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LEPIDOPTERA (Gr. ?emir, a scale or hu...

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Originally appearing in Volume V16, Page 469 of the 1911 Encyclopedia Britannica.
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LEPIDOPTERA (Gr. ?emir, a scale or husk, and 7rrepov, a wing), a term used in zoological classification for one of the largest and best-known orders of the class Hexapoda (q.v.), in order that comprises the insects popularly called butterflies and moths. The term was first used by Linnaeus (1735) in the sense still accepted by modern zoologists, and there are few After Edwards, Riley and Howard's Insect Life, vol. 3 (U.S. Dept. Age.). Flo. I.—e, Crytophasa unipuctata, Donov., Australia. a, Larva; c, pupa, natural size; b, 2nd and 3rd abdominal segments of larva; d, cremaster of pupa, magnified. groups of animals as to whose limits and distinguishing characters less controversy has arisen. Characters.—The name of the order indicates the fact that the wings (and other parts of the body) are clothed with flattened ideas. His daughter, Suzanne Louise, was "adopted by the cuticular structures—the scales (fig. 7)—that may be regarded able- number of Lepidoptera take no food in the imaginal state; as modified arthropodan " hairs." Such scales are not peculiar in these the maxillae are reduced" or altogether atrophied. The second maxillae are intimately fused together to form the labium, to the Lepidoptera—they are found also on many of the Aptera, which consists only on the Psocidae, a family of Corrodentia, on some Coleoptera of a reduced men-(beetles) and on the gnats (Culicidae), a family of Diptera. The turn, bearing some-most distinctive structural features of the Lepidoptera are to times vestigial lobes be found in the jaws. The mandibles are mere vestiges or and palps. Thelways e sepahair ovef entirely absent; the second maxillae are usually reduced to a two or three segnarrow transverse mentum which bears the scale-covered ments and are labial palps, between which project the elongate first maxillae, clothed withescales. grooved on their inner faces, so as to form when apposed a The"form arinal tion of the terminal tubular proboscis adapted for sucking liquid food. segment of the labial All Lepidoptera are hatched as the cruciform soft-bodied pale afford valuable type of larva (fig. 1, a) known as the caterpillar, with biting characters in classimandibles, three pairs of thoracic legs and with a variable In the thorax of number (usually five pairs) of abdominal prolegs, which carry the Lepidoptera the complete or incomplete circles of hooklets. The pupa in a foremost segment or A. single family only is free (i.e. with the appendages free from the prothoraxd isnot very body), and mandibulate. In the vast majority of the order able , small onanthe meso- mov- it is more or less obtect (i.e. with the appendages fixed to the thorax. In many cuticle of the body) and without mandibles (fig. 1, c). families it carries a pair of small erectile Structure.—The head in the Lepidoptera is sub-globular in shape plates—the patagia with the compound eyes exceedingly well developed, and with a —which have been pair of ocelli or " simple eyes " often present on the vertex. It is regarded as serially FIG. 4.—Arrangement of the jaws. in a connected to the thorax by a relatively broad and membranous homologous with the typical Moth. Somewhat f. the aws. and " neck." The feelers are many-jointed, often they are complex, wings. The meso- in part after E. Burgess and V. L. Kellogg the segments bearing thorax is extensive; processes arranged in its scutum forming (Amer. Nat. xiv. xxix.). a comb-like manner most of the dorsal A, Front view of head. and furnished with thoracic area and c, Clypeus. nu me r o us sensory small plates—teg- e, Compound eye. hairs (fig. 2). The u la a--are often m, Vestigial mandible. complexity of the present at the base 1, Labrum. feelers is carried to of the forewings, as g, Galeae of 1st maxillae. " its highest develop- in Hymenoptera. P, Labial palp. Magnified, B. [head. ment in certain male The tegulae which b, Base of first maxilla dissected out of the moths chat have a are beset with long p, Vestigial pale. wonderful power of hair-like scales are g, Galea. Further magnified. discovering their often conspicuous: C, Part transverse section showing how the females by smell or The metathorax is channel (A) of the proboscis is formed From Riley and Howard, Insect Life, vol. 7 (U.S. Dept. some analogous sense. smaller than the by the interlocking of the grooved inner Agr.). Often the feelers are metathorax. The faces of the