarities of structure comparable with those we have
mentioned as so frequently existing in the maxillary palps. Difference of
opinion exists as to the mandibles, some entomologists declaring them to be
entirely absent, while others state that a small tubercular process that
may be seen in some species on each side of the labrum is their
representative. The prothorax is very small, the notum is the largest piece
but is quite short, the side-pieces are very small, and the sternum appears
to consist only of membrane. The mesothorax is much the largest segment of
the body; its sternum {475}is large, but is nearly perpendicular in
direction, and is much concealed by the elongate, free front coxae, which
repose against it. The metathorax is intermediate in size between the pro-
and meso-thorax; its side-pieces are rather large, but the sternum is
membranous, with a heart-shaped piece of more chitinous consistence in the
middle, entirely covered by the middle coxae. The side-pieces both of the
meso- and meta-thorax are large, and are closely connected; the middle and
posterior coxae are very large, elongate, and prominent, and the middle
pair slope backwards, so that their tips are in contact with the tips of
the hind pair. The abdomen is cylindric and rather slender; it looks as if
formed of eight segments in addition to the terminal segment; this latter
in the male usually bears remarkably modified appendages. The first ventral
plate is sometimes, if not always, entirely membranous; indeed the texture
of the segments is in general very delicate, so that they shrivel up to an
extent that renders their comprehension from dried specimens very
difficult. The legs are always elongate, the coxae attaining in some forms
a remarkable length, and the tibiae and tarsi are armed with many spines;
the tarsi are five-jointed, slender, frequently very elongate, terminated
by two large claws and an apparatus, placed between them, consisting of a
pair of hair-like processes with a membranous lobe.
[Illustration: FIG. 321.—Front view of head of _Anabolia furcata_ after
removal of labrum. _o_, Ocellus; _an_, base of antenna; _au_, eye; _cm_,
cardo; _st_, stipes; _l_, external lobe; _pt_, support of palpus; _pm_,
palpus of maxilla; _g_, condyle of articulation of the absent mandible;
_ha_, channel of haustellum; _h_, haustellum; _sp_, apex of channel of
haustellum (not explained by Lucas); _ch_, chitinous point of external lobe
of second maxilla; _pl_, labial palp. (After Lucas.)]
The structure of the mouth-parts of the Phryganeidae has given rise to much
difference of interpretation; it has recently been investigated by R.
Lucas[401] in connexion with _Anabolia furcata_ (Fig. 321). He agrees with
other observers that mandibles are present in the pupa, but states that no
rudiment of them exists in the imago. He calls the peculiar structure
formed by the combination of the maxillae and {476}labium a haustellum. He
looks on the Trichoptera as possessing a mouth intermediate between the
biting and sucking types of Insect-mouths. He considers that the
Phryganeidae take food of a solid, as well as of a liquid, nature by means
of the haustellum, but the solid matter must be in the form of small
particles, and then is probably sucked up by the help of saliva added to
it. Lucas says also that in the larvae certain parts of the salivary glands
serve the function of spinning organs, and it is from these that the
salivary glands of the imago are formed; those salivary glands of the larva
that are not spinning glands disappearing entirely.
[Illustration: FIG. 322.—_Anabolia nervosa._ A, Larva extracted from its
case; B, one of the dorsal spaces of the abdominal segments more strongly
magnified.]
The eggs are deposited in a singular manner; they are extruded in a mass
surrounded by jelly; there may be as many as one hundred eggs in such a
mass. This is sometimes carried about by the female after its extrusion
from the interior of the body, but is finally confided to a suitable place
in stream, spring, or pool. It is said that the female occasionally
descends into the water to affix the egg-mass to some object therein, but
this requires confirmation, and it is more probable that the egg-mass is
merely dropped in a suitable situation. As soon as the larvae are hatched
they begin to provide themselves with cases; they select small pieces of
such material as may be at hand in the water, and connect them together by
means of silk spun from the mouth. Particulars as to these tubes we will
defer till we have considered the larvae themselves. These have the general
appearance of caterpillars of moths; in order to move about they must put
their head and the three pairs of legs at the front of the body out of
their tube or case, and they then look very like case-bearing caterpillars.
