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When the weather is wet or cold the Ascalaphi repose on the stems of grass, with their wings placed in a roof-like manner, with the head downwards, and are then very successful in concealing themselves by the positions they assume, and by sidling round the stems to escape from enemies. Some information as to their metamorphosis has been obtained, though knowledge of this point is far from complete even as regards our European species of the typical genus Ascalaphus. For a long time it was supposed that a larva mentioned by Bonnet in his writings was that of Ascalaphus, but Brauer[386] is of opinion that such is not the case, and as he has described the metamorphoses of A. macaronius he is no doubt correct. The eggs (Fig. 303, A), forty or fifty in number, are laid in two parallel rows on the stems of grass. The larvae (Fig. 304, larva of Helicomitus ?) are in general appearance somewhat like those of Myrmeleon; they are carnivorous in their habits, like the ant-lions, and have similar extraordinarily developed mandibles. Efforts to rear the young larvae failed, but they were kept alive for some time by supplying them with Aphidides found on Centaurea jacea. The cocoon is globular, and the change from the nymph state to the imago is made in the cocoon, the structure of the mandibles of the pupa being peculiar, and specially adapted to the purpose of opening the cocoon.[387] The larvae of Ascalaphides, although so like the ant-lions in appearance, do not form pitfalls for the capture of their prey, but lurk under leaves on the ground, or under stones; they do not move backwards, but progress forwards in an ordinary manner; the habit of backward movement that we noticed in Myrmeleon being probably correlative with the habit of forming pitfalls. Hagen states[388] that the larvae of Ascalaphides and Myrmeleonides, in addition to their peculiarities of form and mandibular structure, are distinguished from those of other Hemerobiidae by the hind legs having the tibia and tarsus united without articulation. Westwood[389] has recently given an account of the young larvae of a Ceylonese Ascalaphid of doubtful species, but possibly Helicomitus insimulans; these were observed by Mr. Staniforth Green to have the very peculiar habit of sitting together in a long row on the stem of a plant, with the jaws widely extended and the body of each one covered by the head of
the individual next it (Fig. 303, B). The little creatures waited patiently in this position until a fly walked between the mandibles of one of them, then these formidable weapons immediately closed, and did not relax their hold until the fly was sucked dry. If Westwood is correct, the young larva of this species differs much from the adult one, the back of the head being broad and the setigerous processes of the body very much more developed. Nearly thirty genera of Ascalaphides are known.[390] In the genus Haplogenius we find an exception to the usual rule that the wings in repose are held in a roof-like manner, it having been noticed by Bates that in the species in question the wings are held expanded as in the dragon-flies.
F . 304. Larva of Helicomitus insimulans (?). (After Westwood.)
Guilding has described[391] a very peculiar mode of oviposition on the part of Ulula macleayana in the island of St. Vincent; the eggs are said to be deposited by the female in circles on the extremity of a twig, and nearer the base of this there is placed a kind of barrier to repel intruders. "The female may be seen expelling from her ovary these natural barriers with as much care as her real eggs." Guilding's description was accompanied by drawings of the eggs, barriers and larvae, but unfortunately these were never published, and no further information has been obtained on the subject. Hagen[392] suggests that the barriers may be somewhat similar to the long stalks on which the eggs of Chrysopa (Fig. 314) are placed.
Sub-Fam. 3. Nemopterides. Head more or less produced and beak-like. Hind wings of peculiar form, being elongate and somewhat strap-like.