flexible maxillae. FIG. 2.a, Feeler of Saturniid Moth (Telea excessively complex legs are of the typical t, Air-tube: polyphemus). b, c, Tips of branches, highly in male moths whose h e x a p o d a n form n, Nerve. magnified. maxillae are so re- with five-segmented m, Muscle-fibres. Highly magnified. duced that they take feet; the shins often no food in the imaginal state. The nature of the jaws has already bear terminal and median spurs articulated at their bases and the been briefly described. Functional mandibles of peculiar form entire'limbs are clothed with scales. (fig. 3, A) are present in the remarkable small moths of the genus The wings of theLepidoptera may be said to dominatethe-structure Micropleryx (or Eriocephala), and there are vestiges of these jaws of the insect; only exceptionally, in certain female moths, are they in other moths of low type, but the minute structures in the higher vestigial or absent (fig. 17). The forewing, with: its prominent apex, Lepidoptera that were formerly described as mandibles are now is longer than the believed to belong to the labrum, the true mandibles being perhaps hindwing, and the represented by rounded prominences, neuration in both not articulated with the head-capsule. (see figs. 5 and 6) is Throughout the order, as a whole, for the most part the jaws are adapted for sucking longitudinal, only a liquid food, and the suctorial pro- few transverse neeboscis (often erroneously called- a vures, which are, in " tongue ") is formed as was shown fact, branches of the by J. C. Savigny in 1816 by two median trunk, elongated and flexible outgrowths of marking off a disthe first maxillae, usually regarded coidal areolet or as' representing the outer lobes or "cell " (fig. 5, a). galeae (fig. 4, A, B, g). These struc- The five branches of tures are grooved along their inner the radial nervure faces and by means of a series of (figs. 5, 6, 3) (see interlocking hair-like bristles can be HEXAPODA) are joined together so as to form a usually present in After A. Walter (Jen. Zeiss. Jr. tubular sucker (fig. 4, C). At their the forewing, but Natu,w. vol. 18). extremities they are beset with club- the hindwing, in Fie. 3.—A, Mandible, and like sense-organs, whose apparent most families, has B, 1st maxilla of Micropleryx function is that of taste. The pro- only a single radial (Eriocephala). Magnified. boscis when in use is stretched out nervure; its anal a, Palp. d, Stipes. in front of the head and inserted area is, however, a, Galea. Cardo of into the corolla of a flower or else- often more strongly b Lacinia. e ,, maxilla. where, for the absorption of liquid c, nourishment. When at rest, the proboscis is rolled up into a close spiral beneath the head and between the labial palps (fig. 4, A, P). Only in the genus Micropleryx mentioned above is the lacinia of the maxilla (as A. Walter has shown) developed (fig. 3, B, c). The maxillary palp is usually a mere vestige (fig. 4, B, p) though it is conspicuous in a few families of small moths. A consider- a 2 t ~ si ? S2 sir $3 a After A. S. Packard, Mem. Nat, Acad. Sci. vol. vii: two wings of a side are usually kept together during flight by a few stout bristles—the frenulum—(fig. 5, f) projecting from the base of the costa of the hindwing and fitting beneath a membranous fold or a few thickened scales—the retinaculum—on the under surface of the forewing. In butterflies there is no frenulum, but a costal outgrowth of the hindwing subserves the same function. In the most primitive for example–the testes retain the primitive paired arrangement. These details have been worked out by various students, among whom W. H. Jackson and W. Petersen deserve special mention. Summing up the developmental history of the genital ducts, Jackson remarks that there is " an Ephemeridal stage, which ends towards the close of larval life, an Orthopteran stage, indicated during the quiescent period preceding pupation, and a Lepidopteran stage which begins with the commencement of pupal life." Development—Many observations have been made on the embryology of the Lepidoptera; for some of the more important moths a small lobate outgrowth—the jugum (fig. 6, j.)