LIFE-STORY N SECTS G, H, CARPENTER Oi 1 CD i ru [r CD m o The Cambridge Manuals of Science and Literature THE LIFE-STORY OF INSECTS CAMBRIDGE UNIVERSITY PRESS ftonlron: FETTER LANE, E.G. C. F. CLAY, MANAGER 100, PRINCES STREET ILontion: H. K. LEWIS, 136, GOWER STREET, W.C. WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND Berlin: A. ASHER AND CO. lLdp>ig: F. A. BROCKHAUS eto $orl;: G. P. PUTNAM'S SONS ant) Calcutta: MACMILLAN AND CO., LTD. All rights reserved B C Frontispiece. Transformation of a Gnat (Culex). Magnified 5 times. A. Larva. (The head is directed downwards and the tail- siphon with spiracle points upwards to the surface of the water.) B. Pupal Cuticle from which the Imago is emerging. (The pair of 'respiratory trumpets' on the thorax of the pupa are conspicuous. The wings of the Imago are crumpled, and the hind feet are not yet withdrawn.) C. Adult Gnat. Female. THE LIFE-STORY OF INSECTS BY GEO. H. CARPENTER Professor of Zoology in the Royal College of Science, Dublin Cambridge : o at the University Press '9*3 I I 4 PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS With the exception of the coat of arms at the foot, the design on the title page Is a reproduction of one used by the earliest known Cambridge printer John Slberch 1521 PREFACE rpHE object of this little book is to afford an out- line sketch of the facts and meaning of insect transformations. Considerations of space forbid any- thing like an exhaustive treatment of so vast a subject, and some aspects of the question, the physiological for example, are almost neglected. Other books already published in this series, such as Dr Gordon Hewitt's House-flies and Mr O. H. Latter's Bees and Wasps, may be consulted with advantage for details of special insect life-stories. Recent re- searches have emphasised the practical importance to human society of entomological study, and insects will always be a source of delight to the lover of nature. This humble volume will best serve its object if its reading should lead fresh observers to the brookside and the woodland. G. H. C. DUBLIN, July, 1913. CONTENTS CHAP. PAGE I. Introduction . II. Growth and Change III. The Life-stories of some Sucking Insects 16 IV. From Water to Air V. Transformations, Outward and Inward . VI. Larvae and their Adaptations VII. Pupae and their Modifications . 79 VIII. The Life- story and the Seasons . 89 IX. Past and Present the Meaning of the Story 105 Outline Classification of Insects . 1'22 Table of Geological Systems . 123 Bibliography . 124 Index 129 LIST OF ILLUSTRATIONS Stages in the Transformations of a Gnat Frontispiece FIG. PAGE 1. Stages of the Diamond-back Moth (Plutella cruciferarum) ...... 3 2. Head of typical Moth 5 3. Head of Caterpillar ...... 5 4. Common Cockroach (Blatta orientalis) . . 12 5. Nymph of Locust (Schistocera americana) . 13 6. Aphis pomi, winged and wingless females . 19 7. Mussel Scale-Insect (Mytilaspis pomorum) . 21 8. Emergence of Dragon-fly (Aesclma cyanea) . 29-31 9. Nymph of May-fly (Chloeon dipterum) . . 33 10. Imaginal buds of Butterfly .... 39 11. Imaginal buds of Blow-fly .... 43 12. Carrion Beetle (Silpha) and larva . . 51 13. Larva of Ground-beetle (Aepus) ... 52 14. Willow-beetle (Pliyllodecta) and larva . . 53 15. Cabbage-beetle (Psylliodes) and larva . . 54 16. Corn Weevil (Calandra) and larva ... 55 17. Ruby Tiger Moth (Pliraymatol>ia fuliginosa) . 61 18. Larvae and Pupa of Hive-bee (Apis mellifica) . 65 19. Larva of Gall-midge (Contarinia nasturtii) . 68 20. Crane-fly (Tipula oleracea) and larva . . 69 21. Maggot of House-fly (M-usca domestica) . . 71 22. Ox Warble-fly (Hypoderma bovis) with egg, larva, and puparium . . . . . . . 75 23. Pupa of White Butterfly (Fieri*) ... 85 CHAPTER I INTRODUCTION AMONG the manifold operations of living creatures few have more strongly impressed the casual observer or more deeply interested the thoughtful student than the transformations of insects. The schoolboy watches the tiny green caterpillars hatched from eggs laid on a cabbage leaf by the common white butterfly, or maybe rears successfully a batch of silkworms through the changes and chances of their lives, while the naturalist questions yet again the ' how ' and ' why ' of these common though wondrous life-stories, as he seeks to trace their course more fully than his predecessors knew. Everyone is familiar with the main facts of such a life-story as that of a moth or butterfly. The form of the adult insect (fig. 1 a) is dominated by the wings two pairs of scaly wings, carried respectively on the middle and hindmost of the three segments that make up the thorax or central region of the insect's body. Each of these three segments carries a pair of legs. In front of the thorax is the head on which the pair of long jointed feelers and the pair .c. i. 1 2 THE LIFE-STORY OF INSECTS [CH. i of large, sub-globular, compound eyes are the most prominent features. Below the head, however, may be seen, now coiled up like a watch-spring, now stretched out to draw the nectar from some scented blossom, the butterfly's sucking trunk or proboscis, situated between a pair of short hairy limbs or palps (fig. 2). These palps belong to the appendages of the hindmost segment of the head, appendages which in insects are modified to form a hind-lip or labium, bounding the mouth cavity below or behind. The proboscis is made up of the pair of jaw-append- ages in front of the labium, the maxillae, as they are called. Behind the thorax is situated the abdo- men, made up of nine or ten recognisable segments, none of which carry limbs comparable to the walking legs, or to the jaws which are the modified limbs of the head-segments. The whole cuticle or outer covering of the body, formed (as is usual in the group of animals to which insects belong) of a horny (chitinous) secretion of the skin, is firm and hard, and densely covered with hairy or scaly outgrowths. Along the sides of the insect are a series of paired openings or spiracles, leading to a set of air-tubes which ramify throughout the body and carry oxygen directly to the tissues. Such a butterfly as we have briefly sketched lays an egg on the leaf of some suitable food-plant, and there is hatched from it the well-known crawling (0 Fig. 1. a, Diamond-back Moth (Plutella cruciferamm) ; 6, young caterpillar, dorsal view; c, full-grown caterpillar, dorsal view; d, side view ; e, pupa, ventral view. Magnified 6 times. From Journ. Dept. Agric. Ireland, vol. i. 1-2 4 THE LIFE-STORY OF INSECTS [CH. larva 1 (fig. 1 b, c, d) called a caterpillar, offering in many superficial features a marked contrast to its parent. Except on the head, whose surface is hard and firm, the caterpillar's cuticle is as a rule thin and flexible, though it may carry a protective arma- ture of closely set hairs, or strong sharp spines. The feelers (fig. 3 At) are very short and the eyes are small and simple. In connection with the mouth, there are present in front of the maxillae a pair of mandibles (fig. 3Mn), strong jaws, adapted for biting solid food, which are absent from the adult butterfly, though well developed in cockroaches, dragon-flies, beetles, and many other insects. The three pairs of legs on the segments of the thorax are relatively short, and as many as five segments of the abdomen may carry short cylindrical limbs or pro-legs, which assist the clinging habits and worm-like locomotion of the caterpillar. No trace of wings is visible ex- ternally. The caterpillar, therefore, differs markedly from its parent in its outward structure, in its mode of progression, and in its manner of feeding ; for while the butterfly sucks nectar or other liquid food, the caterpillar bites up and devours solid vegetable substances, such as the leaves of herbs or trees. It is well-known that between the close of its larval life and its attainment of perfection as a butterfly, 1 The term larva is applied to any young animal which differs markedly from its parent. I] INTRODUCTION B Ma Fig. 2. Fig. 2. A. Head of a typical Moth, showing proboscis formed by flexible maxillae (, cylin- drical, worm-like ; and their legs are relatively short, the build of the insect being adapted for rapid motion through the soil. The grubs of the Chafers (Scarabaeidae) are