F . 305. Nemoptera ledereri. Asia Minor. (After Selys.) A, The imago; B, its head seen from in front and magnified
The Nemopterides are a small group of delicate, graceful Insects. About thirty species are known. Knowledge of the group is still very imperfect. A larva has been found of a most remarkable nature that probably belongs to it; it was described under the name of Necrophilus arenarius, and considered to be a fully-developed Insect. This larva occurs in the tombs and pyramids of Egypt where sand has accumulated. The perfect Insects of the genus Nemoptera are, however, found in open places amongst bushes, and flit about in a very graceful manner. Several species are found in Southern
F 306 Presumed larva of Nemoptera (Necrophilus arenarius) After Roux Pyramids of EgyptEurope and the Mediterranean region (Fig. 305, N. ledereri), but none come so far north as Central Europe. Formerly the genus Nemoptera was considered to be allied to Panorpa on account of the beak-like front of the head. The parts of the mouth are, however, different from those of Panorpa, and it seems more probable that if the Nemopterides have to be merged in any of the divisions of Hemerobiidae, they will be placed in Chrysopides or Osmylides. The species of the sub-family were for a long time believed to be peculiar to the continental regions of the Old World, but a species has recently been discovered in Northern Chili.[393]
Sub-Fam. 4. Mantispides. Prothorax elongate; the raptorial front legs inserted at its anterior part.
The members of this small group are readily recognised by the peculiar structure of the front legs; these organs resembling those of the Orthopterous family Mantidae, so that the earlier systematic entomologists, deceived by this resemblance, placed the Mantispides in the Order referred to.
F . 307. Mantispa areolaris. Brazil. (After Westwood.)
The Mantispides possess four membranous wings, either sub-equal in size or the posterior pair smaller than the front pair and not folded; the veins of these wings are rather numerous, as are also the cells they form; there is considerable difference amongst the species in this latter respect, owing to the transverse veinlets differing in their abundance. The antennae are short, not in the least thickened at the tip. The head is not produced into a beak. The anterior legs, placed
quite at the front part of the thorax, have the coxae very long; the femur is somewhat incrassate, and is armed on one side with spines; the tibia is shaped and articulated so as to fold closely on to the spines, and to thus constitute a formidable and perfect prehensile organ, the tarsus being merely a small appendage.
F . 308. Mantispa styriaca. A, Larva newly hatched, or first form; B, mature larva. (After Brauer.)
Only a few species of Mantispa are found in Southern Europe; but the group has representatives in most of the warmer regions of the world, and will probably prove to be rather numerous in species. The front legs are used for the capture of prey in the same way as the somewhat similar front legs of the Mantidae. The transformations have been observed by Brauer[394] in the case of one of the European species, M. styriaca. The eggs are numerous but very small, and are deposited in such a manner that each is borne by a long slender stalk, as in the lacewing flies. The larvae are hatched in autumn; they then hibernate and go for about seven months before they take any food. In the spring, when the spiders of the genus Lycosa have formed their bags of eggs, the minute Mantispa larvae (Fig. 308, A) find them out, tear a hole in the bag, and enter among the eggs; here they wait until the eggs have attained a fitting stage of development before they commence to feed. Brauer found that they ate the spiders when these were quite young, and then changed their skin for the second time, the first moult having taken place when they were hatched from the egg. At this second moult the larva undergoes a considerable change of form; it becomes unfit for locomotion, and the head loses the comparatively large size and high development it previously possessed. The Mantispa larva—only
one of which flourishes in one egg-bag of a spider—undergoes this change in the midst of a mass of dead young spiders it has gathered together in a peculiar manner. It undergoes no further change of skin, and is full fed in a few days; after which it spins a cocoon in the interior of the egg-bag of the spider, and changes to a nymph inside its larva-skin. Finally the nymph breaks through the barriers—larvaskin, cocoon, and egg-bag of the spider—by which it is enclosed, and after creeping about for a little, appears in its final form as a perfect Mantispa. Thus in this Insect hypermetamorphosis occurs; the larval life consisting of two different instars, one of which is specially adapted for obtaining access to the creature it is to prey on. It should be noticed that though this Insect is so destructive to the young spiders, the mother spider shows no hostility to it, but allows the destroying larva to enter her bag of eggs without any opposition; she appears, indeed, to be so unconscious of the havoc that is going on amongst her young that in one case she continued to watch over and protect the egg-bag in which the destruction was taking place during the whole of the period of the larval development and half the period of pupation of the Mantispa.