The part of the body that usually remains under cover is different in
texture and colour, and frequently bears outstanding processes, or
filaments, containing tracheae for the purpose of extracting air from the
water. Some peculiar spaces of a {477}different texture may be seen on
certain larvae (Fig. 322, B); these may possibly be also connected with
respiration. On each side of the extremity of the body there is a rather
large hook by which the creature attaches its dwelling to its body, and
there are also frequently present three large bosses on the anterior
abdominal segment, which are supposed to assist towards the same end. The
hold it thus obtains is so firm that it cannot be dragged out by pulling
from the front; fishermen have, however, discovered a way of extracting it
by a strategic operation: the cases are, as a rule, partially open behind,
and by putting a blunt object in and annoying the larva it is induced to
relax the hold of its hooks and advance forwards in the case, or even to
leave it altogether. The firm hold of the larva is maintained in spite of
the fact that the body does not fill the case. It is necessary that water
should pass freely into and out of the case, and that there should be some
space for the respiratory filaments to move in. The mouth of the case is
open, and the posterior extremity is arranged by the larva in such manner
as to allow a passage for the water; various ingenious devices are adopted
by different species of larvae with the object of protecting the hind end
of the body, and at the same time of permitting water to pass through the
case.
The mode of changing the skin, or the frequency with which this occurs in
the larval state of the caddis flies has not been recorded. The duration of
life in this stage is usually considerable, extending over several months:
indeed in our climate many species pass the winter in this stage,
completing the metamorphosis in the following spring or summer; and as one
generation each year appears to be the rule, it may be assumed that the
larval condition in such cases lasts from seven to ten months. During this
stage the Insects are chiefly vegetable feeders, some being said to feed on
minute algae; animal diet is not, however, entirely avoided, and it is said
by Pictet that not only do some of the Phryganeidae eat other Insects, but
that they also sometimes devour their companions.
[Illustration: FIG. 323.—A, Pupa of _Phryganea pilosa_. (After Pictet.) B,
Mandibles of pupa of _Molanna angustata_.]
{478}At the end of the larval period of existence the creature closes its
case by a light web spun at each end, taking care not to prevent the
ingress and egress of the water; it sometimes adds a stone or piece of
stick, and having thus protected itself, changes to a nymph. During the
first part of this metamorphosis the creature is completely helpless, for
there is so great a difference between the external structures of the larva
and nymph as to make the latter a new being, so far as these organs are
concerned. The changes take place in the interior of the larval skin, and
as they are completed this latter is shed piecemeal. The resulting pupa or
nymph greatly resembles the perfect Insect, differing consequently very
much from the larva. Pictet, who paid special attention to the nymph
condition of these Insects, concludes, however, that many of the organs of
the nymph are actually formed within the corresponding parts of the larva,
and has given a figure that, if trustworthy, shows that the legs of the
nymph, notwithstanding the great difference between them as they exist in
the larva and in the perfect Insect, are actually formed within the legs of
the larva; each nymphal leg being rolled up in the skin of the
corresponding larval leg, in a spiral, compressed manner, and the only
articulations that can be detected in the leg being those of the tarsus.
The head of the nymph is armed in front with two curious projections that
are, in fact, enormously developed mandibles (Fig. 323, B); they serve as
cutting implements to enable the nymph to effect its escape from its
prison; they are cast off with the nymph-skin, the perfect Insect being
thus destitute of these organs. The abdomen of the nymph differs from that
of the perfect Insect in possessing external respiratory filaments; the
nymphs of some species have also the middle legs provided with
swimming-hairs, that do not exist in the imago.
As a rule the larvae bring the respiratory filaments into contact with the
water by moving the abdomen, but Fritz Müller found[402] that those of a
_Macronema_ move the gills themselves—after the manner of Ephemeridae—with
much rapidity. Many kinds of larvae of Phryganeids possess at the posterior
extremity of the body exsertile pouches in the form of finger-like, or even
branched, processes into which tracheae do not enter. Müller observed that
in the _Macronema_ alluded to these pouches were generally not
{479}exserted; when, however, the larva ceased to move the tracheal gills,
then these pouches were protruded. He is inclined to consider them
blood-gills. Similar structures are found in _Eristalis_ and some other
Dipterous larvae that have to breathe under difficulties.
The imagines of certain species possess filaments—or something of the
sort—on the abdomen. Palmén, who has examined these organs in
_Hydropsyche_, thinks that they are the remains of gills that existed in
the larva and pupa, and that they are functionless in the imaginal instar.
M‘Lachlan thinks that in _Diplectrona_, where the filaments are elongate,
they may be functionally active even in the imago.[403]
The skin of the nymph is at first very soft, but it soon hardens, and when
about fifteen or twenty days have elapsed the nymph opens its case by means
of the mandibular processes, and swims through the water with its back
downwards till it reaches some solid object by which it can ascend to the
air; the nymph skin then swells and splits, and the thorax of the imago
protrudes; this is soon followed by the disengagement of the head and other
parts, and the imago having thus escaped, the nymph skin remains a complete
model of the external structure of the nymph, and contains a considerable
number of tracheae. This sketch of the metamorphosis of a caddis-fly does
not apply in all its details to all the forms of caddis-flies, there being
exceptions, as we shall mention hereafter.