also root-eaters, but they are less active in their habits than the wireworms, vi] LARVAE AND THEIR, ADAPTATIONS 53 and the cuticle of their somewhat stout bodies is, for the most part, pale and flexible ; only the head and legs are hard and horny. Usually an evident corre- spondence can be traced between the outward form of any larva and its mode of life. For example, in the family of the Leaf-beetles (Chrysomelidae) some larvae feed openly on the foliage of trees or herbs, Fig. 14. (a) Willow-beetle (Plnjllodecta vulga- tixsima) and its larva (/>). Magnified 5 times. After Carpenter, Econ. Proc. R. Dublin Soc. vol. i. while others burrow into the plant tissues. The exposed larvae of the Willow-beetles (Phyllodecta, fig. 14) have their somewhat abbreviated body seg- ments protected by numerous spine-bearing, firm tu- bercles. But the grub of the 'Turnip Fly ' (Phyllotreta) 54 THE LIFE-STORY OF INSECTS [CH. which feeds between the upper and lower skins of a leaf, or of Psylliodes chrysocephala (fig. 15), which burrows in stalks, has a pale, soft cuticle like that of a caterpillar. In the larvae of the little timber-beetles and their Fig. 15. (a) Cabbage-beetle (Psylliodes chrysocephala) magnified 5 times, and its larva (b) magnified 12 times. allies (Ptinidae), including the 'death-watches' whose tapping in old furniture is often heard, a marked shortening of the legs and reduction in the size of the head accompany the whitening and softening of the vi] LARVAE AND THEIR ADAPTATIONS 55 cuticle. This shortening of the legs is still more marked in the larvae of the Longhorn Beetles (Ceram- lyvcidae) burrowing in the wood of trees or felled trunks; here the legs are reduced to small vestiges. Fig. 16. a, Grain Weevil (Calandra (jrunaritt) ; b, larva ; c, pupa. Magnified 7 times. After Chitten- den, Yearbook U.S. Dept. Agric. 1894. Finally in the large family of the Weevils (Curculioni- dae, fig. 1(3) and the Bark-beetles (Scolytidae), the grubs, eating underground root or stem structures, mining 56 THE LIFE-STORY OF INSECTS [CH. in leaves or seeds, or tunnelling beneath the bark of trees, have no legs at all, the place of these limbs being indicated only by tiny tubercles on the thoracic segments. Such larvae as these latter are examples of the type called cruciform by A. S. Packard (1898) who as well as other writers has laid stress on the series of transitional steps from the campodeiform to the cruciform type afforded by the larvae of the Coleoptera. A fact of much importance in the transformations of beetles as pointed out by Brauer (1869) is that in a few families, the first larval instar is campodeiform, while the subsequent instars are cruciform. We may take as an example of such 'hypermetamorphosis' the life-story of the Oil or Blister-beetles (Meloidae) as first described by J. H. Fabre (1857), and later with more elaboration by H. Beauregard (1890). From the egg of one of these beetles is hatched a minute armoured larva, with long feelers, legs, and cerci, whose task is, for example, to seize hold of a bee in order that the latter may carry it, an uninvited guest, to her nest. Safely within the nest, the little 'triungulin' beetle-grub moults; the second instar has a soft cuticle and relatively shorter legs, which, as the larva, now living as a cuckoo-parasite, proceeds to gorge itself with honey, soon appear still further abbreviated. Later comes a stage during which legs are entirely wanting, the larva then resting and vi] LARVAE AND THEIR ADAPTATIONS 57 taking no food. The last larval instar again has short legs like the grub of the second period. In connection with this life-history we notice that the newly-hatched larva is not in the neighbourhood of its appropriate food. Hence the preliminary armoured and active instar is necessary in order to reach the feeding place ; this journey accomplished, the cruciform condition is at once assumed. In all cases indeed we may say that the particular larval form is adapted to the special conditions of life. A few examples from other orders of endo- pterygote insects will illustrate this point. The campodeiform type is relatively unusual, but most of the Neuroptera have larvae of this kind, active, armoured creatures with long legs, though devoid of the tail-processes often associated with similar larvae among the Coleoptera. Such are the 'Ant-lions/ larvae of the exotic lacewing flies, which hunt small insects, digging a sandy pit for their unwary steps in the case of the best-known members of the group, some of which are found as far north as Paris. In our own islands the 'Aphis-lions/ larvae of Hemerobius and Chrysopa, prowl on plants infested with 'green-fly' which they impale on their sharp grooved mandibles, sucking out the victims' juices, and then, in some cases, using the dried cuticle to furnish a clothing for their own bodies. Among these insects, while the mouth of the imago is of the 58 THE LIFE-STORY OF INSECTS [OH. normal mandibulate type adapted for eating solid food, the larval mouth is constricted and the slender mandibles are grooved for the transmission of liquid food. Turning to eruciform types of larva, we find the caterpillar (fig. 1 ft, c, d) distinguished by its elongate, usually cylindrical body with feeble cuticle, short thoracic legs and a variable number of pairs of abdominal pro-legs, universal among the moths and butterflies forming the great order Lepidoptera, and usual among the saw-flies, which belong to the Hyme- noptera. The vast majority of caterpillars feed on the leaves of plants and their long worm-like bodies with the series of paired pro-legs, are excellently adapted for their habit of clinging to twigs, and crawling along shoots or the edges of leaves as they go in search of food. Of great importance to a cater- pillar is its power of spinning silk, consisting of fine threads solidified from the secretion of specially modified salivary glands whose ducts open in the insect's mouth at the tip of the tubular tongue which forms a spinneret. On the same bush caterpillars of moths and of saw-flies may often be seen feeding together. The lepidopterous caterpillar, in our countries at least, has never more than five pairs of pro-legs, situated on the third, fourth, fifth, sixth, and tenth abdominal segments ; each of these pro-legs bears a number of vi] LARVAE AND THEIR ADAPTATIONS 59 minute booklets, arranged in a circular or crescentic pattern, which assist the caterpillar in clinging to its food-plant. The saw-fly caterpillar, on the other hand, may have as many as eight pairs of pro-legs, the series beginning on the second abdominal segment ; here, however, the pro-legs have no booklets. Among the Lepidoptera, we notice a reduction in the number of pro-legs in the 'looper' caterpillars of Geometrid moths. Here only two pairs are present, those on the sixth and tenth abdominal segments. Conse- quently, as the caterpillar can cling only by the thorax and by the hinder region of the abdomen, the middle region of the body is first straightened out and then bent into an arch-like form, as the insect makes its progress by alternate movements of stretch- ing and 'looping.' Caterpillars, with their relatively soft bodies, feeding openly on the leaves of plants, are exposed to the attacks of many enemies, and the various ways in which they obtain protection are well worth studying. A clothing of hairs 1 or spines is often present, and it is interesting to find that many species of our native Tiger and Eggar Moths (Arctia- dae and Lasiocampidae) which pass the winter in the larval stage, have caterpillars with an especially 1 The ' hairs ' of an insect are not in the least comparable to the hairs of mammals, being in truth, modified portions of the cuticle, secreted by special cells. 