—from the dorsum of the forewing is present, but it can be of little service in keeping the two wings tlogether. A jugum may be also present on the hindwing. The legs, which are generally used for clinging rather than for walking, have five-segmented feet and are covered with scales. In some families the front pair are reduced and without tarsal segments. Ten abdominal segments are recognizable in many Lepidoptera, but the terminal segments are reduced or modified to form external organs of reproduction. In the male, according to the interpretation of C. Pey- s toureau, the lateral plates belonging to the ninth segment form paired claspers beset with harpes, or series of ridges or teeth, while the tergum of the tenth segment forms a dorsal hook —the uncus—and its sternum a ventral process or scaphium. In the female the terminal segments form, in some cases, a protrusible ovipositor, but g the typical hexapodan ovi- After Packard, Men. Nat. Acad. Sci. vol. vii. positor with its three pairs of processes is undeveloped Moth (Hepialid). j, Jugum. Ner- As already mentioned, vures numbered as in fig. 5. Note the characteristic scales on that there are five branches to the the wings, legs and body radial nervure (No. 3) in both fore- of the Lepidoptera are and hindwing, and that the median cuticular structures. A trunk nervures (No. 4) traverse the complete series of transi- discoidal areolet. tional forms can be traced between the most elaborate flattened scales (fig. 7, B) with numerous longitudinal striae and a simple arthropod " hair." Either a " hair " or a scale owes its origin to a special cell of the ectoderm (hypodermis), a process from which grows through the general cuticle and forms around itself the substance of the cuticular appendage. The scales on the wings are arranged in regular rows (fig. 7, A), and the general cuticle is drawn out into a narrow neck or collar around the base of each scale. The scales can be easily rubbed from the surface of the wing, and the series of collars in which the scales rest are then evident (fig. 7, A, c) on the wing-membrane. On the wings of many male butterflies there are specially modified scales—the androconia (fig. 7, C)—which are formed by glandular cells and diffuse a scented secretion. In some cases, the androconia are mixed among the ordinary scales; in others they are associated into conspicuous " brands " (see fig. 66). The admirable colours of the wings of the Lepidoptera are due partly to pigment in the scales—as in the case of yellows, browns, reds and blacks—partly to " interference " effects from the fine striae on the scales—as with the blues, purples and greens. A few points of interest in the in- ternal structure of the Lepidoptera deserve mention. The mouth opens into a sub-globular, muscular pharynx which is believed to suck the liquid _ food through the proboscis, and force it along the slender gullet into a crop-like enlargement or diverticulum of the B fore-put known as a " food-reservoir" androconium more highly and gives off sub-oesophageal ganglia rare magnified. closely approximated; there are two or three thoracic and four (rarely five) abdominal ganglia. In the female each ovary has four ovarian tubes, in which the large egg-cells are enclosed in follicles and associated with nutritive cells. There is a special bursa which in the Hepialidae opens with the vagina on the eighth abdominal sternum. In the Micropterygidae, Eriocraniidae and the lower Tineides, the duct of the bursa leads into the vagina, which still opens on the eighth sternum. But in most Lepidoptera, the bursa opens by a vestibule on the eighth sternum, distinct from the vagina, whose opening shifts back to the ninth, the duct of the bursa being connected with the vagina by a canal which opens opposite to the spermatheca. In the male, the two testes are usually fused into a single mass, and a pair of tubular accessory glands open into the vasa deferentia or into the ejaculatory duct. In a few families—the Hepialidae and Saturniidae results of these see HEXAPODA. The post-embryonic development of Lepidoptera is more familiar, perhaps, than that of any other group of animals. The egg shows great variation in its outward form, the outer envelope or chorion being in some families globular, in others flattened, in others again erect and sub-conical or cylindrical; while its surface often exhibits a beautifully regular series of ribs and furrows. Throughout the order the larva is of the form known as the caterpillar (fig. I, a, b, fig. 8 B) characterized by the presence of three pairs of jointed and clawed legs on the thorax and a variable number of pairs of abdominal " prolegs "—sub-cylindrical outgrowths of the abdominal segments, provided with a complete or incomplete circle of hooklets at the extremity. There are ten abdominal segments—the ninth often small and concealed; prolegs are usually present on the third, fourth, fifth, sixth and tenth of these segments. The head of the caterpillar (fig. 9) is large with firmly chitinized cuticle ; it carries usually twelve simple eyes or ocelli, a pair of short feelers (fig. 9 At) and a pair of strong mandibles (fig. 9, Mn), for the caterpillar feeds by biting leaves or other plant-tissues. The first maxillae, so highly developed in the imago, are in the larva small and inconspicuous appendages, each bearing two short jointed processes,—the galea and the palp (fig. 9, Mx). The second maxillae form a plate-like labium on whose surface projects the Lm Mx spinneret which is usually regarded as a modified hypopharynx (fig. 9, FIG. 9.