Chapter XX: Introduction: Habits–classification–structure–chilognatha–chilopoda (14)

{395}The association of ocelli with wings is nearly constant in Psocidae. The genus _Clothilla_—allied to _Atropos_—possesses very rudimentary wings but no ocelli. Hagen, however, found[315] that in a certain locality no less than 12 per cent of the individuals of this species were provided with ocelli,—a most extraordinary variation.

In some of these apterous forms there is found on each side of the prothorax a tubercular prominence which, according to Hagen, can be considered only as the rudiment of a wing that never develops. Though no existing Insect is known to possess rudimentary wings on the prothorax, we have previously mentioned (p. 344) that in the Carboniferous epoch appendages of the nature alluded to were not very rare.

A genus of living forms—_Hyperetes_—in which the three thoracic segments are well developed, but in which there are no alar appendages or rudiments, is considered by Hagen to be more primitive than the Psocidae found in amber to which we shall subsequently allude.

The number of described species of Psocidae does not reach two hundred; we have, however, thirty species or more in Britain.[316] Nietner observed about the same number in the immediate vicinity of his house in Ceylon. The isolated and remote Hawaiian group of islands is remarkably rich in Psocidae. Two thousand is a moderate estimate of the number of existing species. The largest forms yet discovered belong to the Brazilian genus _Thyrsophorus_; they attain, however, a breadth of only about one inch with the wings fully expanded. The Cuban genus _Embidopsocus_ is said to be of great interest from its approximation to Embiidae. It is at present very inadequately known.

One (or more) very minute Insects of this family—_Clothilla pulsatoria_ according to Hagen, _Atropos[317] divinatoria_ according to some other authors—is widely known under the name of the death-watch, owing to its being believed to make a peculiar {396}ticking noise, supposed to be prophetic of the decease of some individual—a human being we fancy, not a death-watch. It is difficult to believe that so minute and soft an Insect can produce a sound audible to human ears, and many entomologists are of opinion that the sound in question is really produced by a beetle—of the genus _Anobium_—which lives in wood, and that as the beetle may be concealed in a hole, while the _Clothilla_ is seen running about, the sound is naturally, though erroneously, attributed to the latter. But the rapping of the _Anobium_ is well known, is produced while the Insect is at large, and is said to be a different noise from that of the Psocid; evidence too has been given as to the production of the sound in a workbox when the Psocid was certainly present, and the most careful search failed to reveal any beetle.

The Rev. W. Derham, who two hundred years ago was Rector of Upminster, in Essex, and was well known as a distinguished writer and philosopher, gave an account of the ticking of death-watches to the Royal Society.[318] This gentleman was a most accurate and minute observer; he was well acquainted with the ticking of the greater death-watch—_Anobium_—which he describes very accurately, as well as the acts accompanying it, the details he mentions being exactly such as occur at the present time. He not only heard the ticking of the Psocid or lesser death-watch, but repeatedly witnessed it. He says: "I am now so used to, and skilful in the matter as to be able to see, and show them, beating almost when I please, by having a paper with some of them in it conveniently placed and imitating their pulsation, which they will readily answer." He also states that he could only hear them beating when it was done on paper, and that this death-watch will tick for some hours together without intermission, with intervals between each beat, so that it much resembles the ticking of a watch. The act of ticking was {397}accompanied by rapping the front of the head on the paper, but Mr. Derham could not be sure that the sound was produced in that manner, because each stroke was also accompanied by a peculiar shudder, or recoil. After a prolonged ticking he observed that another individual of the other sex made its appearance. The species figured by Mr. Derham more resembles a _Hyperetes_ than it does either of our two known book-lice, _Atropos_ and _Clothilla_.

Numerous species of Psocidae are preserved in amber; Hagen[319] has made a careful study, based on a considerable number of specimens, of about thirteen such species. They belong to no less than nine genera and five sub-families. _Sphaeropsocus_ is the most remarkable; this Insect has a well-developed prothorax, as is the case in the wingless Psocids, and a pair of large wings or tegmina meeting by a straight suture along the back, as is usual in beetles, though quite unknown in existing Psocidae. Another species, _Amphientomum paradoxum_, has the body and appendages covered with scales like a butterfly or moth; other species, found in gum-copal or still living, have scales on various parts of the body, but not to so great an extent as this amber species. The genus _Amphientomum_ is still represented in Ceylon and elsewhere by living forms; Packard has figured some of the scales;[320] they appear to be extremely similar to those of Lepidoptera or Thysanura. The facts connected with this fauna of amber Psocidae would seem to show that the family was formerly more extensive and important than it is at present; we should therefore expect to find numerous fossil forms in strata of date {398}anterior to that of the amber; but this is not the case, all that is known as to fossil Psocidae being that Scudder has recently ascribed traces of an Insect found in the Tertiary rocks of Utah to this family as a distinct genus.

FAM. V. PERLIDAE.