Dewitz has described[404] the first appearance and development of the wings
in larvae of Phryganeidae. Each one appears at first in the form of a small
thickening of the hypodermis, accompanied outwardly by a minute depression
of the chitin (Fig. 324, A). He compares the structure in the earliest
stage to the entothoracic projections into the interior of the body. The
rudiment grows as the larva increases in size, the chitinous portion being
duly shed at the ecdyses. When the rudiment is larger and more complex, a
mesoderm layer appears in it (Fig. 324, B); this is derived from a
nerve-sheath near the rudiment. During the resting state of the larva—after
its case has been closed, but before the pupal form has appeared—the wing
assumes the form and position shown in C, Fig. 324. Dewitz's description of
the process leaves much to be desired, and it is doubtful whether in {480}C
the position of the wing on the exterior of the body is due to the
stripping off of the chitinous integument, or to a process of eversion, or
to both.
[Illustration: FIG. 324.—Development of wings of Phryganeidae. (After
Dewitz.) A, Portion of body-wall of young larva of _Trichostegia_; _ch_,
chitin, forming at _r_ a projection into the hypodermis _m_; _r_ and _d_
forming thus the first rudiment of the wing. B, The parts in a largely
grown larva; _a_, _c_, _d_, _b_, the much grown hypodermis separated into
two parts by _r_, the penetrating extension of the chitin; _v_, mesoderm.
C, Wing-pad of another Phryganeid freed from its case at its change to the
pupa; _b_, _d_, outer layer of the hypodermis, _m_, of the body-wall; _v_,
inner layer without nuclei.]
[Illustration: FIG. 325.—Cases of British Trichoptera. A, Of _Odontocerum
albicorne_; A^1, its termination; B, quadrangular case of _Crunoecia
irrorata_; B^1, mouth of case.]
There are about 500 species of this family of Insects known as inhabiting
the European region, and about 150 of this number occur in Britain. These
are arranged by M‘Lachlan[405]—whose zealous and persevering work at this
neglected family of Insects is beyond praise—in eight sub-families, on a
system in which the structure of the maxillary palpi plays a principal
part; they are called Phryganeides, Limnophilides, Sericostomatides,
Leptocerides, Oestropsides, Hydropsychides, Rhyacophilides, Hydroptilides.
The first three of these form the division "Inaequipalpia."
PHRYGANEIDES.—This group includes the largest forms of the family, and
appears to be almost confined to the temperate regions of the northern
hemisphere, though a few species are already known from the corresponding
districts of the southern hemisphere. This feature in their geographical
{481}distribution is, however, by no means peculiar to them, for a similar
discontinuity of distribution exists in numerous other groups of Insects,
and even in other divisions of the Phryganeidae.
The Phryganeides almost without exception inhabit still waters, and it is
more specially to them that the brief sketch of metamorphosis given in the
preceding pages will be found to apply. The larva always has the
respiratory filaments simple and thread-like, though elongate, and lives in
a case that it carries about; this case is open at both ends, and the larva
is said to occasionally cut off the end having the least diameter and
increase the other end, thus accommodating the habitation to its own
growth.
LIMNOPHILIDES.—These Insects have only three, instead of four, joints in
the maxillary palpi, but in most other respects agree with the
Phryganeides. There is, however, greater variety in the habits of the
larvae, though all live in free cases. In the genus _Enoicyla_ (Fig. 326)
we meet with the anomaly of a Trichopterous Insect that lives amongst moss
and dead leaves, far away, it may be, from water. The cases of the
Limnophilides are constructed of a great variety of materials, and are
often decorated with shells containing living inmates.
In the genus _Apatania_ the phenomenon of parthenogenesis is thought to
occur, there being at least two species in which no male specimen has ever
been discovered, though M‘Lachlan has made special efforts to discover the
sex of _A. muliebris_. It should, however, be stated that these species
have not been extensively investigated; _A. arctica_ has been detected in
the Arctic regions, and _A. muliebris_ has occurred in several localities
in Europe, in Britain chiefly near Arundel in a lake of intensely cold
water.
[Illustration: FIG. 326.—Metamorphoses of _Enoicyla pusilla_. (After
Ritsema.) A, Case of full-grown larva; B, larva and case magnified; C,
larva extracted; D, wingless adult female; E, male.]