The larval history of a second species of the Mantispides, Symphrasis varia, is partially known;[395] this Insect lives parasitically in the nests of a South American wasp, and each larva when full fed spins a cocoon in one of the cells of the Hymenopteron.
Sub-Fam. 5. Hemerobiides.—Wings in repose forming an angular roof over the body; the antennae moniliform or pectinate, not clavate.
The Hemerobiides consist of several minor groups about whose number and characters systematists are not very well agreed, and about some of which very little is known. We merely mention the latter, giving details as to some of the better known only.
1. The Dilarina are a small group found chiefly in the Old World, where, however, they have a wide distribution. North and South America have each one species. They are distinguished by their antennae, which, in the male, are pectinate somewhat like those of many Lepidoptera, this character being of extremely rare occurrence in the Neuroptera; the abdomen of the female terminates in a long ovipositor. The metamorphoses are not known.
2. Nymphidina: Australian Insects resembling Myrmeleonides, but having antennae without club. Metamorphoses not known.
3. Osmylina: a group of delicate and elegant Insects of small or moderate size, distinguished by the possession of three simple eyes placed on the middle of the head just above the antennae. A species of this group, Osmylus chrysops (maculatus of some authors), is an inhabitant of Britain (Fig. 212); its larva is to some extent amphibious. The metamorphoses have been observed by Dufour, Brauer, and Hagen;[396] it lurks under stones in or close to the water, or in moss, or on the stems of aquatic plants, and pierces and empties small Insects with its sucking-spears, which are very elongate. The young are hatched from the egg in the autumn and hibernate before becoming full grown; when this moment arrives the larva spins a round cocoon of silk mixed with sand. The pupa, or nymph, in general appearance somewhat resembles the perfect Insect, except that it is shorter and has the short wing-pads clinging close to the body. Dufour denied the existence of abdominal spiracles in either larva or imago, but, according to Hagen, they are certainly present in both. It would appear that in the larva the alimentary canal is not open beyond the chylific ventricle, and that its terminal section is modified to form a spinning apparatus.
F . 309. Osmylus chrysops. A, Larva; B, side view of head of larva (after Brauer); C, pupa (after Hagen)
Osmylus and its allies, including Sisyra, are now frequently treated as a separate sub-family, Osmylides, equivalent to the Chrysopides. In it is placed a very anomalous Insect—Psectra dispar—of great rarity. The male has only two wings, the posterior pair being the merest rudiments, though the female has the four wings normally developed. Individuals of the male have been found[397] in widely separated localities in the Palaearctic region—Somersetshire being one of them—and also in North America.
The genus Sisyra forms for some Neuropterists the type of a separate group called Sisyrina, though by others it is placed, as we have said, with the Osmylina, though it is destitute of ocelli. The larvae of at least one species of this genus are aquatic, and have been found in abundance living in Spongilla (Ephydatia) fluviatilis, a fresh-water sponge; when discovered their nature was not at first recognised, as they possess on each ventral segment a pair of articulated appendages, looking like legs, but which are considered to be more of the nature of gills. The sucking-spears of this Insect are so long and slender as to look like hairs; whether the little animal draws its nutriment from the sponge, or merely uses this latter as a place of shelter, is not ascertained.
F . 310. A, Larva of Sisyra fuscata, ventral aspect; B, an abdominal appendage. (After Westwood.)
F 311 Larva of Hemerobius sp from Kent A, The larva bare; B, the same, partially concealed by the remains of its victims, etc ; a portion of the covering has been removed in order to show the head.
4. Hemerobiina: a somewhat numerous group of small or more rarely moderate-sized Insects, with moniliform antennae, no ocelli, a complex and comparatively regular system of wing-nervures; the veinlets are especially numerous at the margins, owing to the mode of forking of the nervures there (Fig. 298, Drepanepteryx phalaenoides). The larvae of most of the species of which the habits are known live on Aphides, which they suck dry, and at least one species, in all probability several, has the habit of covering itself with the skins of the victims it has sucked; to these remains it adds other small debris, and the whole mass completely covers and conceals the Insect (Fig. 311, B). The larva is furnished at the sides with projections which serve as pedicels to elongate divergent hairs, and these help to keep the mass in place on the back of the Insect; some fine threads are distributed through this curious mantle and serve to keep it from disintegration, but whether they are fragments of spiders' webs or are spun by the Insect itself is not quite clear.