_Insects of moderate or large size, furnished with four membranous wings;
these are usually complexly reticulate; the hind pair are much the
larger, and have a large anal area of more simple venation, which becomes
plicate when folded. The coxae are small, the legs widely separated. The
larvae are aquatic in habits; the metamorphosis is slight._

The Perlidae form a small family of Insects unattractive in their general appearance. The life-history of each individual consists of two abruptly contrasted portions; the earlier stage being entirely aquatic, the later aerial. Hence the Perlidae come into the amphibious division of Neuroptera. The definition we have given above would, except as regards the texture of the front wings and the aquatic habits of the larvae, apply to many Insects of the Order Orthoptera. The Phryganeidae, another {399}family of Neuroptera, have aquatic larvae and wings somewhat similar in form to those of the Perlidae, but the members of the two families cannot be confounded, as the Phryganeidae have hairy front wings and large and contiguous coxae.

The antennae of the Perlidae are long, very flexible, and composed of a very large number of joints. The parts of the mouth vary a good deal. The mandibles and maxillae are usually rather small, and all the parts of the mouth are of feeble consistence or even membranous; the maxillary palpi are, however, well developed and exserted from the mouth, five-jointed. The labium is short and but little conspicuous. The mandibles in some forms are almost membranous, but in other genera they are firmer and are toothed. The labium is composed of a very large mentum, beyond which is a large piece, usually undivided, bearing the four terminal lobes; the three-jointed palpus is seated on the side of the large middle sclerite, which is no doubt of composite nature. Considerable variety as to the lower lip prevails. The head is broad and flat; there is an indistinctly-indicated clypeus, three—more rarely two—ocelli, and on each side an eye neither very large nor perfect. The prothorax is free, and has a flat, margined notum. The meso- and the meta-thorax are large, equal segments. The pro-, meso-, and meta-sternum are large pieces; between the first and second, and between the second and third there is an intervening membrane. The metasternum is much prolonged backwards, and has on each side a peculiar slit; similar orifices exist on the other sterna (Fig. 254, _o_). Newport, who has examined them in _Pteronarcys_, says that they are blind invaginations of the integument; he calls them the sternal or furcal orifices.[321] According to this naturalist these very peculiar openings pass into the body "as strong bone-like tubes, diverging from the axis to the periphery of the body in the immediate vicinity of some of the principal tracheae, but that they do not in any way communicate with them, as they terminate abruptly as caecal structures." He thinks them analogous with the endo-skeleton of other Insects; a view which cannot be considered sufficiently established. Laboulbène states[322] that when _Perla parisina_ is seized and placed on its back, it does not move, but emits a liquid at the base of the articulation of the legs. {400}This suggests that it may come from these sternal orifices. The abdomen consists of ten dorsal plates, the first being short, and of nine ventral; the dorsal plates are much more ample transversely than the ventral. Frequently the hind body is terminated by two long, many-jointed cerci, looking like antennae. The coxae are small, not prominent, and are directed outwards. The legs are slender, the tibiae often grooved. The tarsi are three-jointed, terminating in two claws and a more or less distinct pad. In the genus _Isopteryx_ an auditory organ has been described as existing in the legs, in a position similar to that of the analogous structures in Termitidae and Blattidae. The wings when closed repose flat on the back, and fold and overlap so that only one is seen (Fig. 251); in this state the costal portion of each front wing is turned downwards, so as to protect to some extent, the sides of the body.

The early stages are known, but have not been described minutely, and there appears to be very little information as to the youngest life. All the species are, when immature, aquatic in their habits; the larvae greatly resemble the perfect Insects in form, though differing in not possessing wings and in the ocelli being merely opaque spaces. They have rather large compound eyes; the future wings are represented by lobe-like prolongations—varying in length according to age—of the meso- and meta-notum. In the Nemourae the cerci are absent in the imago though present in the young. The larvae of Perlidae are carnivorous {401}and are able to swim well, the legs being provided with abundant swimming hairs; they, however, as a rule, prefer to walk at the bottom of the pool, or on rocks or boulders in the water they live in.

One of the most peculiar features of the Perlidae is their respiratory system. Unfortunately the greatest differences of opinion have prevailed on various matters in connexion with this subject, and there are several points about which it is not possible at present to express a decided opinion.

The larvae have no stigmata; it appears to be generally agreed that there is in them no means of admitting air to the tracheal system by means of orifices. Some breathe entirely through the integument, the process being aided by the accumulation of tracheae at the spots where the breathing orifices should be, and where the integument is more delicate. Others, however, possess gills in the form of protruded bunches of filaments, connected with tracheae in the manner shown in Fig. 253. These filamentous branchiae occur in numerous species of the family, and are situate on various parts of the body, but many species are destitute of them in genera, other members of which possess the filaments. In some Nemourae instead of bunches of filaments there are tubular projections on the prothoracic segment; and in _Dictyopteryx signata_ similar structures occur even in the cephalic region, Hagen stating[323] that there exists a pair on the submentum and another on the membrane between the head and the thorax. In the imago state, stigmata are present in the normal fashion, there being two thoracic and six abdominal pairs. In several species the filaments persist in the imago, so that in these cases we meet with the curious condition of the coexistence of branchiae with a well-developed and functionally active system of spiracles; this is the more curious because the creatures usually have then nothing to do with the water, it having been ascertained that in these cases the species live out of the water as other terrestrial and aerial Insects do. These instances of persistence {402}of branchiae during the aerial life have been the source of some perplexity; the condition was shown to exist in _Pteronarcys_ by Newport, and has since been demonstrated in various other forms. Newport believed that the imago of _Pteronarcys_ breathes by means of the gills, although it lives out of the water and possesses spiracles; and he informs us that Mr. Barnston observed the Insect when on the wing "constantly dipping on the surface of the water." Hence Newport concluded that _Pteronarcys_ in the winged state is "an amphibious animal." That a winged Insect should live in the air and yet breathe by means of gills would be truly extraordinary, and there can be little doubt that Newport's idea was erroneous. Hagen[324] was able to examine living imagos of the species in question. He found that they avoided the water, and though he placed some individuals therein, yet they did not use the gills. He also informs us that the branchiae have, during life, a shrivelled appearance, indicating that they are not functionally active, but are merely useless organs carried over to the imago from the previous instar, in which they were truly the means of obtaining air. Hagen also ascertained that the spiracles of the imago are in a normal state, being adapted for breathing, even as far back as the seventh abdominal segment.