—Head of Goat Moth Lm). The silk-glands whose ducts Caterpillar (Cossus) from be-open on this spinneret are paired hind. Magnified. (From Miall convoluted tubes lying alongside and Denny after Lyonnet.) the elongate cylindrical stomach. At, Feeler. In the common " silkworm " these Mn, mandible. Elands are five times as long as the Mx, First maxilla. bony of the caterpillar. They are re- Lm, Second maxillae (Labgarded as modified salivary glands, 'um) with spinneret. though the correspondence has been doubted by some students. The body of the caterpillar is usually cylindrical and wormlike, with the 2 segmentation well marked and the cuticle feebly chitinized and 1 flexible. Firm chitinous plates are, however, not seldom present on the prothorax and on the hindmost abdominal segment. The Legments are mostly provided with bristle or spine-bearing tubercles, whose arrangement has lately been shown by H. G. Dyar to give partially trustworthy indications of relationship. On either side of the median line we find two dorsal or trapezoidal tubercles (Nos. I and 2), while around the spiracle are grouped (Nos. 3, 4 and 5) supra-, post-, and pre-spiracular tubercles; below are the subspiraculars, of which there may be two (Nos. 6, 7). The last-named is situated on the base of the abdominal proleg, and yet another tubercle (No. 8) may be present on the inner aspect of the proleg. The spiracles are very conspicuous on the body of a caterpillar, occurring on the prothorax and on the first eight abdominal segments. Various tubercles may become coalesced or aborted (fig. to, B); often, in conjunction with the spines that they bear, the tubercles serve as a valuable protective armature for the caterpillar. Much discussion has taken place as to whether the abdominal prolegs are or are not developed directly from the embryonic abdominal appendages. In the more lowly families of Lepidoptera, these organs are provided at the extremity with a complete circle of hooklets, but in the more highly organized familie:, only the inner half of this circle is retained. The typical Lepidopteran pupa, or " chrysalis," as shown in the higher families, is an obtect pupa (fig. II) with no trace of mandibles, the appendages being glued to the body by an exudation, and A B B, after Grote, Mitt. aus dem Roemer Museum, No. 6. is explained in the text. Note that in FIG. 11.—Pupa B No. 2 is much reduced and disappears of a Butterfly after the first moult. 4 and g are (Amathusia phi- coalesced, and 6 is absent. dippus). motion being possible only at three of the abdominal intersegmental regions, the fifth and sixth abdominal segments at most being " free." A flattened or pointed process—the cremaster—often prominent at the tail-end, may carry one or several hooks (fig. I, d) which serve to anchor the pupa to its cocoon or to suspend butterfly-pupae from their pad of silk (fig. II). In the lower families the pupa (fig. I, c) is only incompletely obtect, and a greater number of abdominal segments can move on one another. The seventh abdominal segment is, in all female lepidopterous pupae, fused with those behind it; in the male " incomplete " pupa this becomes " free " and so may the segments anterior to it, in both sexes, for-ward to and including the third. The presence of circles of spines on the abdominal segments enables the " incomplete " pupa as a whole to work its way partly out of the cocoon when the time for the emergence of the imago draws near. In the family of the Eriocraniidae (often called the Micropterygidae) the pupa resembles that of a caddis-fly (Trichopteron) being active before the emergence of the imago and provided with strong mandibles by means of which it bites its way out of the cocoon. The importance of the pupa in the phylogeny and classification of the Lepidoptera has lately been demonstrated by T. A. Chapman in a valuable series of papers. Sometimes organs are present in the pupa which are undeveloped in the imago, such as the maxillary palps of the Sesiidae (clearwing moths) and the pectination on the feelers of female Saturniids. E. B. Poulton has drawn attention to the ancestral value of such characters. Habits and Life-Relations.