F . 312. Portions of wings of Drepanepteryx phalaenoides. A, Underface of basal parts of the two wings; a, base of front wing; b, of hind wing B, Portion of front wing, showing the apparent interruption of nervures
The genus Drepanepteryx consists of several species, and appears to be best represented in the Antipodes; we have, however, one species in Britain—D. phalaenoides (Fig. 298)—an extremely interesting member of our fauna. This Insect has, like several of its congeners, a moth-like appearance, and it has a peculiar structure for bringing the hind and fore wings into correlation, the costa at the base of the hind wing being interrupted and prominent, furnished with setae (Fig. 312, A), and playing in a cavity on the under-surface of the front wing. This character is of great interest in connexion with analogous structures of a more perfect nature existing in various moths. M‘Lachlan has described and figured[398] a more primitive, though analogous, condition of the wings in Megalomus hirtus, also a species of British Hemerobiina. Another very curious feature of D. phalaenoides is shown in Fig. 312, B, there being a narrow space on the hind part of the front wing from which the colour is absent, while the nervures appear to be interrupted; they are, however, really present, though transparent; the nature of this peculiar mark is quite unknown, but is of considerable interest in connexion with the small transparent spaces that exist on the wings of some butterflies.
Sub-Fam. 6. Chrysopides, Lacewing-flies. Fragile Insects with elongate, setaceous antennae.
F . 313.—Chrysopa flava. Cambridge.
F 314 Eggs of Chrysopa A, Five eggs on a leaf; B, one egg, more magnified (After Schneider )
F . 315. Larva of Chrysopa sp. Cambridge. A, The Insect magnified; B, foot more magnified; C, terminal apparatus of the claws, highly magnified
The lacewing-flies—also called stink-flies and golden-eyes—are excessively delicate Insects, of which we possess about 15 species in Britain. Their antennae are more slender and less distinctly jointed than they are in Hemerobiides, and the Chrysopides are more elongate Insects. The peculiar metallic colour of their eyes is frequently very conspicuous, the eyes looking, indeed, as if they were composed of shining metal; this fades very much after death. Although not very frequently noticed, the Chrysopides are really
common Insects, and are of considerable importance owing to their keeping "greenfly" in check.
F 316 Chrysopa (Hypochrysa) pallida, larva (After Brauer )
The eggs are very remarkable objects (Fig. 314), each one being supported at the top of a stalk many times as long as itself; in some species (C. aspersa) the eggs are laid in groups, those of each group being supported on a common stalk. The larvae (Fig. 315) are of a very voracious disposition, and destroy large quantities of plantlice by piercing them with sucking-spears, the bodies of the victims being afterwards quickly exhausted of their contents by the action of the apparatus connected with the spears. The larvae of two or three species of Chrysopa cover themselves with the skins of their victims after the manner of the larvae of Hemerobius; but most of the larvae of Chrysopa are unclothed, and hunt their victims after the fashion of the larvae of Coccinellidae, to which these Chrysopa larvae bear a considerable general resemblance. These larvae have a remarkable structure at the extremity of their feet, but its use is quite unknown (Fig. 315, B, C). Some larvae of the genus make use of various substances as a means of disguise or protection. Dewitz noticed[399] that some specimens he denuded of their clothing and placed in a glass, seized small pieces of paper with their mandibles and, bending the head, placed the morsels on their backs; here the pieces remained in consequence of the existence of hooked hairs on the skin. Green algae or cryptogams are much used for clothing, and Dewitz supposes that the Insect spins them together with webs to facilitate their retention. According to Constant and Lucas[400] the
larvae of Chrysopa attack and kill the larvae of Lepidoptera and Phytophagous Hymenoptera. The curious form we figure (Fig. 316) has been hatched from eggs found by Brauer on Pinus abies in Austria. The eggs were of the stalked kind we have described; the young escaped from them in the autumn, twelve days after deposition, but did not take any food till the following spring.