{482}SERICOSTOMATIDES, like the Limnophilides, is a group rich in species;
the larvae are chiefly found in streams. They form portable cases out of
sand and stones (Fig. 325, B, case of _Crunoecia irrorata_) in preference
to vegetable matter. It is here that the genus _Helicopsyche_, which for
long was an enigma to naturalists, is now placed. This genus consists of
Insects whose larvae form spiral cases, similar to small snail shells, of
sand or minute stones. These objects occur in various parts of the world.
Fritz Müller[406] has informed us that the larva inhabiting one of them,
when it withdraws entirely within its abode to repose, takes the precaution
of anchoring its snail-like habitation, fixing it to a rock or stone by
spinning some temporary silken threads. The respiratory filaments in this
group are filiform.
[Illustration: FIG. 327.—Cases of _Helicopsyche shuttleworthi_. (After von
Siebold.) A, Natural size; B, C, magnified.]
LEPTOCERIDES.—The first group of the division Aequipalpia; so that there
are five-jointed maxillary palpi in both sexes; these organs are frequently
developed in a remarkable manner. The antennae are usually extremely long
and slender. The case of the larva is portable (Fig. 325, A, case of
_Odontocerum_); the respiratory filaments are not very conspicuous; they
form short tufts placed on various parts of the abdomen. Müller[406] has
called attention to a species whose larva lives in Brazil between the
leaves of Bromeliae on trees.
The OESTROPSIDES is a small group, and has recently been reduced by
M‘Lachlan to the rank of an inferior division.
HYDROPSYCHIDES.—An extensive group, in which the larvae are believed to be
chiefly of carnivorous habits. They vary, according to species, as to the
nature of the respiratory filaments, and live in fixed abodes; these are
less tubular than is the rule with the portable cases, and are formed from
pieces of sand and stone spun together and fixed to larger stones under
water. Sometimes several larvae live together in loosely compacted
structures of this kind, and only form true cases when about to undergo
their metamorphosis. Müller describes[406] a Brazilian species of
_Rhyacophylax_ as forming a case in which the mouth-end has a {483}large
funnel-shaped verandah, covered by a beautiful silken net. This larva lives
in the rapids of various rivulets, and the entrance to the verandah is
invariably directed towards the upper part of the rivulet, so as to
intercept any edible material brought down by the water. Several of these
larvae, moreover, build their cases so that they form a transverse row on
the upper side of a stone; as many as thirty cases may be placed in one of
these rows, and sometimes several rows are placed parallel with one
another. This same larva has the habit of coming out of its case when
necessary, and suspending itself in the water—as some caterpillars do in
the air—by means of a silken thread. Other members of the Hydropsychides
form tubes or covered ways of silk, earth and mud attached to stones, and
in which they can move freely about. Some of the Hydropsychidae have been
ascertained with certainty to be carnivorous in the larval state. A species
of the genus _Hydropsyche_ has been found by Howard[407] to help itself in
the task of procuring food by spreading a net in the water in connexion
with the mouth of its case. This net is woven in wide meshes with extremely
strong silk, and supported at the sides and top by bits of twigs and small
portions of the stems of water-plants. Small larvae brought down by the
current are arrested by this net for the advantage of the larva that lurks
in the tube. The breathing organs of the larvae of Hydropsychides are
apparently of a varied character, and would well repay a careful study. Mr.
Morton informs the writer that some of our British species of
_Philopotamus_ and _Tinodes_ have no gills either in the larval or pupal
state, and probably respire by means of modified tracts in the integument.
In some of the allied genera, _e.g._ _Polycentropus_, the larvae are
destitute of gills, but the pupae possess them.
[Illustration: FIG. 328.—Case, with head of larva and snare of North
American Hydropsychid. (After Riley and Howard.)]
The RHYACOPHILIDES is another group in which the larval habitations are
fixed. Some of these larvae have no respiratory filaments, breathing only
by means of the stigmata, but others have tufts of filaments. These Insects
have a peculiarity in their {484}metamorphosis inasmuch as the larva,
instead of lying free, constructs a cocoon in its case or other habitation
in which to change to a nymph. In the larvae that do not make use of a
portable case the abdominal hooks are not essential, and are replaced by
other organs differing much in structure, being sometimes apparently of a
sensitive nature, in other forms possibly respiratory. Müller tells us of a
carnivorous larva of this group in which the anterior legs are armed with
powerful forceps for predatory purposes.
The HYDROPTILIDES comprise the most minute of the Phryganeidae, and their
species will probably prove to be very numerous in well-watered tropical
regions, though few have yet been described from there. The perfect Insects
(Fig. 320) bear an extreme resemblance to small moths of the group
Tineidae. The larvae (Fig. 329) are destitute of respiratory filaments, and
construct portable cases of a variety of forms, some resembling seeds.
