Chapter XXI: Introduction: Habits–classification–structure–chilognatha–chilopoda (15)

A good work on the British Odonata is still a desideratum.[344] In Britain about forty-six species are believed to be native. They are said to be of late years less numerous than they used to be. Notwithstanding their great powers of flight, dragon-flies are destroyed by birds of various kinds; several hawks are said to be very fond of them, and _Merops persicus_ to line its nest with their wings. The number of Insects killed by dragon-flies in places where they are abundant must be enormous; the nymphs, too, {425}are very destructive in the waters they inhabit, so that dragon-flies have no doubt been no mean factor in maintaining that important and delicate balance of life which it is so difficult for us to appreciate. The nymphs are no doubt cannibals, and this may perhaps be an advantage to the species, as the eggs are sometimes deposited in large numbers in a limited body of water, where all must perish if the nymphs did not, after exhausting other food, attack one another. Martin, speaking of the Odonata of the Département de l'Indre in France, says:[345] "The eggs, larvae, and nymphs are the prey of several fishes, snakes, newts, Coleoptera, aquatic Hemiptera, and of some diving birds. Sometimes the destruction is on a considerable scale, and one may notice the dragon-flies of some piece of water to diminish gradually in numbers, while the animals that prey on them increase, so that a species may for a time entirely disappear in a particular spot, owing to the attacks of some enemy that has been specially prosperous, and also eager in their pursuit. De Selys found that from a pond filled with carp, roach, perch, and eels, several of the dragon-fly denizens disappeared directly the bream was introduced." On the other hand, there can be little doubt that the nymphs are sometimes injurious to fish; it has been recorded that in a piscicultural establishment in Hungary 50,000 young fishes were put into a pond in spring; in the following autumn only fifty-four fish could be found, but there were present an enormous quantity of dragon-fly nymphs.

Odonata are among the few kinds of Insects that are known to form swarms and migrate. Swarms of this kind have been frequently observed in Europe and in North America; they usually consist of species of the genus _Libellula_, but species of various other genera also swarm, and sometimes a swarm may consist of more than one species. _L. quadrimaculata_ is the species that perhaps most frequently forms these swarms in Europe; a large migration of this species is said to occur every year in the Charente inférieure from north to south.[346] It is needless to say that the instincts and stimuli connected with these migrations are not understood.

The nymphs are capable, under certain circumstances, of accommodating themselves to very peculiar conditions of life. The Sandwich Islands are extremely poor in stagnant waters, and {426}yet there exist in this remote archipelago several highly peculiar species of Agrioninae. Mr. R. C. L. Perkins has recently discovered that the nymphs of some of these are capable of maintaining their existence and completing their development in the small collections of water that accumulate in the leaves of some lilies growing on dry land. These nymphs (Fig. 271) have a shorter mask than occurs, we believe, in any other Odonata, and one would suppose that they must frequently wait long for a meal, as they must be dependent on stray Insects becoming immersed in these tiny reservoirs. The cannibal habits of the Odonata probably stand these lily-dwellers in good stead; Mr. Perkins found that there were sometimes two or three nymphs of different sizes together, and we may suspect that it sometimes goes hard with the smaller fry. The extension in the length of the body of one of these lily-frequenting Agrions when it leaves the water for its aerial existence is truly extraordinary.

The Odonata have no close relations with any other group of Insects. They were associated by Latreille with the Ephemeridae, in a family called Subulicornia. The members of the two groups have, in fact, a certain resemblance in some of the features of their lives, especially in the sudden change, without intermediate condition, from aquatic to aerial life; but in all important points of structure, and in their dispositions, dragon-flies and may-flies are totally dissimilar, and there is no intermediate group to connect them. We have already, said that the Odonata consist of two very distinct divisions—Anisopterides and Zygopterides. The former group comprises the subfamilies Gomphinae, Cordulegasterinae, Aeschninae, Corduliinae, and Libellulinae,—Insects having the hinder wings slightly larger than the anterior pair; while the Zygopterides consist of only two subfamilies—Calepteryginae and Agrioninae; they have the wings of the two pairs equal in size, or the hinder a little the smaller. The two groups Gomphinae and Calepteryginae are each, in several respects, of lower development than the others, and authorities are divided {427}in opinion as to which of the two should be considered the more primitive. It is therefore of much interest to find that there exists an Insect that shares the characters of the two primitive subfamilies in a striking manner. This Insect, _Palaeophlebia superstes_ (Fig. 272), has recently been discovered in Japan, and is perhaps the most interesting dragon-fly yet obtained. De Selys Longchamps refers it to the subfamily Calepteryginae, on account of the nature of its wings; were the Insect, however, deprived of these organs, no one would think of referring _Palaeophlebia_ to the group in question, for it has the form, colour, and appearance of a Gomphine Odonate. Moreover, the two sexes differ in an important character,—the form of the head and eyes. In this respect the female resembles a Gomphine of inferior development; while the male, by the shape and large size of the ocular organs, may be considered to combine the characters of Gomphinae and Calepteryginae. The Insect is very remarkable in colour, the large eyes being red in the dead examples. We do not, however, know what may be their colour during life, as only one pair of the species is known, and there is no record as to the life-history and habits. De Selys considers the nearest ally of this Insect to be _Heterophlebia dislocata_, a fossil dragon-fly found in the Lower Lias of England.

Numerous fossil dragon-flies are known; the group is well represented in the Tertiary strata, and specimens have been found in amber. In strata of the Secondary age these Insects {428}have been found as far back as the Lower Lias; their remains are said to exist in considerable variety in the strata of that epoch, and some of them to testify to the existence at that period of dragon-flies as highly specialised as those now living. According to Hagen[347] _Platephemera antiqua_ and _Gerephemera simplex_, two Devonian fossils, may be considered as dragon-flies; the evidence as to this appears inadequate, and Brongniart refers the latter Insect to the family Platypterides, and considers Platephemera to be more allied to the may-flies.

One of the most remarkable of the numerous discoveries lately made in fossil entomology is the finding of remains of huge Insects, evidently allied to dragon-flies, in the Carboniferous strata at Commentry. Brongniart calls these Insects Protodonates,[348] and looks on them as the precursors of our Odonata. _Meganeura monyi_ was the largest of these Insects, and measured over two feet across the expanded wings. If M. Brongniart be correct in his restoration of this giant of the Insect world, it much resembled our existing dragon-flies, but had a simple structure of the thoracic segments, and a simpler system of wing-nervures. On p. 276 we figured _Titanophasma fayoli_, considered by Scudder and Brongniart as allied to the family Phasmidae, and we pointed out that this supposed alliance must at best have been very remote. This view is now taken by M. Brongniart himself,[349] he having removed the Insect from the Protophasmides to locate it in the Protodonates near Meganeura. There appears to be some doubt whether the wings supposed to belong to this specimen were really such, or belonged rather to some other species.

{429}CHAPTER XIX

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._

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.

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.

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.

Wing-sheath of left side, gills of right side, removed; _g_, tracheal gills. (After Vayssière.)]

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.

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}

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.

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.

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.

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.

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.

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.

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.

_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}

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.

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.

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.

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.

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._

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]

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}

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._

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._