The Chrysopides are widely distributed over the earth's surface. They form an important part of the fauna of the Hawaiian islands.
Sub-Fam. 7. Coniopterygides. Minute Insects with very few transverse nervules in the wings; having the body and wings covered by a powdery efflorescence.
These little Insects are the smallest of the Order Neuroptera, and have the appearance of winged Coccidae; their claim to be considered members of the Neuroptera was formerly doubted, but their natural history is quite concordant with that of the Hemerobiid groups, near which they are now always placed. Löw has made us acquainted with the habits and structure of an Austrian species, Coniopteryx lutea Wallg., but for which he has proposed the new generic name Aleuropteryx; the larvae are found on Pinus mughus at Vienna feeding on Aspidiotus abietis, which they pierce with suckingspears, after the fashion of the Hemerobiides; when full fed they spin a cocoon formed of a double layer of silk, in which metamorphosis takes place in a manner similar to that of other Hemerobiidae. The better-known genus Coniopteryx differs from Aleuropteryx in having the sucking-spears short and nearly concealed by the front of the head, which is somewhat prolonged.
F . 317. Coniopteryx psociformis. Cambridge. (After Curtis.) A, The insect with wings expanded, magnified; B, with wings closed, natural size
We may conclude this sketch of the Hemerobiid groups by remarking that fossil remains of specimens of most of them have been detected in the Tertiary strata, and that in the Secondary strata these groups are represented by only a small number of fossils, which are referred specially to Hemerobiina, Nymphidina, and Chrysopides.
F 318 A, Larva of Coniopteryx tineiformis (?) (After Curtis ) B, Head and prothorax of larva of Coniopteryx sp ; C, upper surface of head of larva of Coniopteryx (after Löw), much magnified
CHAPTER XXI
NEUROPTERA CONTINUED TRICHOPTERA, THE PHRYGANEIDAE OR CADDIS-FLIES
Fam. XI. Phryganeidae—Caddis-flies.
(TRICHOPTERA OF MANY AUTHORS)
Wings more or less clothed with hair, nervures dividing at very acute angles, very few transverse nervules; hind pair larger than the front, with an anal area which is frequently large and in
repose plicately folded. Antennae thread-like, porrect, of many indistinct joints. Mandibles absent or obsolete. Coxae elongate and free but contiguous. Metamorphosis great; larvae caterpillarlike, usually inhabiting cases of their own construction. Pupa resembling the perfect Insect in general form, becoming active previous to the last ecdysis. Wingless forms of the imago excessively rare.
The caddis-flies are Insects of moth-like appearance, found in the neighbourhood of water; their larvae live in this element, where they may sometimes be found in abundance. Phryganeidae are not very attractive Insects, and there are few of large size; Hence they have been much neglected by entomologists, and very little is known about the exotic forms of the family. The habitations constructed by the larvae are, many of them, of a curious nature, and usually attract more attention than do the creatures they serve to protect.
The Phryganeidae form the division or series Trichoptera; the two terms are therefore synonymous; those entomologists who consider these Insects to form a distinct Order use the latter appellation for it.
The perfect Insect, though the wings are usually ample, has but feeble powers of flight, and rarely ventures far from the water it was reared in; it has a moth-like appearance, and the wings in repose
F 319 Halesus guttatipennis Britain (After M‘Lachlan ) F . 320. Hydroptila angustella ♀. Britain. (After M‘Lachlan.)meet, at an angle, in a roof-like manner over the back (Fig. 326, E).