Müller has given particulars of a curious nature as to the cases of some
Brazilian Hydroptilides; one species moors its dwelling to a stone by means
of a long silken cable, by this artifice combining safety with the power of
ranging over a considerable extent of water. In _Diaulus_ there is only a
narrow slit at each end of the case, but one side of it is provided with
two chimneys to permit the flow of water for respiratory purposes.
[Illustration: FIG. 329.—_Hydroptila maclachlani._ B, Case with larva
magnified; A, larva more magnified. (After Klapálek.)]
The larva of _Oxyethira_ (Fig. 330) is a curious form, possessing
comparatively long legs, and a head and thorax slender in comparison with
the distended hind body. The cases are fastened, for the purposes of
pupation, to a leaf of a water-lily.
Some very curious anomalies as regards the development of the wings exist
in the Phryganeidae; _Anomalopteryx_, for instance, has the wings quite
short and useless for flight in the male, while in the other sex they are
ample; in _Enoicyla_—the curious Insect figured on p. 481, in which the
larvae are of terrestrial habits—we find the females with only rudiments
{485}of wings, while in _Thamastes_ the posterior wings are absent in both
sexes. These anomalies are at present quite inexplicable; and we may here
mention that we are in complete ignorance as to the functional importance
of many of the peculiarities of the Phryganeidae. We do not know why the
mouth is reduced from the normal state, the maxillary palpi being, on the
other hand, extraordinarily developed; we do not know the importance of the
numerous spines and of the spurs on the legs, nor of the hairs on the
wings, although these are amongst the most characteristic of the special
features of this group of Insects.
[Illustration: FIG. 330.—_Oxyethira costalis._ A, Larva in case; B, cases
fastened to leaf for pupation. (After Klapálek.)]
FOSSILS.—Abundant remains of Phryganeidae belonging to the Tertiary epoch
have been discovered. They are common in amber, and it is a remarkable fact
that a larval case has been found in amber. This seems almost inexplicable,
except on the assumption that such larvae were of arboreal habits, a
condition that, at the present time, must be excessively rare, though the
terrestrial habits of _Enoicyla_ warrant us in believing it may occur. In
the Tertiary Lake Basin at Colorado the remains of Phryganeidae in the
imago state are extremely abundant, so that it is curious that but few such
remains have been found in Europe. In Auvergne the so-called indusial
limestone, which is two or three yards thick over a wide area, is
considered to be composed chiefly of the cases of larvae of this family.
In the Mesozoic epoch some wings found in the lower Purbeck strata are
considered to be those of Phryganeidae; similar wings have been found in
the Lias, but this is the only evidence of the existence of the family at
that period except a tube, supposed to be a larval case, detected in the
Cretaceous of Bohemia. Earlier than this nothing has been discovered that
can be connected with the family, so that at present the palaeontological
evidence appears unfavourable to the view held by some that the
Phryganeidae may be considered forms allied to the early {486}conditions of
the Lepidoptera. It should be noted that the head in Phryganeidae is the
most important part from a systematic point of view, and that fossils have
been chiefly identified from the wings; this is a much more doubtful
character, as the wings of the Phryganeidae have a simple system of
neuration, and in shape have nothing very characteristic.
EXTINCT ORDER PALAEODICTYOPTERA.
This seems to be the fittest place to notice the existence of some fossil
remains from the Palaeozoic rocks that cannot be fitly, or certainly,
assigned to any of our existing Orders, and to which the above name has
consequently been given. These remains consist chiefly of wings in a more
or less imperfect state of preservation, and it is therefore quite doubtful
whether the course of assigning them to a separate Order supposed to be
extinct be correct. This is all the more doubtful when we recollect that an
Insect fossil, _Eugereon bockingi_, having the head with mouth-parts of a
Hemipterous or Dipterous nature, has been found, the wings attached to it
being such as, had they been found separate, would have been considered to
be Neuropterous, or at any rate allied thereto. About forty-two forms of
Palaeodictyoptera are assigned by Scudder to a section called
Neuropteroidea, and may therefore be considered to have a special
resemblance to our Neuroptera. These Neuropteroidea comprise numerous
genera and no less than six families. Scudder's view is at the best
tentative, and is not very favourably received by some entomologists.
Brauer has, indeed, objected altogether to the formation of this Order
Palaeodictyoptera, and Brongniart has published a list of the Palaeozoic
Insects in which a system of arrangement different to that of Scudder is
adopted. In his most recent work[408] Brongniart assigns some of these
Neuropteroidea to the families Platypterides and Protodonates, which we
have previously discussed. The whole subject of these Palaeozoic Insect
remains is still in its infancy, and it would not be proper to accept any
view as final that has yet been stated, nor would it be fair to dismiss the
subject as unimportant because of the great divergence of opinion amongst
the authorities who have investigated it.