60 THE LIFE-STORY OF INSECTS [CH. dense hairy covering (fig. 17). Experiments have shown that hairy and spiny insects are distasteful to birds and other creatures that prey readily on smooth-skinned species, a conclusion that might well have been expected. Certain smooth caterpillars however appear to be protected by producing some nauseous secretion, which renders them unpalatable. Many of these, as the familiar cream yellow and black larva of the Magpie Moth (Abraxas grossu- lariata), are very conspicuously adorned, and furnish examples of what is known as 'warning coloration/ on the supposition that the gaudy aspect of such insects serves as an advertisement that they are not fit to eat, and that birds and other possible devourers thus learn to leave them alone. On the other hand, smooth caterpillars which are readily eaten by birds are usually ' protectively' coloured, so as to resemble their surroundings and remain hidden except to careful seekers. Many such caterpillars are green, the upper surface, which is naturally exposed to the light, being darker than the lower which is in shadow. When the caterpillar is large, the green area is often broken up by pale lines, longitudinal as on the larvae of many Owl Moths (Noctuidae) or oblique, as on the great caterpillars of most Hawk Moths (Sphingidae). Such an arrangement tends to make the insect less easily seen than were it to display a continuous area of the same colour. The 'looper' caterpillars vi] LARVAE AND THEIR ADAPTATIONS 61 mentioned above afford remarkable examples of 'protective' resemblance, for many of them show a marvellous likeness to the twigs of their food-plant, tubercles on the insect's body resembling closely the little outgrowths of the plant's cortex. It has been shown by E. B. Poulton (1892) that many cater- pillars are, in their early stages, directly responsive to their surroundings as regards colour. Usually green when hatched, they remain green if kept among leaves or young shoots of plants, while they turn red, a Fig. 17. c, Ruby Tiger Moth (PJiragmatobia fuliginosa); a, cater- pillar ; b, cocoon. After Lugger, Insect Life, vol. n. brown, or blackish if placed among twigs of these respective hues. This eifect appears to be due to a direct response of the subcutaneous tissue to the rays of light reflected from the surrounding objects. The sensitiveness dies away as the caterpillar grows older, since little or no change of hue in response to a change of environment could be induced after the penultimate moult. Among those families of the Lepidoptera which 62 THE LIFE-STORY OF INSECTS [CH. are usually regarded as low in the scale of organisa- tion, caterpillars are very generally protected by the habit of feeding in some concealed situation. For example, the great larvae of the Goat Moth (Cossus) and the whitish caterpillars of the Clear- wing Moths (Sesiidae) burrow through the wood of trees, eating the timber as they go. The little irritable caterpillars of the Bell Moths (Tortricidae) roll leaves, fastening the edges together with silk, and thus make for themselves a shelter ; or they bore their way into seeds or fruits, like the larva of the Codling Moth that is the cause of ' worm-eaten' apples, too well-known to orchard-keepers. Very many small caterpillars mine between the two skins of a leaf, eating out the soft green tissue, and giving rise to a characteristic blister in form of a spreading patch or a narrow sinuous track through the leaf. The caterpillars of the Clothes-moths (Tineidae) make for themselves garments out of their own excrement, the particles fastened together by silk. In such curious cylindrical cases they wander over the wool or fur, feeding and indirectly supplying themselves with clothing at the same time. The case-forming habit of the Clothes-moth cater- pillars leads us naturally to consider the similar habit adopted by their allies the Caddis-larvae which live in the waters of ponds and streams, for the Caddis- flies (Trichoptera) have much in common with the vi] LARVAE AND THEIR ADAPTATIONS 63 more primitive Lepidoptera. The caddis-larva is as a rule of the cruciform type, but with well-developed thoracic legs, and with hook-like tail-appendages ; by means of the latter it anchors itself to the extremity of its curious 'house.' It is of interest to note that in the earlier stages of some caddises lately described V and figured by A. J. Siltala (1907), the legs are rela- tively very long, and the larva is quite campodeiform in aspect. Some of these caddis-grubs retain the campodeiform condition and do not shelter perma- nently in cases, as their relations do. Different genera of caddises differ in their mode of building. Some fasten together fragments of water-weeds and plant refuse, others take tiny particles of stone, of which they make firmly compacted walls, others again lay hold of water-snail shells, which may even contain live inhabitants, and bind these into a limy rampart behind which their bodies are in safe hiding. The silk with which the * caddis-worms' fasten together the materials for their houses is produced from spinning-glands which like those of the Lepido- ptera open into the mouth. The survey of the various types of beetle-larvae enumerated above (pp. 50-56) concluded with a short description of the legless grub, which is the young form of a weevil or a bark-beetle. This is a larva in which the head alone has its cuticle firm and hard ; the rest of the body is covered with a pale, flexible 64 THE LIFE-STORY OF INSECTS [CH. cuticle, so that the grub is often described as ' fleshy.' This type of larva is by no means confined to certain families of the beetles, it is frequently met with, in more or less modified form, in two other important orders of insects, the Hymenoptera and the Diptera. Among the Hymenoptera this is indeed the predomi- nant larval type. We have just seen that a cater- pillar is the usual form of larva among the saw-flies, but in all other families of the Hymenoptera we find the legless grub. A grub of this order may usually be distinguished from the larva of a weevil or other beetle, by its relatively smaller head and smoother, less wrinkled cuticle; it strikes the observer as a feebler, more helpless creature than a beetle-grub. And it is of interest to note that this somewhat degraded type of larva is remarkably constant through a great series of families gall-flies, ichneumon-flies, wasps, bees (fig. 18), ants that vary widely in the details of their structure and in their habits and mode of life. Almost without exception, however, they make in some way abundant provision for their young. The feeble, helpless, larva is in every case well sheltered and well fed ; it has not to make its own way in the world, as the active armoured larva of a ground-beetle or the caterpillar of a butter- fly is obliged to do. Among those saw-flies whose larvae feed through- out life in a concealed situation, we find an interesting vi] LARVAE AND THEIR ADAPTATIONS (35 transition between the caterpillar and the legless grub. For example, the giant saw-flies (so called 'Wood- wasps') have larvae that burrow in timber, and these larvae possess relatively large heads, some- what flattened bodies with pointed tail-end, and very greatly reduced legs. The feeble legless grub, cha- racteristic of the remaining families of the Hymeno- ptera, is provided for in a well-nigh endless variety of an c CO Fig. 18. Young Larva (/'/,), Full-grown Larva (SL) and Pupa (N) of Hive-bee (Apis mcllifica}. co, cocoon; sp, spiracles; ci>t. Ayric. insect life-stories. The fly- -with its large globular head, bearing the extensive compound eyes, the highly modified feelers with their exquisitely feathered slender sensory tips, and the complex suctorial jaws ; with its compact thorax bearing the glassy fore-wings 72 THE LIFE-STORY OF INSECTS [CH. alone used for flight, though the hind-wings modified into tiny drumstick-like 'halters' are the organs of a fine equilibrating sense is perhaps the most special- ised, structurally the 'highest' of all insects. Yet in a week or two this swift, alert, winged creature is developed from the degraded maggot, white, legless, headless, that buries itself in putrid flesh, 'feeding on corruption.' The broad end of the maggot is the tail, while the narrow extremity marks the position of the mouth. Above this are a pair of very short feelers (fig. 21 c), while from the aperture project the tips of the mouth- hooks (fig. 21 e,f), formidable, black, claw-like struc- tures, articulated to the strong pharyngeal sclerites and moved by powerful muscles, tearing up the fibres of the flesh. On either side of the prothorax is an anterior spiracle, a curious branching or