—The attractiveness of the Lepidoptera and the conspicuous appearance of many of them have led to numerous observations on their habits. The method of feeding of the imago by the suction of liquids has already been mentioned in connexion with the structure of the maxillae and the food-canal. Nectar from flowers is the usual food of moths and butterflies, most of which alight on a blossom before thrusting the proboscis into the corolla of the flower, while others —the hawk moths (Sphingidae) for example—remain poised in theair in front of the flower by means of excessively rapid vibration of the wings, and quickly unrolling the proboscis sip the nectar. Certain flowers with remarkably long tubular corollas seem to be specially adapted for the visits of hawk moths. Some Lepidoptera have other sources of food-supply. The juices of fruit are often sought for, and certain moths can pierce the envelope of a succulent fruit with the rough cuticular outgrowths at the tips of the maxillae, so as to reach the soft tissue within. Animal juices attract other Lepidoptera, which have been observed to suck blood from a wounded mammal; while putrid meat is a familiar " lure " for the gorgeous " purple emperor " butterfly (A pat ura iris). The water of streams or the dew on leaves may be frequently sought by Lepidoptera desirous of quenching their thirst, possibly with fatal results, the insects being sometimes drowned in rivers in large numbers. Members of several families of the Lepidoptera—the Hepialidae, Lasiocampidae and Saturniidae, for example—have the maxillae vestigial or aborted, and take no food at all after attaining the winged condition. In such insects there is a complete " division of labour " between the larval and the imaginal instars, the former being entirely devoted to nutritive, the latter to reproductive functions. Of much interest is the variety displayed among the Lepidoptera in the season and the duration of the various instars. The brightly coloured vanessid butterflies, for example, emerge from the pupa in the late summer and live through the winter in sheltered situations, reappearing to lay their eggs in the succeeding spring. Many species, such as the vapourer moths (Orgyia), lay eggs in the autumn, which remain unhatched through the winter. The eggs of the well-known magpie moths (Abraxas) hatch in autumn and the caterpillar hibernates while still quite small, awaiting for its growth the abundant food-supply to be afforded by the next year's foliage. The codlin moths (Carpocapsa) pass the winter as resting full-grown larvae, which seek shelter and spin cocoons in autumn, but do not pupate until the succeeding spring. Lastly, many of the Lepidoptera hibernate in the pupal stage; the death's head moth (Acherontia) and the cabbage-white butterflies (Pieris) are familiar examples of such. The last-named insects afford instances of the " double-brooded " condition, two complete life-cycles being passed through in the year. The flour moth (Ephestia kiihniella) is said to have five successive generations in a twelvemonth. On the other hand, certain species whose larvae feed in wood or on roots take two or three years to reach the adult stage. The rate of growth of the larva depends to a great extent on the nature of its food, and the feeding-habits of caterpillars afford much of interest and variety to the student. The contrast among the Lepidoptera between the suctorial mouth of the imago and the biting jaws of the caterpillar is very striking (cf. figs. 4 and q), and the profound transformation in structure which takes place is necessarily accompanied by the change from solid to liquid food. The first meal of a young caterpillar is well known to be often its empty egg-shell; from this it turns to feed upon the leaves whereon its provident parent has laid her eggs. But in a few cases hatching takes place in winter or early spring, and the young larvae have then to find a temporary food until their own special plant is available. For example, the cater-pillars of some species of Xanthia and other noctuid moths feed at first upon willow-catkins. On the other hand, the caterpillars of the pith moth (Blastodacna) hatched at midsummer, feed on leaves when young, and burrow into woody shoots in autumn. All who have tried to rear caterpillars know that, while those of some species will feed only on one particular species of plant, others will eat several species of the same genus or family, while others again are still less particular, some being able to feed on almost any green herb. It is curious to note how certain species change their food in different localities, a caterpillar confined to one plant in some localities being less particular elsewhere. Individual aberrations in food are