The head is small, with the front inflexed; it has two large compound eyes, and usually three ocelli; the antennae are slender, thread-like, and occasionally attain a great length. The parts of the mouth are very peculiar, the labrum and the palpi—especially the maxillary palps—being well developed, while the lobes of the maxillae and labium are amalgamated and therefore indistinct. The labrum is more or less elongate, and is more mobile than is usual in mandibulate Insects; it is held closely applied to the maxillae. These latter are small, have usually only a single small free lobe; they are united to one another and to the labium by membrane in such a manner as to form a channel along the middle of the mouth, the labrum forming the roof of this channel. The palpi are in some cases (Sericostomatides) of a remarkable nature; their joints vary in number from three to five, and differ sometimes in the sexes of the same species. The lower lip appears as a plate supporting the labial palpi, which are three-jointed and do not exhibit any peculiarities of structure comparable with those we have mentioned as so frequently existing in the maxillary palps. Difference of opinion exists as to the mandibles, some entomologists declaring them to be entirely absent, while others state that a small tubercular process that may be seen in some species on each side of the labrum is their representative. The prothorax is very small, the notum is the largest piece but is quite short, the side-pieces are very small, and the sternum appears to consist only of membrane. The mesothorax is much the largest segment of the body; its sternum is large, but is nearly perpendicular in direction, and is much concealed by the elongate, free front coxae, which repose against it. The metathorax is intermediate in size between the pro- and meso-thorax; its side-pieces are rather large, but the sternum is membranous, with a heart-shaped piece of more chitinous consistence in the middle, entirely covered by the middle coxae. The side-pieces both of the meso- and meta-thorax are large, and are closely connected; the middle and posterior coxae are very large, elongate, and prominent, and the middle pair slope backwards, so that their tips are in contact with the tips of the hind pair. The abdomen is cylindric and rather slender; it looks as if formed of eight segments in addition to the terminal segment; this
latter in the male usually bears remarkably modified appendages. The first ventral plate is sometimes, if not always, entirely membranous; indeed the texture of the segments is in general very delicate, so that they shrivel up to an extent that renders their comprehension from dried specimens very difficult. The legs are always elongate, the coxae attaining in some forms a remarkable length, and the tibiae and tarsi are armed with many spines; the tarsi are five-jointed, slender, frequently very elongate, terminated by two large claws and an apparatus, placed between them, consisting of a pair of hair-like processes with a membranous lobe.
F 321 —Front view of head of Anabolia furcata after removal of labrum. o, Ocellus; an, base of antenna; au, eye; cm, cardo; st, stipes; l, external lobe; pt, support of palpus; pm, palpus of maxilla; g, condyle of articulation of the absent mandible; ha, channel of haustellum; h, haustellum; sp, apex of channel of haustellum (not explained by Lucas); ch, chitinous point of external lobe of second maxilla; pl, labial palp. (After Lucas.)
The structure of the mouth-parts of the Phryganeidae has given rise to much difference of interpretation; it has recently been investigated by R. Lucas[401] in connexion with Anabolia furcata (Fig. 321). He agrees with other observers that mandibles are present in the pupa, but states that no rudiment of them exists in the imago. He calls the peculiar structure formed by the combination of the maxillae and labium a haustellum. He looks on the Trichoptera as possessing a mouth intermediate between the biting and sucking types of Insectmouths. He considers that the Phryganeidae take food of a solid, as well as of a liquid, nature by means of the haustellum, but the solid matter must be in the form of small particles, and then is probably sucked up by the help of saliva added to it. Lucas says also that in the larvae certain parts of the salivary glands serve the function of
spinning organs, and it is from these that the salivary glands of the imago are formed; those salivary glands of the larva that are not spinning glands disappearing entirely.
F .