fan-like out- growth (fig. 21&), with a variable number of tiny open- ings which are probably of little use for the admission of air to the tubes. In many maggots the mouth- hooks and the front spiracles become more and more complex in form in the successive instars. The cuticle, white and smooth to the unaided eye, is seen on microscopic study to be set with rows of tiny spines which assist the maggot's movements through its food-mass. At the tail-end the large hind spi- racles are conspicuous on a flattened dorsal area of the ninth abdominal segment ; each shows a hard vi] LARVAE AXD THEIR ADAPTATIONS 73 brown plate, traversed by three slits. And as we watch this curious degraded larva thrusting its nar- row head-end into the depths of its ofttimes loathsome food-supply, we understand the advantage of access to the air-tube system being mainly confined to the hinder end of the body. Maggots, differing from that of the Bluebottle only in minor details, are the larval forms of a vast multitude of allied species and display great varia- tion in the nature of their food. Most, however, hide their soft defenceless bodies in some substance which affords shelter as well as food. The Bluebottle mag- got burrows into flesh, that of the House-fly into horse-dung or vegetable refuse. The maggot of the Cabbage-fly eats its way into the roots of cruciferous plants, that of the Mangel-fly works out a broad blister between the two skins of a leaf, into which the newly-hatched larva crawls directly from the egg. A large number of species, forming an entire sub- family (the Tachininae) have larvae that feed as parasites within the bodies of other insects. The habit of parasitism by maggots in back-boned animals has led to some remarkable modifications of the larva and to curious adventures in the course of the life-story. The Bot-fly of the Horse (Gastrophttus eqw) and the Warble-fly of the Ox (Hypoderma bovis, fig. 22) lay eggs attached to the hairs of grazing animals, which, at least in the case of 74 THE LIFE-STORY OF INSECTS [OH. Gastrophilus, lick the newly-hatched larvae into their mouths. The 'bot,' or maggot of Gastrophilus, comes to rest in the horse's stomach ; often a whole family attach themselves by their mouth-hooks to a small patch of the mucous coat of that organ. The maggot is relatively short and stout, with rows of strong spicules surrounding the segments, and with spiracles capable of withdrawal through a cup-like inpushing of the tail-region of the body, so that the parasite is preserved from drowning when the host drinks water. The young maggot of Hypoderma (fig. 22 e) is elongate and slender, spends its first two stages burrowing in the gullet wall and then wandering through the dorsal tissues of its host; ultimately it arrives beneath the skin of the back and assumes for its third and fourth instars a broad barrel-like form (fig. 22 b). The supply of free oxygen within the ox's tissues being now insufficient, the warble-maggot bores a circular hole through the skin and rests with the tail spiracles directed upwards towards the outer air. When fully grown the maggot works its way through the hole in the host's skin, and falling to the ground pupates in some sheltered spot, the life cycle occupying about a year. Similarly the Horse-bot escapes from the host's intestine with the excrement, and pupates on the ground. A curious modification of the maggot is noticeable in the larva of the Hover-flies (Syrphus). These, vi] LARVAE AND THEIR ADAPTATIONS 75 unlike most of their allies, live exposed on the foliage of plants, where they feed by preying on aphids. Fig. 22. Ox Warble-fly (Hiipoderma bovia), a, female ; b, full-grown maggot from back of ox, dorsal view ; c, egg; rf, empty puparium, ventral view ; e, young maggot from gullet, ventral view. Mag- nified (lines show natural size), ad, after Theobald, 2nd Report Econ. Zool. (Brit. Mus.). 76 THE LIFE-STORY OF INSECTS [CH. In agreement with this manner of life, the cuticle is roughly granulated, often greenish or reddish in hue, and the maggot, despite its want of definite head and sense organs, moves actively and purposefully about, often rearing up on its broad tail-end with an aphid victim impaled on its mouth-hooks. In a previous chapter reference was made to the exopterygote insects, stone-flies, dragon-flies, and may-flies, whose preparatory stages live in the water. Among the endopterygote orders many Neuroptera and Coleoptera, all Trichoptera, a very few Lepi- doptera and many Diptera, have aquatic larvae. One or two examples of the adaptations of dipteran larvae to life in the water may well bring the present chapter to a close. Many members of ' the hover-fly family (Syrphidae) have maggots with the tail-spiracles situated at the end of a prominent tubular process. Among the best-known of syrphid flies are the drone- flies (Eristalis), often seen hovering over flowers, and presenting a curious likeness to hairy bees. The larva of Eristalis is one of the most remarkable in the whole order, the 'Rat-tailed maggot' found in the stagnant water of ditches and pools. It has a cylin- drical body with the hinder end drawn out into a long telescopic tube, a more slender terminal section being capable of withdrawal into, or protrusion from, a thicker basal portion. At the extremity of the slender tube is a crown of sharp processes, forming vi] LARVAE AND THEIR ADAPTATIONS 77 a stellate guard to the spiracles. These processes can pierce the surface-film of the water, and place the trachea! system of the maggot in touch with the pure upper air; while its mouth may be far down, feeding among the foul refuse of the ditch, it can still reach out to the medium in which the end of its life- story must be wrought out. Reverting to the first great division of the Diptera, we find varied adaptations to aquatic life among many grubs that possess a definite head. The larva of a Gnat (Culex 1 ) has projecting from the hind region of the abdomen a long tubular outgrowth, at the end of which are the spiracles, guarded by three pointed flaps forming a valve. When closed these pierce the surface-film of the water in which the larva lives ; when opened a little cup-like depression is formed in the surface-film, from which the larva hangs. Or having accumulated a supply of air, it can disengage itself from the surface-film and dive through the water, its tracheal system safely closed. Another mode of breathing is found in the 'Blood- worms' and allied larvae of the Harlequin-midges (Chironomidae) whose transformations are described in detail by Miall and Hammond (1900). These larvae have tAvo pairs of cylindrical, spine-bearing pro-legs- one on the prothorax and the other on the hind- most abdominal segment; the latter structures serve 1 See Frontispiece, A. 78 THE LIFE-STORY OF INSECTS [CH. to fix the larva in the muddy tube which it inhabits at the bottom of its native pond. The penultimate abdominal segment has four long hollow outgrowths, which contain blood, and have the function of gills, while the hindmost segment has four shorter out- growths of the same nature. Enabled thus to breathe dissolved air, the Chironomus larva needs not, like the Culex or the Eristalis, to find contact with the atmosphere beyond the surface-film. Most remarkable, in many respects, of all aquatic larvae are the grubs of the Sand-midges (Simulium). These live entirely submerged and, having no special gills, carry out an exchange of gases through the general surface of the cuticle between the dissolved air in the water and the cavities of the air- tube system. The body is shaped like a flask swollen slightly at the hinder end and possesses a median pro-leg just behind the head, also another at the tail, which serves to attach the larva to a stone or to the leaf of an aquatic plant. The head has, in addition to feelers and jaws, a pair of processes with wonderful fringes which by their motion set up currents in the water, and bring food particles within reach of the mouth. A number of the larvae usually live in a community. Their power of spinning silken threads by which they can work their way back when acci- dentally dislodged from their resting-place, has been vividly described by Miall (1895). vi] LARVAE AXD