of special interest in suggesting the starting-point for a change in the race. When we consider the vast numbers of the Lepidoptera and the structural modifications which they have undergone, their generally faithful adherence to a vegetable diet is remarkable. The vast majority the smut-covered bark of the tree, it roams about during its later stages, devouring several coccids every day. So nutritious is the food, that four or five successive broods follow each other through the summer. The habit just mentioned of forming some kind of protective covering out of foreign substances spun together by silk is practised by caterpillars of different families. The clothes moth larvae (Tinea, fig. 14), for example, make a tubular dwelling out of caterpillars eat leaves, usually devouring them openly, and, if of large size, quickly reducing the amount of foliage on the plant. But many small caterpillars keep, apparently for the sake of concealment, to the under surface of the leaf, while others burrow into the green tissue, forming a characteristic sinuous " mine " between the two leaf-skins. In several families we find the habit of burrowing in woody stems,—the " goat " (Cossus, fig. 8) and the cicarwings (Sesiidae), for example, while others, like the larvae of the swift moths (Hepialidae) live underground devouring roots (fig .12). The richer nutrition in the green food is usually shown by the quicker growth of the numerous cater-pillars that feed on it, as compared with the slower development of the wood and root-feeding species. Aquatic larvae are very rare among the Lepidoptera The caterpillars of the pyralid " china-mark " moths (Hydrocampa, fig. 13), however, live under water, feeding on duckweed (Lonna) and breathing atmospheric air, a film of which is enclosed in a spun-up shelter beneath the leaves, while the larvae of Paraponyx, which feed on Stratiotes, have closed spiracles and breathe dissolved air by means of branchial filaments along the sides of the body: campa aquatilis (water moth). We may now turn to instances of more anomalous modes of feeding. The clothes moths (Tineids) have invaded our dwellings and found a congenial food-stuff for their larvae in our garments. A few small species of the same group are reared in meal and other human food-stores; so are the caterpillars of some pyralid moths (Ephestia), while others (Asopia, Aglossa) feed upon kitchen refuse. Two species of crambid moths (Aphomia sociella and Galleria melonella) find a home in bee-hives, where their caterpillars feed upon the wax, while the waxy secretion from the body of the great American lantern-fly (Fulgora candelaria) serves both as shelter and food for the caterpillar of the moth Epipyrops anomala. Very few caterpillars have developed a thoroughly carnivorous habit. That of Cosmia trapezina feeds on oak and other leaves, but devours smaller caterpillars which happen to get in its way, and if shaken from the tree, eats other larvae while climbing the trunk. Xylina ornithopus and a few other species are said to be always carnivorous when opportunity offers; the small looping caterpillar of a " pug " moth (Eupithecia coronata) has been observed to eat a larva three times as big as itself. The caterpillars of Orthosia pistacina live together in peace while their food is moist, but devour each other when it dries up; this is true cannibalism—a term which should not be applied to the habit of preying on another species. A few carnivorous caterpillars do not attack other caterpillars, but prey upon insects of another order; among these Fenescia tarquinius, which eats aphides, and Erastria scitula, which feeds upon scale insects, must be reckoned as benefactors to mankind. The life-history of the latter moth has been worked out by H. Rouzaud. It inhabits the shores of the Mediterranean, and its caterpillar devours the coccids upon various fruit-trees, especially the black-scale (Lecanium oleae) of the olive. The moth, which is a small noctuid, the white markings on whose wings give it the appearance of a bird-dropping when at rest in the daytime, appears in May, and lays her eggs, singly and far apart, upon the trees infested by the coccids. When hatched, the young caterpillar selects a large female coccid, eats its way through the scale, and devours the insect beneath; having done this it makes its way to a fresh victim. As it increases in size it forms a case for itself made of the scales of its victims, excrement, &c., bound together by silk which it spins, and, protected by this covering, which closely resembles After Marlatt (after Riley), Bull. 4, Div. Ent. U.S. Dept. Agr. of its case. Magnified. of the pellets of wool