Great difference of opinion has prevailed as to the relations of the branchiae to the stigmata, it having been contended that the falling off of some of the branchiae left the stigmatic orifices. The facts appear to be only consistent with the conclusion that the two are totally independent organs. This subject has been investigated by Palmén,[325] who finds that in Perlidae—contrary to what occurs in may-flies—the species are either entirely destitute of gills, or these organs are persistent throughout life. It is not to be inferred from this that the gills in the {403}perennibranchiate Perlidae are as conspicuous as they are in the exceptional _Pteronarcys_: for it appears that at the final moult the gills usually become very much contracted and concealed by the new integument; in some cases they merely appear as slight prominences in the neighbourhood of the stigmata.

Pictet, Dufour, Newport, and Imhof[326] have studied the internal anatomy. The alimentary canal is remarkable for the enormous oesophagus; there is no distinction between this and the crop. A proventriculus is quite absent, and there are no chitinous folds in the position it usually occupies. The true stomach is small, and only commences in the fourth abdominal segment. It has a prolonged lobe on each side in front, and in addition to this eight sacs; thus there are formed ten diverticula, fastened to the posterior part of the oesophagus by ligaments. The terminal portion of the stomach is small, and apparently only distinguished from the short intestine by the point of insertion of the Malpighian tubes; these vary in number from about twenty to sixty. There are two pairs of large salivary glands. In _Pteronarcys_ the caecal diverticula of the stomach are wanting. In some Perlidae the terminal parts of the gut are more complex than in _Perla maxima_; Newport figures both an ilium and colon very strongly differentiated, and states that these parts differ much in _Perla_ and _Pteronarcys_. According to him the stomach is embraced by a network of tracheae, and Imhof tells us that he found the stomach to contain only air.

The brain is small, but, according to Imhof, consists of four amalgamated divisions; the infra-oesophageal ganglion is small, {404}and placed very near the brain. There are three thoracic and six abdominal ganglia on the ventral chain. The nerves to the wings are connected with the longitudinal commissures of the ventral chain by peculiar, obliquely-placed, short commissures. The reproductive glands are peculiar, inasmuch as in each sex the pair of principal glands is connected together in the middle. The testes thus form an arch consisting of a large number of sub-spherical or pear-shaped follicles; the vasa deferentia are short in _Perla maxima_, and there are no vesiculae seminales; the ejaculatory duct is divided into three parts by constrictions. In _Pteronarcys_ and in _Perla bicaudata_, according to Newport and Dufour, the vasa deferentia are very long and tortuous, and there are elongate vesiculae seminales. The arrangement of the extremely numerous egg-tubes is analogous to that of the follicles of the testes, so that, as Dufour says, there is but a single ovary; connected with the short, unpaired portion of the oviduct, there is a large receptaculum seminis, and near the terminal orifice of the duct there is in _P. maxima_ an eight-lobed accessory gland.

The eggs are produced by Perlidae in enormous numbers: they are rather small, but peculiar in form, and possess at one extremity a micropyle apparatus, covered by a glassy substance through which Imhof could find no orifice. On the other hand, the chorion on another part of the egg is perforated by several canals.

{405}The Perlidae being of aquatic habits in their early stages, and, notwithstanding their ample wings, very poor adepts in the art of flying, are rarely found at any considerable distance from their native element. They are specially fond of running water, and delight in the neighbourhood of waterfalls, or other spots where the current is broken by obstacles so that a foaming water results. It is probable that the larvae which breathe by means of gills find an advantage in living in strongly-aerated water. Mountain streams and torrents are therefore specially affected by them; but Pictet informs us that they do not like the waters descending from glaciers. The food of the larvae is believed to be chiefly young may-flies, or other small, soft creatures, and it may possibly be owing to the absence of these that the Perlidae do not affect the glacier streams. Although Perlidae are remarkable for their capacity for enduring cold, it is possible that they may require warmth of the water at some period of their development, and this the glacier-streams cannot offer to them. They are among the earliest Insects to appear in the spring in Europe. Mr. Barnston says that on the Albany river in Canada the nymph of _Capnia vernalis_ comes up frequently in the cracks of the ice and casts its skin there; "it frequently comes up when the thermometer stands at freezing." Of _Nemoura glacialis_, which inhabits similar localities, he says that "it appears in the spring (end of March or beginning of April) when the ice becomes honeycombed, and even before then, at the same time as _Capnia vernalis_. It pairs in the crevices of decaying ice. The male has long antennae, and his wings are generally rumpled as if glued together." Newport entertained the idea that those Perlidae that live at low temperatures are of lower organisation than the other forms of the family.