THEIR ADAPTATIONS 79 Examples might be multiplied, but enough have been given to enforce the conclusion that the forms of insect-larvae are wondrously varied, and that frequently, within the limits of the same order or even family, modifications of type may be found which are suited to various modes of life adopted by different insects. A survey of the multitudes of insect larvae grubs, caterpillars, maggots living on land, on plants, underground, in the water ; feeding on leaves, in stems, on roots, on carrion, on refuse ; by hunting or by lurking after prey ; as parasites or as scavengers, brings home to us most strongly the con- clusion that each larva is fitted to some little niche in the vast temple of life, each is specially adapted to its part in the great drama of being. CHAPTER VII PUPAE AND THEIR MODIFICATIONS THE pupal stage is characteristic of the life-story of those insects whose larvae have wing-rudiments in the form of inpushed imaginal discs, and in all these insects there is, as we have seen, considerable divergence in form between larva and imago. In 80 THE LIFE-STORY OF INSECTS [CH. the pupa the wings and other characteristically adult structures are, for the first time, visible outwardly; is the instar which marks the great crisis in trans- formation. The pupa rests, as a rule, in a quiescent condition, and during the early period of this stage the needful internal changes, the breaking down of many larval tissues, and their replacement by imaginal organs, go on. Both outwardly and inwardly therefore, the insect undergoes, at the pupal stage, a reconstruction necessitated by the differences in form and often in habit, between the larva and the winged adult ; and the greater these differences, the more profound must be the changes that mark the pupal stage. From the prominence of imaginal structures in the pupa, it is at once seen that the pupa of any insect must resemble the adult more nearly than it resembles the larva. But in different groups of in- sects we find different degrees of likeness between pupa and imago. In a beetle pupa (see fig. 16 c), the appendages feelers, jaws, legs, wings stand out from the body as do those of the perfect insect. This type is called a free pupa. The pupal cuticle has to be shed for the emergence of the imago, but the pupa is already a somewhat reduced model of the final instar, with abbreviated wings and doubled-up legs. A free pupa is characteristic of the Coleoptera, Neuroptera, Trichoptera, Hymenoptera and many vii] PUPAE AND THEIR MODIFICATIONS 81 Diptera. In some cases the pupa requires to be specially adapted for a peculiar mode of life; for example, a special arrangement of breathing organs may be necessary for life under water, and there must needs be temporary pupal structures, not represented in the imago. On the other hand, in the pupae of most Lepido- ptera and of some Diptera, there is more or less coalescence between the cuticle of the appendages and the cuticle of the body generally, so that the appendages do not stand out, but being, as it were, glued down to the body, are somewhat masked (see fig. 1 e and fig. 23). Consequently the obtect pupa, as this type is called, does not resemble its imago as fully as a free pupa does. The outline of the wings for example in a butterfly's pupa can in some cases be traced only with difficulty. T. A. Chapman has shown (1893) that the completely obtect pupa characterises the more highly developed families of Lepidoptera, while in the more primitive families the pupa is incompletely obtect. If the pupa of a butterfly or moth be lifted and held in the hand, a bending or wriggling motion of the abdomen can be observed. In the incompletely obtect pupa, this motion is evi- dent in a greater number of segments than in the completely obtect, the number concerned varying from five to two in different families. In the nymphalid butterflies, the pupa is often called a c. i. 6 82 THE LIFE-STORY OF INSECTS [CH. 1 chrysalis' on account of the golden hue displayed by the cuticle, and the term ' chrysalis ' is sometimes bestowed indiscriminately on any kind of pupa. It has been shown by Poulton (1892) and others, that the colour of a butterfly pupa is to some extent affected by the surroundings of the caterpillar just before its last moult. Reference has been made (p. 58) to the power of spinning silk possessed by many larvae; often the principal use of this silk is to form some protection for the pupa, the larva before its last moult con- structing a cocoon within which the pupa may rest safely. Many larvae bury themselves in the earth, and the pupa lies in an earthen chamber, the lining particles of soil fastened together by fine silken threads. Larvae that feed in wood, like the cater- pillar of the Goat-moth (Cossus) make a cocoon of splinters spun together, while hairy caterpillars, such as those of the Tiger-moths, work some of their hairs in with the silk to make a firm cocoon (fig. 17 >). On the other hand, those caterpillars known as * silk- worms ' make a dense cocoon of pure silk, consisting of two layers, the outer of coarse and the inner of fine threads. Silken cocoons very similar in appear- ance are spun by the larvae of small Ichneumon-flies. Many pupae lie in a loose cocoon formed of a few interlacing threads, as for example the conspicuous black and yellow banded pupa of the Magpie-moth vii] PUPAE AND THEIR MODIFICATIONS 83 (Abraxas grossulanata) and the pupae of various leaf-beetles. Others again spin together the edges of leaves with connecting silken threads. The grubs of bees and wasps which are reared in the comb- chambers of their nests seal up the opening of the chamber with a lid, partly silk (fig. 18 GO) and partly excretion, when ready to pass into the pupal state An additional external 'capping' may be also sup- plied by the workers. The pupae of butterflies are especially interesting, as illustrating the extreme reduction of the silken cocoon. The pupa of a ' swallowtail ' (Papilionid) or a 'white' (Pierid) butterfly (fig. 23) may be found attached to a twig of its food-plant or to a wall, in an upright position, its tail fastened to a pad of silk and a slender silken girdle encircling its thorax. The pupa of a ' Tortoiseshell ' or 'Admiral' (Xymphalid) butterfly hangs head downwards from a twig, sup- ported only by the tail-pad of silk, which, useless as a shelter, serves only for attachment. The pupa is fastened to this pad by a spiny hook or process, the cremastcr (fig. 23 cr\ on the last abdominal segment. The cremaster is a characteristic structure in the pupa of a moth or butterfly. C. Y. Riley (1880) and W. Hatchett- Jackson (1890) have shown that it cor- responds with a spiny area, the suranal plate, which lies above the opening of the caterpillar's intestine. The means by which the suspended pupa of a 6-2 84 THE LIFE-STORY OF INSECTS [CH. nymphalid butterfly attaches its cremaster to the silken pad which the larva has spun in preparation for pupation, is worthy of brief attention. The caterpillar, hanging head downwards, is attached to the silken pad by its hindmost pair of pro-legs or claspers and by the suranal plate, and the cuticle is slowly worked off from before backwards, so as to expose the pupa. Were the process of moulting to be simply completed while the insect hangs by the claspers, the pupa would of course fall to the ground. But there is enough adhesion between the pupal and larval cuticles at the hinder end of the body, especially by means of the everted lining of the hind- gut, for the pupa to be supported while it jerks its cremaster out of the larval cuticle and works it into the meshes of the silken pad. The moult is thus completed and the pupa hangs securely all the time. In the numerous cases where the pupa is enclosed in a cocoon, the cremaster serves to fix the pupa to the surrounding silk. Chapman (1893) has drawn attention to the fact that among the more highly organised moths the pupa remains in the cocoon, the emergence being entirely left to the imago, while the pupae of the more primitive moths work their way partly out of the cocoon before the final moult begins. In the latter case, the cremaster is anchored by a strand of silk which allows a certain degree of emergence, and the pupa has rows of spines on its vn] PUPAE AND THEIR MODIFICATIONS 85 abdominal segments, of which a greater number Fig. 23. Pupa of White Butterfly (Pier is), side view ; /, feeler ; w, wing ; sp, spiracle; p, anal pro-leg; cr, cremas- ter. Magnified 8 times. In part after Hatchett-Jackson, Trans. Linn. Soc. 1900, and Tutt's British Butterflies. 