{487}CHAPTER XXII
HYMENOPTERA—HYMENOPTERA SESSILIVENTRES—CEPHIDAE—ORYSSIDAE—SIRICIDAE—
TENTHREDINIDAE OR SAWFLIES
ORDER IV. HYMENOPTERA.
_Wings four, membranous, without scales, usually transparent, never very
large, the posterior pair smaller than the anterior; the cells formed by
the nervures irregular in size and form, never very numerous (less than
twenty on the front, than fifteen on the hind, wing). Mandibles
conspicuous even when the other parts of the mouth form a proboscis. The
side-pieces of the prothorax are disconnected from the pronotum and
overlap the prosternum, usually entirely concealing it. The females are
furnished at the extremity of the body with either saw, sting, or
ovipositor; these parts may either be withdrawn into the body or be
permanently protruded. The metamorphosis is great and abrupt, the chief
changes being revealed in the pupa disclosed at the last moult of the
larva; this moult is frequently delayed till long after growth has been
completed. In the pupa the parts of the perfect Insect are seen nearly
free, each covered in a very delicate skin._
The term Hymenoptera includes ants, bees, wasps, sawflies, and the tribes
of innumerable Ichneumon-flies. The Order is of enormous extent, consisting
even at present of tens of thousands of described and named species, and
yet these are but few in comparison with those that remain unknown. It has
good claims to be considered the "highest" Order of Insects. Sir John
Lubbock says: "If we judge animals by their intelligence as evinced in
their actions, it is not the gorilla and the chimpanzee, but the bee, and
above all the ant, which approach nearest to {488}man."[409] The mechanical
perfection of the structures of the individuals, and the rapid and
efficient manner in which their functions are discharged, are very
remarkable. In many species of Hymenoptera the individuals have the habit
of living together in great societies, in which the efforts of the members
are combined for the support of the whole society and for the benefit of a
younger generation. To fit them for this social life the bodies of the
larger number of the individuals are more or less changed in structure, so
that they become workers. These workers are in all cases imperfect females;
besides carrying on the ordinary work of the society, they tend and feed
the young. The duty of reproduction is restricted to a single female,
called a queen, or to a small number of such individuals in each society.
The males occupy an unimportant position in the society, and are usually
much shorter-lived than the workers and queens. The social Hymenoptera do
not form a single zoological group, but are of three different kinds—wasps,
bees, and ants. There are numerous non-social, or solitary, wasps and bees.
[Illustration: FIG. 331.—_Bombus lucorum._ A, Adult larva; B, pupa; C,
imago, female. Britain.]
In the Order Hymenoptera—especially in the higher forms—the males and
females are often different in appearance and structure. In the ants, one
of the social groups, the workers, or imperfect females, are quite
wingless. There are numerous other groups in which species, not social, are
found, having the females wingless while the males have wings. In a few
species there is an apterous condition of the male, perhaps usually only as
a {489}dimorphic form. In the parasitic division there are species that are
apterous in both sexes. The structure of the outer skeleton, or external
part of the body, exhibits some peculiarities, the chief of which is the
detachment of the side-pieces of the prothorax and their great development.
Not less remarkable is the abstraction of a segment from the abdomen to
become, as it were, part of the thorax; while between the first and second
true segments of the abdomen there exists a joint, or articulation, of the
utmost mechanical perfection, enabling the operations of stinging and
piercing to be executed with an accuracy that cannot be surpassed.
As a result of the detachment of the thoracic side-pieces, the front legs
and the structures connected with them are disjoined from the notum, as
shown in Fig. 332, and act in connexion with the head, while the dorsal
portion of the segment remains attached to the great thoracic mass. The
head is quite free from the thorax and very mobile; the upper organs of the
mouth—the labrum and the mandibles—are not subject to modifications equal
to those exhibited by the maxillae and lower lip, which parts in the bees
are prolonged to form a suctorial apparatus that may exceed in length the
whole body of the Insect. The mandibles remain cutting or crushing
implements even when the maxillae and lower lip are modified to form a
suctorial apparatus of the kind we have mentioned; so that in the higher
forms—ants, bees, and wasps—the mouth-pieces are completely differentiated
for two sets of functions, one industrial, the other nutritive.
[Illustration: FIG. 332.—_Tenthredo_, with head fully extended: _a_,
pleuron; _b_, pronotum; _c_, membrane; _d_, mesonotum.]