It is a remarkable fact that several Perlidae frequently have—like _Nemoura glacialis_—the wings of the male much reduced in size; this being the contrary of the rule that usually prevails among Insects to the effect that, when there is a difference in the powers of flight, or even in the size of the wings, it is the male that is superior. Mr. J. J. Lister met with a very interesting Perlid at Loch Tanna in Arran at the beginning of April 1892. In this Insect, which is, according to Mr. M‘Lachlan, a form of _Isogenus nubecula_, the wings of the female (Fig. 258, B) are reduced to a size much less than those of ordinary {406}Perlidae, while those of the male (Fig. 258, A) are mere useless rudiments. Morton has pointed out that in Scotland more than one species of _Taeniopteryx_ occasionally produces micropterous males, and he associates this phenomenon with the early time of their appearance "almost in winter."[327] In _Nemoura trifasciata_ this reduction of the wings takes another but equally curious form; the hind wings of the male being long enough to cover the body, while the anterior pair are reduced to mere rudiments.

The phenomena of micropterism in Perlidae are well worthy of more detailed investigation. Mr. Morton informs the writer that the male of _Perla maxima_ (Fig. 251) in North Britain has the wings so short that they cannot be of any use as organs of flight. In Central Europe the wings are ample, as shown in our figure. In _Perla cephalotes_ the male is short-winged in both Britain and Central Europe; of the male of _Dictyopteryx microcephala_ only the micropterous form is known to exist. In _Isogenus nubecula_ (Fig. 258) it appears that the wings of the female are always more ample than those of the male of the same locality, and that local micropterism affects the two sexes unequally. Within the Arctic circle this Insect is usually of the Scotch form, though the male there occasionally has more ample wings.

It has been observed that in some Perlidae the eggs, after they have been extruded, are carried about by the female; for what reason is not at all known. They are said to be enclosed in a membranous capsule at the apex of the abdomen. The number of eggs deposited is sometimes very large, amounting to five or six thousand, and they are often of very minute size.

About twenty-four species of Perlidae occur in Britain.[328] The {407}species from all parts of the world existing in collections probably scarcely exceed two hundred. The insignificance of this number is no doubt chiefly due to the fact that these unattractive Insects are rarely captured by collectors, and are so fragile that unless good care is taken of them, specimens soon go to destruction after being dried. Perlidae are known to occur in most parts of the world, so that the number of species really existing may reach two or three thousand. They are known to anglers as stone-flies and creepers and are a favourite bait for trout.

The family in its character comes near to the Orthoptera, especially to the more simple forms of Phasmidae, but the two groups differ in the texture of the front wings and in the structure of the mouth-parts, as well as in the different proportions of the mesothorax and metathorax. According to Pictet, in the Australian genus _Eusthenia_ the trophi (Fig. 259) approach nearer to those of the Orthoptera, so that it appears possible that a more intimate connexion will be found to exist as more forms are discovered. Of the groups we include in Neuroptera, Perlidae are in structure most allied to Sialidae, but the development in the two groups exhibits very important distinctions. Brauer treats the Perlidae as forming a distinct Order called Plecoptera, a name applied to the family by Burmeister many years ago.

Several species of Perlidae, considered to belong to existing genera, have been found in amber. A fossil from the Eocene deposits in the Isle of Wight and another from the Miocene of Continental Europe are referred to the family. Brauer has recently described[329] some fossils from the Jurassic formation in East Siberia as forming three genera, now extinct, of Perlidae.

Brongniart informs us[330] that several fossils have been found {408}in the Carboniferous strata of Commentry that justify us in asserting that allies of Perlidae then existed. He considers these Carboniferous Insects to have belonged to a separate family, Protoperlides. The fragments are, however, so small that we must await further information before forming a definite opinion as to these Protoperlides.

{409}CHAPTER XVIII

AMPHIBIOUS NEUROPTERA _CONTINUED_—ODONATA, DRAGON-FLIES

FAM. VI. ODONATA—DRAGON-FLIES.

(LIBELLULIDAE OF SOME AUTHORS)

_Elongate Insects with very mobile head and large eyes, with small and
inconspicuous antennae ending in a bristle; with four elongate wings
sub-equal in size and similar in texture, of papyraceous consistency and
having many veinlets, so that there exists a large number of small cells.
All the legs placed more anteriorly than the wings. The earlier stages of
the life are aquatic; there is great change in the appearance of the
individual at the final ecdysis, but there is no pupal instar._

The dragon-flies form a very natural and distinct group of Insects. All the species are recognised with ease as belonging to the family. They are invariably provided with wings in the perfect state, and many of them are amongst the most active of Insects. Their anatomy is, in several respects, very remarkable.

The head is large and is concave behind; it is attached to the thorax in such a way that it rotates on two cervical sclerites that project forwards, and in some cases almost meet in a point in front; hence it possesses extreme mobility, the power of rotation being very great.

The eyes are always large; in some cases they are even enormous, and occupy the larger part of the area of the head: the upper facets of the eye are in many cases larger than the lower, and in a few forms the line of division is sharply marked transversely. There are three ocelli, which, when the size of the compound eyes is not too great, are placed in the usual {410}manner as a triangle on the vertex; but in the forms where the compound eyes are very large the portion of the head between is, as it were, puffed out so as to form a projection just in front of where the eyes meet, and one ocellus is then placed on each side of this projection, an antenna being inserted quite close to it; the third ocellus is placed in front of the projection we have mentioned, by which it is often much concealed; this anterior ocellus is in some cases of unusually large size, and oval or transverse in form.