The eggs are deposited in a singular manner; they are extruded in a mass surrounded by jelly; there may be as many as one hundred eggs in such a mass. This is sometimes carried about by the female after its extrusion from the interior of the body, but is finally confided to a suitable place in stream, spring, or pool. It is said that the female occasionally descends into the water to affix the egg-mass to some object therein, but this requires confirmation, and it is more probable that the egg-mass is merely dropped in a suitable situation. As soon as the larvae are hatched they begin to provide themselves with cases; they select small pieces of such material as may be at hand in the water, and connect them together by means of silk spun from the mouth. Particulars as to these tubes we will defer till we have considered the larvae themselves. These have the general appearance of caterpillars of moths; in order to move about they must put their head and the three pairs of legs at the front of the body out of their tube or case, and they then look very like casebearing caterpillars. The part of the body that usually remains under cover is different in texture and colour, and frequently bears outstanding processes, or filaments, containing tracheae for the purpose of extracting air from the water. Some peculiar spaces of a different texture may be seen on certain larvae (Fig. 322, B); these may possibly be also connected with respiration. On each side of the
322. Anabolia nervosa. A, Larva extracted from its case; B, one of the dorsal spaces of the abdominal segments more strongly magnified.extremity of the body there is a rather large hook by which the creature attaches its dwelling to its body, and there are also frequently present three large bosses on the anterior abdominal segment, which are supposed to assist towards the same end. The hold it thus obtains is so firm that it cannot be dragged out by pulling from the front; fishermen have, however, discovered a way of extracting it by a strategic operation: the cases are, as a rule, partially open behind, and by putting a blunt object in and annoying the larva it is induced to relax the hold of its hooks and advance forwards in the case, or even to leave it altogether The firm hold of the larva is maintained in spite of the fact that the body does not fill the case. It is necessary that water should pass freely into and out of the case, and that there should be some space for the respiratory filaments to move in. The mouth of the case is open, and the posterior extremity is arranged by the larva in such manner as to allow a passage for the water; various ingenious devices are adopted by different species of larvae with the object of protecting the hind end of the body, and at the same time of permitting water to pass through the case.
The mode of changing the skin, or the frequency with which this occurs in the larval state of the caddis flies has not been recorded. The duration of life in this stage is usually considerable, extending over several months: indeed in our climate many species pass the winter in this stage, completing the metamorphosis in the following spring or summer; and as one generation each year appears to be the rule, it may be assumed that the larval condition in such cases lasts from seven to ten months. During this stage the Insects are chiefly vegetable feeders, some being said to feed on minute algae; animal diet is not, however, entirely avoided, and it is said by Pictet that not only do some of the Phryganeidae eat other Insects, but that they also sometimes devour their companions.
F 323 A, Pupa of Phryganea pilosa (After Pictet ) B, Mandibles of pupa of Molanna angustata
At the end of the larval period of existence the creature closes its case by a light web spun at each end, taking care not to prevent the ingress and egress of the water; it sometimes adds a stone or piece of stick, and having thus protected itself, changes to a nymph. During the first part of this metamorphosis the creature is completely helpless, for there is so great a difference between the external structures of the larva and nymph as to make the latter a new being, so far as these organs are concerned. The changes take place in the interior of the larval skin, and as they are completed this latter is shed piecemeal. The resulting pupa or nymph greatly resembles the perfect Insect, differing consequently very much from the larva. Pictet, who paid special attention to the nymph condition of these Insects, concludes, however, that many of the organs of the nymph are actually formed within the corresponding parts of the larva, and has given a figure that, if trustworthy, shows that the legs of the nymph, notwithstanding the great difference between them as they exist in the larva and in the perfect Insect, are actually formed within the legs of the larva; each nymphal leg being rolled up in the skin of the corresponding larval leg, in a spiral, compressed manner, and the only articulations that can be detected in the leg being those of the tarsus. The head of the nymph is armed in front with two curious projections that are, in fact, enormously developed mandibles (Fig. 323, B); they serve as cutting implements to enable the nymph to effect its escape from its prison; they are cast off with the nymphskin, the perfect Insect being thus destitute of these organs. The abdomen of the nymph differs from that of the perfect Insect in possessing external respiratory filaments; the nymphs of some species have also the middle legs provided with swimming-hairs, that do not exist in the imago.