86 THE LIFE-STORY OF INSECTS [CH. retain the power of mutual motion than in those pupae which do not come out of their cocoons. While the pupa on the whole resembles the imago that is to emerge from it, there are not a few cases in which a special structure necessary for some contingency in pupal life is retained or adopted in this stage. A butterfly pupa, like the imago, has no mandibles, but in the case of the Caddis-flies (Trichoptera) and two families of small moths, the most primitive of all Lepidoptera, the pupa, like the larva, has well-developed mandibles. These enable the caddis pupa to bite its way out of the shortened larval case in which it has pupated, and then to swim upwards through the water ready for the caddis-fly's emergence into the air. Pupae that are submerged require special breathing-organs. In the previous chapter (p. 77) mention was made of the gnat's aquatic larva with its tail-spiracles adapted for procuring atmospheric air through the surface-film. The pupa of the gnat l also has * respiratory trumpets ' serving the same purpose, but these are a pair of processes on the prothorax, so that the pupa, which is fairly active, hangs from the surface-film with its abdomen pointing downwards through the water. This change of position is correlated with the necessity for the imago to emerge into the air ; were the pupa to hang head downwards as the larva does, the gnat would 1 See Frontispiece, B. vn] PUPAE AND THEIR MODIFICATIONS 87 perforce have to dive into the water. With the beautifully adapted transfer of the functional spiracles, their position is appropriately arranged for the gnat's emergence at the surface, and the empty pupal cuticle floats serving the insect as a raft. On this it rests securely and the crumpled wings have opportunity to expand and harden before the insect takes to flight. The aquatic pupae of other Diptera, many species of the midges Chironomus and Simulium for example, breathe dissolved air by means of tufts of thread-like gills, which arise on either side of the prothorax. The pupae of Simulium rest in their curious little cup- like dwellings, attached to submerged stones or plants. The Chironomus pupa is usually found in an elongate gelatinous case adhering to a stone. From this case the pupa rises to the surface of the water, that the midge may emerge into the air. Miall and Hammond (1900) describe the arrangement by which, when the pupal stage ends, and these gills are no longer required, their connection with the air-tube system is severed 'without undue violence.' The walls of the fine air-tubes that pass into the gills are specially strengthened, but just below the pupal cuticle these walls are exceedingly thin and delicate. Thus when the pupal cuticle is cast, they are readily broken there, and the cuticle of the midge forming beneath has a spiracular opening into the main air-trunk, ready for use during the insect's aerial life. 88 THE LIFE-STORY OF INSECTS [OH. Among those Diptera whose larva is the headless maggot a most remarkable arrangement for pro- tecting the pupa is to be found. The last larval cuticle, instead of being as usual worked off and cast, after separation from the underlying structures, becomes hard and firm, forming a protective case (puparium) within which by the processes of histo- lysis and histogenesis already described the organs of the pupa and imago are built up. This puparium (fig. 22 d) is usually dark in colour, often brown and barrel-shaped, and a subcircular lid splits off from it at the head-end to allow the emergence of the fly 1 . While the maggot breathes by its tail-spiracles, the functional spiracles of the puparium (connected with the tracheal system of the enclosed pupa) are far forward, and these may be situated at the tips of long sometimes branching processes, which recall the thoracic gills of the aquatic pupae men- tioned a few pages above. Adaptations, various and beautiful, to special modes of life, are thus seen to characterise pupae as well as larvae. 1 The presence of this sub-circular lid characterises Brauer's sub- order Cyclorrhapha. Those Diptera in which the pupal cuticle splits iu the normal, longitudinal manner are included in the Orthorrhapha (see p. 67). viii] THE LIFE-STORY AXD THE SEASOXS 89 CHAPTER VIII THE LIFE-STORY AND THE SEASONS A NUMBER of interesting questions are associated with the seasonal cycle of an insect's life-history. In a previous chapter (iv. pp. 30, 34) reference has been made to the contrast between the long aquatic life of the larval dragon-fly or may -fly, extending over several years, and the short aerial existence of the winged adult restricted in the case of the may-flies to a few hours. Here we see that the feeding activi- ties of the insect are carried on during the larval stage only ; the may-fly in its winged condition takes no food, pairing and egg-laying form the whole of its appointed task. A similar though less extreme shortening of the imaginal life may be noticed in many endopterygote insects. For example, the bot- and warble-flies have the jaws so far reduced that they are unable to feed, and the parasitic life of the maggot (see p. 74) extending over eight or nine months in the body of the horse or ox, prepares for a winged existence of probably but a few days. Again in many moths the jaws are reduced or vesti- gial so that no food can be taken in the winged state, as for example in the 'Eggars' (Lasiocampidae) 90 THE LIFE-STORY OF INSECTS [CH. and the 'Tussocks' (Lymantriidae). It is noteworthy tliat in these short-lived insects the male is often provided with elaborate sense-organs which, we may believe, assist him to find a mate with as little delay as possible ; the male may-fly has especially complex eyes, while the feelers of the male silk-moth or eggar are comb-like or feathery, the branches bearing thousands of sensory hairs. A box with a captive living female of one of these moths, if taken into a wood haunted by the species becomes rapidly sur- rounded by a swarm of would-be suitors, attracted by the odour emitted from the prisoner's scent- glands. Very exceptionally the imaginal stage may be omitted from the life-story altogether. Nearly fifty years ago N. Wagner (1865) made the remarkable discovery that in the larvae of certain gall-midges (Cecidomyidae) the ovaries might become preco- ciously mature and unfertilised eggs might be de- veloped into small larvae observable within the body of the mother-larva; ultimately these abnormally reared young break their way out. In this case therefore there may be a series of larval generations, neither pupa nor imago being formed. Extended observations on the precocious reproductive processes of these midges have lately been published by W. Kahle (1908). A less extreme instance of an ab- breviated life-story was made known by 0. Grimm vin] THE LIFE-STORY AND THE SEASONS 91 (1870) who saw pupae of Harlequin-midges (Chiro- nomus) lay unfertilised eggs, which developed into larvae. Here the imaginal stage only is omitted from the life-history. Not always however is it the imaginal stage of the life-history which is shortened. Refer- ence (p. 18) has already been made to the case of the virgin female aphids, whose eggs develop within the mother's body, so that active, formed young are brought forth. Among the Diptera it is not unusual to find similar cases, the female fly giving birth to young maggots instead of laying eggs. Such is the habit of the great flesh-fly (Sarcophaga), of some allied genera (Tachina, etc.) whose larvae live as parasites on other insects, and occasionally of the Sheep Bot-fly (Oestrus). In such cases we recognise the beginning of a shortened larval period, and Br uce's investigations in 1895, summarised by E. E. Austen (1911), have shown that females of the dreaded African Tsetse flies (Glossinia) bring forth nearly / mature larvae, which pupate