The parts of the mouth are very peculiar, especially the lower lip: we will briefly allude to its characters in the highly modified forms, premising that in the smaller and less active species it is less remarkable. The Libellulidae are carnivorous, their prey being living Insects which are captured by the dragon-fly on the wing; it is believed that the mouth is largely instrumental in the capture, though the flight of these Insects is so excessively rapid that it is difficult, if not impossible, to verify the action of the mouthpieces by actual observation.

{411}

For the purpose of securing the prey a mouth that can change its capacity to a considerable extent and with rapidity is a desideratum, and these qualities are present in the mouths of those Libellulidae that capture their prey while hawking. The upper lip is very mobile, is pendent, and closes the mouth above, while the lower lip entirely closes the under part by means of two mobile plates; these in some forms (_Libellula_) meet together in the mesial line, while in others a third plate separates them in the middle (Fig. 261, B, _li_). These plates are, according to Gerstaecker's view,[331] portions of the much changed labial palpi, the part that separates them in _Aeschna_ being the inner lobes of the labial maxillae; in _Libellula_, where the dilated and valve-like joints of the palpi meet in the middle line, the labial lobes remain small and are overlapped by the dilated portions of the palpi. The maxillae proper (Fig. 261, A) are less peculiar, their chief character being that the inner and outer lobes are not separated, and that the palpus is of only one joint. Some entomologists take, however, another view of this structure, looking on the palp-like outer part (_p_ of our figure) as the true outer lobe of the maxillae, the palpus proper being in that case considered to be entirely absent. The mandibles are very powerful, and armed with largely developed teeth. In the interior of the mouth there is a large, free, semi-membranous lingua, the posterior part of its delicate inferior lamina being connected with the mentum; the upper lamina of the lingua is stronger and is pilose. The antennae of the dragon-flies are always small, and consist of two stouter joints at the {412}base, and a terminal part which is very slender and pointed, and formed of four or five joints.

The prothorax is always small; the pronotum is distinct, though in some forms it is quite concealed in the concavity of the back of the head; the sternum is small; the anatomy of the pleura and basal pieces of the legs is obscure.

The meso- and meta-thorax are very intimately combined, and their relations are such that the former is placed much above the latter. This peculiarity is carried to its greatest extent in some of the Agrioninae (Fig. 262, A), where not only are the wings placed at a considerable distance behind the three pairs of legs, but also the front pair of wings is placed almost directly above the hind pair. In the Anisopterides these peculiarities are much less marked (Fig. 262, B), nevertheless even in them the three pairs of legs are placed quite in front of the wings. This peculiar structure of the wing-bearing segments is accompanied by an unusual development of the pleura, which, indeed, actually form the larger part, if not nearly the whole, of the front region of the dorsal aspect of these two segments. We shall not enter into more minute particulars as to the structure of the thorax, for difference of opinion prevails as to the interpretation of the parts.[332] The abdomen is remarkable for its elongation; it is never broad, and in some genera—_Mecistogaster_, _e.g._—it attains a length and slenderness which are not {413}reached by any other Insects. It consists of ten segments and a pair of terminal calliper-like or flap-like processes of very various sizes and forms.

The wings of the dragon-flies are usually transparent and provided with a multitude of small meshes. The hind wings are about as large as the front pair, or even a little larger; the main nervures have a sub-parallel course, and are placed in greater part on the anterior region of each wing. The relations of the more constant nervures and the cells of which they are parts form a complex subject, and are amongst the most important of the characters used in classifying these Insects. The wings are always elongate in comparison to their breadth and have no folds; they are held partially extended, or are placed so as to project backwards, or backwards and outwards. They exhibit another peculiarity, inasmuch as the front or costal margin is slightly uneven before or near the middle, giving rise to an appearance such as might result from the breaking and subsequent mending of the marginal rib at the spot in question, which is called the nodus. In some forms a peculiar character exists in the shape of a small opaque space called the membranule, lying close to the body of the Insect in the anal area of the wing, as shown in Fig. 260.

The legs are slender and are chiefly remarkable for the beautiful series of hair-like spines with which they are armed, and which in some forms (_e.g._ _Platycnemis_, Fig. 264) are of considerable length. We believe that the legs are of great importance in capturing the prey, they being held somewhat in the position shown in Fig. 262, C. The tarsi are three-jointed. In the male of _Libellago caligata_ the legs exhibit a remarkable condition, the tibiae being dilated, and on the upper side of a vivid red colour, while below they are white. This coloration and form are each unusual in the family. The male of _Platycnemis pennipes_, a British species (Fig. 264), shows a similar dilatation of the tibiae, but to a less extent and without any great difference in the colour of the two faces of the dilatation. This dilatation reaches its maximum in _Psilocnemis dilatipes_ M‘Lach. The position of the legs in relation to the other parts of the body is peculiar to the dragon-flies; the legs seem to be unfit for walking, the Insects never using them for that purpose.