Behind the head there is a large consolidated mass representing the thorax
of other Insects, but made up, as we have already indicated, in an unusual
manner, and which therefore may be called by a special name, the alitrunk
(Fig. 333). The pronotum forms the anterior part of the alitrunk, with
which it is usually very closely connected, being indeed frequently
immovably soldered {490}thereto. It exhibits, however, considerable
variety, and is seen in its simplest and least soldered state in _Cephus_.
In the higher bees the pronotum takes on a form not seen in any other
Insects, being one of the most beautiful sclerites to be found in the class
(Fig. 334, pronotum of _Xylocopa_). We have already remarked that in
Hymenoptera the lower portions of the prothoracic segment are detached from
the upper, so that the pronotum is not supported beneath by a sternum as
usual. In the bees in question the pronotum makes up for the removal of the
corresponding side-pieces and sternum, by becoming itself a complete ring,
its sides being prolonged and meeting in the middle line of the under
surface of the body. At the same time a large lobe is developed laterally
on each side, overlying and protecting the first breathing orifice. The
intermediate stages of this remarkable modification may be observed by
dissecting a small series of genera of bees.
[Illustration: FIG. 333.—Alitrunk of _Sphex chrysis_. A, Dorsal aspect:
_a_, pronotum; _b_, mesonotum; _c_, tegula; _d_, base of anterior, _e_, of
posterior, wing; _f_, _g_, divisions of metanotum; _h_, median (true first
abdominal) segment; _i_, its spiracle; _k_, second abdominal segment,
usually called the petiole or first abdominal segment. B, Posterior aspect
of the median segment: _a_, upper part; _b_, superior, _c_, inferior
abdominal foramen; _d_, ventral plate of median segment; _e_, coxa.]
[Illustration: FIG. 334.—Pronotum of a carpenter bee, _Xylocopa_ sp. East
India.]
Although the prosternum of a Hymenopterous Insect is not usually visible
owing to its being overwrapped by the side-pieces, it is really, as shown
in Fig. 335, B, of complicated form. In _Cimbex_ and some other sawflies
the side-pieces are not so large as usual, but the prosternum is larger and
is exposed. The prothoracic spiracle is rarely visible externally, but its
position is remarkably constant, and is usually indicated by a peculiar
lobe or angle of the pronotum projecting backwards just below {491}the
insertion of the front wing. This stigmatic lobe is frequently fringed with
short hairs.
The mesothorax is the largest of the three divisions of the thorax proper;
its notum is large, and usually divided into two parts by a transverse
suture. The side-pieces are so placed that the epimeron is rather behind
than below the episternum. The mesosternum forms the larger part of the
under-surface of the alitrunk. A very large phragma projects from the
mesothorax into the interior of the body. The mesothoracic spiracle is
usually not visible; its existence was unknown to the older entomotomists,
who were in consequence led to consider the spiracle of the median segment
as belonging to the thorax. The mesothoracic spiracle is, however, easily
seen in _Cimbex_, placed in the suture between the mesothoracic epimeron
and the metathoracic episternum, a little below the insertion of the front
wing; close to this spot the mesophragma, just spoken of, comes, in
_Cimbex_, to the surface. The mesothoracic spiracle is generally
conspicuous in the worker ant. The parts of the metathorax are usually
small, but so much variety prevails in this respect that no general
description can be given.
[Illustration: FIG. 335.—Articulation of front legs of the hornet (_Vespa
crabro_, ♀). A: _a_, side-piece of prothorax overlying the prosternum; _b_,
coxa; _c_, trochanter. B, prosternum proper, as seen from front when
extracted.]
The structure of the posterior part of the alitrunk has given rise to an
anatomical discussion that has extended over three-quarters of a
century,[410] with the result that it is now clear that the posterior part
of what appears to be thorax in Hymenoptera is composed of an abdominal
segment. This part has been called "Latreille's segment," the "median
segment," and the "propodeum." The latter term was proposed by Newman,
under the form of propodeon,[411] and appears to be on the whole the most
{492}suitable term for this part, which is of great importance in
systematic entomology, owing to the extreme variety of characters it
affords. Although it is clear that the propodeum is, in large part, an
abdominal segment, yet its morphology is still uncertain; what parts are
pleural, what tergal, and what may be chitinised spiracular area, or
portions of the metathorax, being undetermined. The ventral portion of the
propodeum affords a strong contrast to the dorsal part, being so small that
it has frequently been described as absent; it is, however, not difficult
to detect it in the position shown at _d_, Fig. 333, B.