soon after birth. In another group of Diptera, the blood-sucking parasites of the Hippoboscidae and allied families, the whole larval development is passed through within the mother's body, and a full-grown larva is born the V f cuticle of which hardens and darkens immediately to form a puparium ; hence these flies are often called, though incorrectly, Pupipara. Still more astonishing is the mode of reproduction in the allied family of 92 THE LIFE-STORY OF INSECTS [CH. the Termitoxeniidae, curious, degraded, wingless ' guests ' of the termites, or ' white ants,' lately made known through the researches of E. Wasmann (1901). Here the individual is hermaphrodite a most ex- ceptional condition among insects and lays a large egg, whence is usually hatched a fully- developed adult! Here then Ave find that all the early stages, usual in the higher insects, are omitted from the life- story. Interesting comparison may be made between the total duration of various insect life-stories. To some extent at least, the length of an insect's life is cor- related with its size, its food, the season of the year when it breeds. Small insects have, as a rule, shorter lives than large ones; those whose larvae devour highly nutritive food generally develop more quickly than those which have to live on dry, poor, sub- stances; life-cycles follow one another most rapidly in summer weather when temperature is high and food plentiful. In early chapters we have already noticed the long aquatic life of the larva and nymph of a dragon-fly, relatively a large insect, and the rapid multiplication of the repeated summer broods of virgin aphids (p. 18). Within the one order of the Coleoptera it is instructive to compare the small jumping leaf-beetles, the 'turnip-flies' of the farmer, whose larvae mine in the green tissues, and complete vin] THE LIFE-STORY AND THE SEASONS 93 their transformations so rapidly that several succes- sive broods appear in the spring and early summer, with the larger click-beetles whose larvae, the equally notorious 'wireworms/ feed on roots for three or four years before they become fully grown. Among the Diptera, the 'leather-jacket' grub of the crane-fly, feeding like the wireworm on roots, has a larval life extending through the greater part of a year, while the maggot of the bluebottle, feeding on a rich meat diet, becomes mature in a few days. As examples of excessively long life-cycles the ' thirteen-year ' and ' seventeen-year ' cicads of North America, described by C. L. Marlatt (189o), are noteworthy. Certain specially populous 'broods' of these insects are known and localised, so that the appearance of the imagos in future years can be accurately predicted. Here again we have to do with bulky insects Avhose subterranean larvae and nymphs feed on compara- tively innutritions roots. In our own climate, it is of interest tojiotice the variation among insects as to the stage which carries the race over the winter. The click-beetles, men- tioned just above, emerge from their buried pupae in summer, hibernate under stones or clods, and lay eggs among the herbage next spring. At the same time of course, owing to the extended term of the larval life, many more individuals of the species are wintering underground as ' wireworms ' of various 94 THE LIFE-STORY OF INSECTS [CH. ages, and these, except in very severe frosts, can continue their occupation of feeding on roots. But in the case of the t turnip-flies ' the food-supply is cut off in winter, and all those beetles of the latest summer brood that survive hibernate in some shel- tered spot, waiting for the return of spring, that they may lay their eggs, and start the life-cycle once again. Among the Diptera, most species pass the winter as pupae, the sheltering puparium being a good protection against most adverse conditions, or as flies. But where there is a prolonged parasitic larval life, as with the bot- and warble-flies, the maggot, warm and well-fed within the body of its mammalian host, affords an appropriate wintering stage. Among the Hymenoptera an especially interesting seasonal life-cycle is afforded by the alternation of summer and winter generations in many Gall-flies (Cynipidae) as H. Adler (1881, 1896) demonstrated for most of our common species. The well-known ' oak-apples' are tenanted in summer by grubs, which after pupation develop into winged males and wingless females. The latter, after pairing, burrow under- ground and lay their eggs in the roots, the larvae causing the presence there of globular swellings or root-galls within which they live, pass through their transformations and develop into wingless virgin females. These shelter until February or March in vin] THE LIFE-STORY AND THE SEASONS 95 their underground chambers, then climb up the tree and lav on the shoots eggs, from which will be hatched c/ the grubs destined to grow within the oak-apples into the summer sexual brood of flies. The Lepidoptera afford examples of hibernation in all stages of the life -history. In this order a few large moths with wood-boring caterpillars, the 'Goat' (Cossus) for example, undergo a development extend- ing over several years, while at the other extreme a few small species may have three or more complete cycles within the twelve months. But in the vast majority of Lepidoptera we find either one or two generations, definitely seasonal, within the year ; the insect is either 'single-brooded' or 'double- brooded.' Almost every winter one or more letters may be read in some newspaper recording the writer's sur- prise at seeing on a sunny day during the cold season, one of our common gaily-coloured butterflies / of the Vanessa group, a ' Tortoiseshell ' or 'Red Ad- miral,' flitting about. Surprise might be greater did the observers realise that the imaginal is the normal hibernating stage for these species. Emerging from the pupa in late summer or autumn, the} 7 shelter during winter in hollow trees, under thatched eaves, in outbuildings or in similar situations, coming out in spring to lay their eggs on the leaves of their caterpillars' food-plants. The larvae feed and grow 96 THE LIFE-STORY OF INSECTS [OH. through the early summer months, in the case of the Small Tortoiseshell ( Vanessa urticae) pupating before midsummer and developing into a July brood of butterflies whose offspring after a late summer life- cycle, hibernate; while for the larger species of the group there is, in our islands, only one complete life- cycle in the year, though the same insects in warmer countries may be double-brooded. C. G. Barrett records (1893, vol. I. pp. 153-4) how in the August of 1879 hundreds and thousands of ' Painted Ladies ' (Pyrameis cardui) migrated into the south of England from the European continent where in many places great swarms had been observed early in the summer. 'These August butterflies, the progeny of the June swarms, coming from a warmer climate, had no in- tention of hibernating, but paired and laid eggs. Some of the larvae were collected and reared indoors [butterflies] emerging in November and December, but out of doors all must have been destroyed by damp or frost, in either the larva or pupa state, for no freshly emerged specimens were noticed in the spring, and no trace of the great migration remained.' In September and October the pedestrian, even in a suburban square, may see moths with pretty brown, white-spotted wings flying around trees. These are males of the common * Vapourer \0rgyia antiqua), in search of the females which, wingless and helpless, rest on the cocoons surrounding the pupae whence vin] THE LIFE-STORY AND THE SEASONS 97 they have just emerged, the cocoons being attached to the branches of the trees where the caterpillars have fed. After pairing, the female lays her eggs among the silk of the cocoon, partly covering them with hairs shed from her body, and then dies. The eggs thus protected remain through the winter, the larvae not being hatched till springtide, when the young leaves begin to sprout forth. The caterpillars, adorned and probably protected by their 'tussocks' of black or coloured bristles, feed vigorously. Their activity and habit of occasional migration from one tree to another, compensates, to some extent, as Miall (1908) has