{414}Several peculiarities in the internal anatomy deserve notice. The alimentary canal in _Libellula_ is about as long as the body, the oesophagus and chylific stomach being elongate, while the intestine is short and divided into only two parts; there is no definite proventriculus. The Malpighian tubules are shorter than usual; they are about forty in number. The male has no vesiculae seminales; the vasa deferentia are elongate, and the ejaculatory duct is very short, being in fact merely a common sinus formed by the terminations of the vasa deferentia. The opening of this duct is situated on the penultimate ventral plate; the organs of intromission are, however, placed much anterior to this, on the under side of the second segment. The mode in which the fertilising fluid is transferred from the ninth to the second segment is not well understood, but it is known that the abdomen is flexed by the Insect so as to bring the ninth ventral plate into contact with the second. The three thoracic ganglia of the nervous chain are all contiguous, though not completely amalgamated; the abdominal ganglia are seven in number, and are all separated, the terminal one being larger than the others. Dufour, after repeated dissections, was unable to find any salivary glands, but Olga Poletajewa[333] states that they exist.

The Odonata must be ranked among the most highly-organised Insects so far as external structure and powers of locomotion are concerned; the peculiar modifications of the thoracic segments and the relative positions of the wings and legs mark a great departure from the normal type of Insect structure. Their prey consists of living Insects, which they capture on the wing by their own superior powers of flight. They destroy a great many Insects, their appetite for food being, as in the cases of the Mantidae and of the tiger-beetles, apparently almost insatiable. They are admirably constructed for the purposes of their predatory lives; they fly with great swiftness and change the direction of their flight with admirable facility. They are, however, dependent on sunshine, and conceal themselves in dull and cloudy weather. The larger Insects of the family belong to the division Anisopterides (Fig. 260, _Anax formosus_) and some of these may, in our own country, usually be seen, in the bright sunshine of the summer and autumn, engaged in hawking in their favourite haunts. Places where other Insects {415}abound are naturally those most frequented; the glades of woods, country lanes and hedge-sides, the borders of streams and the margins of sheets of water are the places they most affect. They inspire the rustics with some feeling of fear, and hence have received the name of "horse-stingers," and in North America are called "devil's darning-needles." The aversion to dragon-flies may perhaps be due to their appearance, which is certainly, in the case of some of our species of _Aeschna_, _Cordulegaster_, and _Gomphus_, very remarkable, consisting of a dark ground-colour with bars and spots of vivid green or yellow, giving, it must be admitted, a peculiar, even savage appearance to the Insects. Whatever the reason may be, they are, it is certain, held in much fear, and it is difficult to induce a country lad to touch one even when it is captured and held by another person. The idea of dragon-flies being dangerous to anything but their Insect victims is, however, entirely erroneous; they may be captured and handled without their inflicting any injury. It is probable that the life of the imago may endure for several weeks if not months. It is known that _Sympycna fusca_—a common European though not British dragon-fly—hibernates in the imago state.

In the case of the large dragon-flies we have mentioned, each individual appears to have a domain, as it were, of its own. Westwood tells us that he has seen what he believed to be the same individual hawking daily for several weeks together over a small pond. The writer observed a specimen of _Cordulegaster annulatus_ to frequent a particular bush, to which it returned—frequently to the same leaf—after an excursion in search of food. The way in which these Insects actually seize their prey has not yet been made clear; it is certain that they capture flying Insects, and it seems most probable, as we have already said, that this is done by means of the legs. These, as we have said, are inserted so as to be very near to the mouth; they are directed forwards, and are held bent at right angles so as to form a sort of net, and are armed with a beautiful system of fine spines; it is probable that if the dragon-fly pursue an Insect on the wing and strike it with the trap, formed by its six legs (Fig. 262, C), then these immediately come together under the mouth, so that the victim, directly it is captured by the leg-trap of its pursuer, finds itself in the jaws of its destroyer. It is perhaps impossible to {416}verify this by actual observation, as the act of capture and transfer is so very brief and is performed in the midst of a rapid dash of flight, but it seems more probable that the prey is first struck by the legs than that the mouth is the primary instrument of capture. The excessive mobility of the head permits the victim to be instantly secured by the mouth, and the captured fly is turned about by this and the front pair of legs, and is nipped rapidly so that the wings and drier parts fall off; the more juicy parts of the prey are speedily squeezed into a little ball, which is then swallowed, or perhaps we should rather say that the mouth closes on it, and submits it to further pressure for the extraction of the juices. We have already noted that many of these large and active dragon-flies, particularly in the Libellulinae and Aeschninae, have their eyes distinctly divided into two parts, the facets in the lower part of the eye being different from those of the upper part. Exner considers[334] that the upper division is for the perception of movement, the lower for the perception of the form of resting objects. Plateau thinks[335] that the dragon-flies perceive only movement, not form.

The splendid acts of flight of the Anisopterid Odonata are accomplished by the aid of a complex arrangement of chitinous pieces at the bases of the wings (Fig. 263). In Insects with considerable powers of flight the hind wings are usually subordinate in functional importance to the anterior, to which they are attached by a series of hooks, or some other simple mechanism, on the wings. {417}In the Odonata the two wings of each pair are quite free, but they are perhaps brought into correlative action by means of a lever of unusual length existing amongst the chitinous pieces in the body wall at the base of the wings (Fig. 263, _b_). The wing muscles are large; according to von Lendenfeld[336] there are three elevator, five depressor, and one adductor muscles to each wing: he describes the wing movements as the results of the correlative action of numerous muscles and ligaments, and of a great number of chitinous pieces connected in a jointed manner.