Although the true first segment of the abdomen is detached from its normal
position and added to the thorax, yet the term abdomen is conventionally
restricted to the part that commences with the true second segment, which,
in counting the number of abdominal segments, is reckoned as being the
first. There are two modes of articulation of the Hymenopterous abdomen
with the alitrunk; in one (Fig. 336, A) the base of the abdomen remains of
the calibre usual in Insects, while in the other (Fig. 336, B) it is
greatly contracted, so that the two parts are connected only by a slender
stalk, the petiole. The petiole, besides articulating in a very perfect
manner with the propodeum by means of certain prominences and notches, is
also connected therewith by means of a slender ligament placed on its
dorsal aspect and called the funiculus, shown in Fig. 333, A, just at the
extremity of the pointing line _k_. This mode of articulation gives great
freedom of motion, so that in some Petiolata (_Ampulex_) the abdomen can be
doubled under the body and the sting brought to the head. It is worthy of
note that even in the Sessiliventres—as the sub-Order with broad-based
abdomen is called—some amount of movement exists at the corresponding spot;
while, as shown in Fig. 336, A, between _a_ and _b_, there exists an
exposed membrane, the homologue of the funiculus.
[Illustration: FIG. 336.—Articulation of abdomen and alitrunk of, A,
_Cimbex_, B, _Vespa_. _a_, Propodeum or median segment; _b_, dorsal plate
of first (second true) abdominal segment or petiole; _c_, spiracle of the
propodeum; _d_, hind coxa; _e_, ventral plate of first (second true)
abdominal segment.]
The number of abdominal segments that can be seen in the {493}perfect
Insect varies greatly. In Tenthredinidae nine can be distinguished, while
in some of the Chrysididae it is difficult to detect more than three behind
the petiole. These distinctions are, however, superficial or secondary,
being due to changes in the later life in connexion with the stings or
borers; in the larvae that have been examined thirteen segments behind the
head have usually been detected.
Nothing is more remarkable in the Hymenoptera than the great differences
that exist in the form of the petiole. This may be very short, as in the
bees, so that the abdomen when not deflexed does not appear to be separated
from the thorax (Fig. 331, C); in this condition it is said to be sessile,
a term which it would be well to replace by that of pseudosessile. In many
of the solitary wasps the petiole is very long. In ants it is replaced by
one or two curiously-shaped small segments called nodes (Fig. 60, B, 2, 3),
and in many ants these are provided with structures for the production of
sound. The abdomen is formed by a system of double imbrications; each
dorsal plate overlaps each ventral plate, and the hind margin of each
segment embraces the front part of the one following; thus this part of the
body has not only great mobility, but is also capable of much distension
and extension. The pleura are apparently absent, but each one has really
become a part of the dorsal plate of the segment to which it belongs. This
is shown to be the case by _Cimbex_, where the division between pleuron and
dorsal plate exists on each segment except the basal one. Owing to this
arrangement, the abdominal stigmata in Hymenoptera appear to be placed on
the dorsal plates.
The organs for mechanical purposes existing at the extremity of the body in
Hymenoptera exhibit a great diversity of form; they are saws, borers,
piercers, or stings. Notwithstanding their great differences they are all,
in their origin, essentially similar, and consist of six parts developed
from limb-like prolongations on the penultimate and antepenultimate
segments of the larva, as described by Packard and Dewitz.[412] These
processes have by some been thought to be not essentially different from
abdominal legs, and Cholodkovsky has recently advocated this opinion.[413]
{494}The legs of bees exhibit modifications for industrial purposes. In the
stinging Hymenoptera the trochanters are usually of a single piece, and
these Insects are called monotrochous; but in most of the other forms the
trochanters are more or less distinctly divided into two parts (Fig. 345,
_b_). The usual number of joints in the tarsus is five, but is subject to
diminution in many cases. In the bees and ants the first joint is altered
in form; in the bees to act as an instrument for gathering or carrying
pollen; in the ants to act, as it were, as a second tibia. Between the
claws there is a very perfect pad, already described and figured on p. 106.
The wings are remarkable for the beautiful manner in which the hinder one
is united to the anterior, so that the two act in flight as a single organ.
The hind wing is furnished with a series of hooks, and the hind margin of
the front wing is curled over so that the hooks catch on to it. In some of
the parasitic forms the wings are almost destitute of nervures, and have no
hooks. The powers of flight in these cases are probably but small, the
wings merely serving to float the Insect in the air. In some Hymenoptera,
especially in Pompilides and Xylocopa, the wings may be deeply pigmented or
even metallic; and in some forms of Tenthredinidae, Ichneumonidae, and
Braconidae the pigmentation assumes the form of definite patterns.