pointed out, for the females' enforced passivity ; only in the larval state can moths with such wingless females extend their range. The cater- pillars spin their cocoons towards the end of summer, and then pupate, the moths emerging in the autumn and the eggs, as we have seen, furnishing the winter stage. After midsummer, the conspicuous cream, black and yellow-spotted 'Magpie' moth (Abraxas grossu- lariata) is common in gardens. The female lays her eggs on a variety of shrubby plants; gooseberry and currant bushes are often chosen. From the eggs caterpillars are hatched in autumn, but these, instead of beginning to feed, seek almost at once for rolled- up leaves, cracks in walls, crannies of bark, or similar places, which may afford winter shelters. Here they c. i. 98 THE LIFE-STORY OF INSECTS [CH. remain until the spring, when they come out to feed on the young foliage and grow rapidly into the con- spicuous cream, yellow and black ' looper ' caterpillars mentioned in a previous chapter (p. 60). These, when fully-grown, spin among the twigs of the food- plant a light cocoon, in which the black and yellow- banded wasp like pupa spends its short summer term before the emergence of the moth. An equally familiar garden insect, the common 'Tiger' moth (Arctia caia) with its 'woolly bear' caterpillar, affords a life-cycle slightly differing from that of the ' Magpie.' The gaudy winged insects are seen in July and August, and lay their eggs on a great variety of plants. The larvae hatched from these eggs begin to feed at once, and having moulted once or twice and attained about half their full size, they rest through the winter, the dense hairy covering wherewith they are provided forming an effective protection against the cold. At the approach of spring they begin to feed again, and the fully-grown i woolly bear' is a common object on garden paths in May and June. Before midsummer it has usually spun its yellow cocoon under some shelter on the ground and changed into a pupa. Another modification with respect to seasonal change is shown by the Turnip moth (Agrotis segetmn) and other allied Xoctuidae (Owl-moths). These are insects with brown-coloured wings, flying after dark vin] THE LIFE-STORY AND THE SEASONS 99 in June. The dull greyish larvae feed on many kinds of low-growing 1 plants, usually hiding- in the earth by day and wandering along the surface of the ground by night, biting off the farmer's ripening corn, or burrowing into his turnips or potatoes. On account of the burrowing habits of this insect it can feed throughout the winter, except when a hard frost puts a temporary stop to its activity. By April it has become fully grown and pupates in an earthen chamber a few inches below the surface. The Turnip moth in our countries is partially double-brooded, a minority of the autumn caterpillars growing more rapidly than their comrades so that they pupate, and a second brood of moths appear in September. These pair and lay eggs, the resulting caterpillars going as Barrett suggests (1896, vol. in. p. 291) 'to reinforce the great army of wintering larvae.' / Such underground caterpillars, to a great extent protected from cold, can continue to feed through the winter. With other species we find that the larva becomes fully grown in autumn, yet lives through the winter without further change. This is the case with the Codling moth (Carpocapsa pomo- ndla\ a well-known orchard pest, which in our countries is usually single-brooded. The moth is flying in May and lays her eggs on the shoots or leaves of apple-trees, more rarely on the fruitlets, into which however the caterpillar always bores by 72 100 THE LIFE-STORY OF INSECTS [OH. the upper (calyx) end. Here it feeds, growing with the growth of the fruit, feeding on the tissue around the cores, ultimately eating its way out through a lateral hole, and crawling upwards if its apple-habi- tation has fallen, downwards if it still remains on the bough, to shelter under a loose piece of bark where it spins its cocoon about midsummer and hibernates still in the larval condition. Not until spring is the pupal form assumed, and then it quickly passes into the imaginal state. In the south of England, as F. V. Theobald (1909) has lately shown, and also in south- western Ireland, this species may be double-brooded, the usual condition on the European continent and in the United States of America. There the mid- summer larvae pupate at once and the moths of an August brood lay eggs on the hanging or stored fruit ; in this case, again, however, the full-grown larva, quickly fed-up within the developed apples, is the wintering stage. Several of the insects mentioned in this survey, like the last-named codling moth, are occasionally double-brooded. As an example of the many Lepi- doptera, which in our islands have normally two complete life-cycles in the year, we may take the very familiar White butterflies (Pieris) of which three species are common everywhere. The appearance of the first brood of these butterflies on the wing in late April or May is hailed as a sign of advanced vin] THE LIFE-STORY AND THE SEASONS 101 spring-time. They pair and lay their eggs on cabbages and other plants, and the green hairy caterpillars feed in June and July, after which the spotted pupae may be found on fences and walls, attached by the silken tail-pad and supported by the waist-girdle. In August and September butterflies of the second brood have emerged from these and are on the wing ; their off- spring are the autumn caterpillars which feed in some seasons as late as November, doing often serious damage to the late cruciferous crops before they pupate. The pupae may be seen during the winter months, waiting for the spring sunshine to call out the butterflies whose structures are being formed beneath the hard cuticle. Reviewing the small selection of life-stories of various Lepidoptera just sketched, we notice an in- teresting and suggestive variety in the wintering stage. The vanessid butterflies hibernate as images ; the 'vapourer' winters in the egg, the magpie as a young ungrown larva, the ' tiger ' as a half-size larva ; the Agrotis caterpillar feeds through the winter, growing all the time ; the codling caterpillar com- pletes its growth in the autumn, and winters as a full-size resting larva ; lastly, the ' whites ' hibernate in the pupal state. And in every case it is note- worthy that the form or habit of the wintering stage is well adapted for enduring cold. Our native ' whites ' afford illustration of another 102 THE LIFE-STORY OF INSECTS [CH. interesting feature often to be noticed in the life- story of double-brooded Lepidoptera. The butterflies of the spring brood differ slightly but constantly from their summer offspring, affording examples of what is called seasonal dimorphism. All three species have whitish wings marked with black spots, larger and more numerous in the female than in the male. In the spring butterflies these spots tend towards reduction or replacement by grey, while in the summer insects they are more strongly defined, and the ground colour of the wings varies towards yellowish. In the 'Green- veined' white (Pieris napi) the characteristic greenish-grey lines of scaling beneath the wings along the nervures, are much broader and more strongly marked in the spring than in the summer generation, whose members are distinguished by systematic entomologists under the varietal name napaeae. The two forms of this insect were discussed by A. Weismann in his classical work on the Seasonal Dimorphism of butterflies (1876). He tried the effect of artificially induced cold conditions on the summer pupae of Pieris napij and by keeping a batch for three months at the temperature of freezing water, he succeeded in completely changing every individual of the summer generation into the winter form. The reverse of this experiment also was attempted by Weismann. He took a female of brt/oniae, an alpine and arctic variety of Pieris napi, showing in an vin] THE LIFE-STORY AND THE SEASONS 103 intensive degree the characters of the spring brood. This female laid eggs the caterpillars from which fed and pupated. The pupae although kept through the summer in a hothouse all produced typical b)'yoiti