Amans[337] has suggested that the mechanism of flight of the dragon-fly would form a suitable model for a flying-machine, to be propelled by electricity.

The Zygopterides—the second of the two divisions of the Odonata—are Insects different in many respects from the large and robust Anisopterides. The division comprises the delicate Insects called "demoiselles," damsel-flies, by the French (Fig. 262, A, and Fig. 264). Great power of flight is not possessed by these more fragile Insects; they flit about in the most gentle and airy manner from stem to stem of the aquatic plants and grasses that flourish in the localities they love. To this group belong the fairy-like Insects of the genus _Calepteryx_, in which various parts of the body and wings are suffused with exquisite {418}metallic tints, while sometimes the two sexes of one species have differently coloured wings. The smallest and most delicate dragon-flies that are known are found in the tropics; some of the genera allied to _Agrion_ consist of Insects of extraordinary fragility and delicacy.

Although the mature Odonata are so pre-eminently endowed for an aerial and active life, yet in the earlier stages of their existence they are very different; they are then, without exception, of aquatic habits; though carnivorous also in this period of their existence, they are sluggish in movement, lurking in concealment and capturing their prey by means of a peculiar conformation of the mouth, that we shall subsequently describe. Their life-histories are only very imperfectly known.

The eggs are deposited either in the water or in the stem of some aquatic plant, the female Insect occasionally undergoing submersion in order to accomplish the act. The young on hatching are destitute of any traces of wings (Fig. 265), and the structure of the thoracic segments is totally different from what it is in the adult, the rectal respiratory system (Fig. 265, _x_) to which we shall subsequently allude, being, however, already present. The wings are said to make their first appearance only at the third or fourth moult. At this time the pleura of the second and third thoracic segments have grown in a peculiar manner so as to form a lateral plate (Fig. 266, B, shows this plate at a later stage), and the wing-pads appear as small projections from the membranes at the upper margins of these pleural plates (Fig. 266, A, B). The plates increase in size during the subsequent stadia, and meet over the bases of the wing-pads, which also become much longer than they were at first. The number of moults that occur during growth has not been observed in the case of any species, but they are believed to be numerous. There is no pupa, nor is there any well-marked quiescent stage preceding the assumption of the winged form at the last ecdysis, although at the latter part of its life the nymph appears to be more inactive than usual. When full grown, the nymph is more like the future perfect Insect than it was at first, and presents the appearance shown in Figs. 266 and 270. At this stage it crawls out of the water and clings to some support such as the stem or leaf of an aquatic plant; a few minutes after doing so the skin of the back of the thoracic region splits, and the imago emerges from the nymphal skin.

{419}

The nymphs never have the body so elongate as the perfect Insect, the difference in this respect being frequently great, and the nymphs of the subfamily Libellulinae being very broad (Fig. 266, nymph of _Ictinus_ sp.); consequently the creature on emergence from the nymph-skin is very much shorter than it will soon become. Extension begins to take place almost immediately; it has been thought by some that this is accomplished by swallowing air; this is, however, uncertain. At first the wings have only the length of the wing-pads of the nymph, and their apical portion is an unformed mass. The colour of the perfect Insect is not present when the emergence takes place. The wing grows quickly until the full length is attained. In the genus _Agrion_ {420}the expansion of the wings is accompanied by frequent elevations and depressions of the body, and occupies an hour or so; the elongation of the abdomen is not so soon completed, and its brilliant colours do not appear for several hours.

The mouth of the nymph bears a remarkable structure called the mask (Fig. 268). It is apparently formed by a backward growth of the bases of the labium and lingua, a hinge being formed between the two at the most posterior point of their growth. The prolonged portions of these parts are free: usually the mask is folded under the head, but it can be unfolded and thrust forward, remaining then attached to the head by means of the more anterior parts of the lingua and by the maxillae, the whole of the elongate apparatus being, when used, extended from this anterior part of its attachment. The front parts of the labium form a prehensile apparatus armed with sharp teeth, so that the structures make altogether a very effectual trap, that can be extended in order to secure the prey.

The fact that the dragon-fly passes suddenly, in the middle of its existence, from an aquatic to an aerial life, makes the condition of its respiratory organs a subject for inquiry of more than usual interest. Réaumur was of opinion that the nymph was, in spite of its aquatic existence, provided with an extensive system of stigmata or orifices for breathing air; this was, however, denied by Dufour, and his opinion seemed to be supported by the fact that other means of obtaining air were discovered to exist in these nymphs. The inquiries connected with the respiration {421}of Odonata are still very incomplete, but some interesting points have been ascertained, the most important of which is perhaps the existence in some forms of a respiratory system in connexion with the posterior part of the alimentary canal (Fig. 269). In the nymphs of Anisopterides the system consists of four main tracheal trunks, traversing the length of the body, and by their ramifications and inosculations forming an extensive apparatus. Connected with the four main trunks we have described, there is a shorter pair confined to the abdomen, where it supplies a large number of branches to the walls of the stomach. The dorsal pair of the main tubes give numerous subsidiary branches to the outside of the rectum, and the ventral pair furnish a smaller number. The walls of the gut are penetrated by the branches, which inside the rectum form numerous loops; these, covered by a membrane, project into the interior in the form of multitudinous papillae (Æschninae). In the Libellulinae the papillae are replaced by more flattened processes or lamellae. The structures attain a remarkable development, there being in _Aeschna cyanea_ upwards of 24,000 papillae.

These rectal gills obtain air from water admitted into the rectum for the purpose; the extremity of the body being armed with projections of variable form, that can be separated to allow ingress and egress of the fluid, or brought into close apposition so as to close the orifice. The water so taken in can, by some species, be ejected with force, and is used occasionally as a means of locomotion. These rectal branchiae can absorb free air, as well as air dissolved in water. If the fluid in which the creatures are placed has been previously boiled, so as to expel the air from it, the nymphs then thrust the {422}extremity of the body out of the water and so obtain a supply of air.

Oustalet and Palmén state that at the last ecdysis the lamellae, or the papillae, do not disappear, but remain quite empty, and are consequently functionless, while on the tracheal trunks there are developed air vesicles to fit the creature for its aerial career. Hagen says that in _Epitheca_ the whole structure of the gills is shed at the final moult.

The subject of the rectal branchiae of _Libellula_ has been discussed and illustrated by Chun,[338] who states that Leydig has made known that in _Phryganea grandis_ a structure is found connecting the rectal branchiae of _Libellula_ with the rectal glands of some other Insects. We have not been able to find a confirmation of this in the writings of Leydig or elsewhere.

In the nymphs of the Zygopterides the highly-developed rectal branchiae found in the Aeschninae and Libellulinae do not exist, and the respiration seems to be of a complex character. In one division of the Zygopterides—Calepteryginae—rectal gills of an imperfect character are said by Hagen and others to exist.[339] The nymphs of the Zygopterides are provided with three mobile processes at the extremity of the body (Fig. 270); these serve the purposes of locomotion. They are believed to possess also a respiratory function, but this must be of an accessory nature, for the nymphs live after the removal of the processes, and indeed reproduce them; the skin of these processes is harder than is usual in Insect gills. In the nymph of _Euphaea_—a genus of Calepteryginae found in tropical Asia—there are also external {423}abdominal gills, and in this case respiration may, according to Hagen,[340] take place in four different manners: (1) by ten pairs of stigmata; (2) by lateral branchiae well furnished with tracheae; (3) by caudal branchiae; (4) by rectal branchiae. It is further said that in this Insect the lateral branchiae persist in the imago.

Although the means of respiration of the nymphs have been fairly well ascertained, yet the mode in which the nymph is prepared for the sudden change from the aquatic to aerial life is still obscure, the condition of the stigmata not being thoroughly elucidated. It appears probable, however, that the young nymph has no stigmata; that these organs appear in the course of its development, being at first quite impervious, but becoming—at any rate in the case of the larger and more important pair—open previous to the final ecdysis. We have mentioned the contradictory opinions of Réaumur and Dufour, and will now add the views of some modern investigators. Oustalet says[341] that there are two pairs of spiracles in the nymphs; the first pair is quite visible to the naked eye, and is situate between pro- and meso-notum; it is in the nymph closed by a membrane. The other pair of spiracles is placed above the posterior pair of legs, is small and completely closed. He does not state what stage of growth was attained by the nymphs he examined. Palmén was of opinion that not only thoracic but abdominal spiracles exist in the nymph,[342] and that they are completely closed so that no air enters them; he says that the spiracles have tracheae connected with them, that at each moult the part closing the spiracles is shed with some of the tracheal exuviae attached to it. The breathing orifices are therefore for a short time at each ecdysis open, being subsequently again closed by some exudation or secretion. This view of Palmén's has been thought improbable by Hagen and Dewitz, who operated by placing nymphs in alcohol or warm water and observing the escape of bubbles from the spots where the supposed breathing orifices are situate. Both these observers found much difference in the results obtained in the cases of young and of old nymphs. Hagen concludes that the first pair of thoracic spiracles are functionally active, and that abdominal stigmata exist though {424}functionless; he appears to be of opinion that when the first thoracic stigma is closed this is the result of the abutting against it of a closed trachea. Dewitz found[343] that in the adult nymph of _Aeschna_ the thoracic stigma is well developed, while the other stigmata—to what number and in what position is not stated—are very small. In a half-grown Aeschnid nymph he found the thoracic stigma to be present in an undeveloped form. On placing a full-grown nymph in alcohol, gas escaped from the stigma in question, but in immature nymphs no escape of gas occurred although they were subjected to a severe test. A specimen that, when submitted to the above-mentioned immersion, emitted gas, subsequently moulted, and thereafter air escaped from the spiracle previously impervious. The observations of Hagen and Dewitz are perhaps not so adverse to the views of Palmén as has been supposed, so that it would not be a matter for surprise if Palmén's views on this point should be shown to be quite correct.

The number of species of Odonata or Libellulidae that have been described is somewhat less than two thousand, but constant additions are made to the number, and when the smaller and more fragile forms from the tropics are collected and worked out it will probably be found that the number of existing species is somewhere between five and ten thousand. They are distributed all over the world, but are most numerous in species in the warmer regions, and their predominance in any one locality is very much regulated by the existence of waters suitable for the early stages of their lives.