passed from their own intestines, while the allied Tortricid caterpillars roll up leaves and spin for themselves cylindrical shelters. The habit of spinning over the food plant a protective mass of web, whereon the caterpillars of a family can live together socially is not uncommon. In the case of the small ermine moths (Hyponomeuta) the caterpillars remain associated throughout their lives and pupate in cocoons on the mass of web produced by their common labour. But the larger, spiny caterpillars of the vanessid butterflies usually scatter away from the nest of their infancy when they have attained a certain size. Spines and hairs seem to be often effective protections for caterpillars; the experiments of E. B. Poulton and others tend to show that hairy caterpillars (fig. 15) are distasteful to birds. Many caterpillars are protected by the harmony of their general green coloration with their surroundings. When the insect attains a large size—as in the case of the hawk moth (Sphingid) caterpillars—the extensive green surface becomes broken up by diagonal dark markings (fig. 46b), thus simulating the effect of light and shade among the foliage. A remarkable result of Poulton's experiments has been the establishment of a reflex effect through the skin on the colour of a caterpillar. Some species of " loopers " (Geometridae, fig. 43) for example, if placed when young among surroundings of a certain colour, become closely assimilated thereto—dark brown among dark twigs, green among green leaves. These colour-reflexes in conjunction with the elongate twig-like shape of the caterpillars and their habit of stretching themselves straight out from a branch, afford some of the best and most familiar examples of " protective resemblance." The " terrifying attitude " of caterpillars, and the supposed resemblance borne by some of them to serpents and 'other formidable vertebrates or arthropods, are discussed in the article MIMICRY. The silk produced by a caterpillar is, as we have seen, often advantageous in its own life-relations, but its great use is in connexion with the pupal stage. In the life-history of many Lepidoptera, the last act of the caterpillar is to spin a cocoon which may afford protection to the pupa. In some cases this is formed entirely of the silk produced by the spinning-glands, and may vary from the loose meshwork that clothes the pupa of the Europe. diamond-back moth (Plutella cruciferarum) to the densely woven cocoon of the silkworms (Boinbycidae and Saturniidae) or the hard shell-like covering of the eggars (Lasiocampidae). Fre- quently foreign substances are worked up with the silk and serve to strengthen the cocoon, such as hairs from the body of the caterpillar itself, as among the " tigers " (Arctiidae) or chips of wood, as with the timber-burrowing larva of the " goat " (Cossus). In many families of Lepidoptera we can trace a degeneration of the cocoon. Thus, the pupae of most owl moths (Noctuidae) and hawk moths (Sphingidae) lie buried in an earthen cell. Among the butterflies we find that the cocoon is reduced to a pad of silk which gives attachment to the cremaster; in the Pieridae there is in addition a girdle of silk around the waist-region of the pupa, but the pupae of the Nym,phalidae (figs. 1 r, 65) simply hang from the supporting pad by the tail-end. Poulton has shown that the colours of some exposed pupae vary with the nature of the surroundings of the larva during the final stage. When the pupal stage is complete the insect has to make its way out of the cocoon. In the lower families of moths it is the pupa which comes out at least partially, working itself onwards by the spines on its abdominal segments; the pupa of the primitive Micropteryx has functional mandibles with which it bites through the cocoon. In the higher Lepidoptera the pupa is immovable, and the imago, after the ecdysis of the pupal cuticle, must emerge. This emergence is in some cases facilitated by the secretion of an acid or alkaline solvent discharged from the mouth or from the hind-gut, which weakens the cocoon—so that the delicate moth can break through without injury. As might be expected, the conditions to which larva and pupa are subjected have often a marked influence on the nature of the imago. An indifferent food-supply for the larva leads to a dwarfing of the moth or butterfly. Many converging lines of experiment and observation tend to show that cool conditions during the pupal stage frequently induce darkening of pigment in the imago, while a warm temperature brightens the colours of the perfect insect. For example, in many species of butterfly that are double-brooded, the spring brood emerging from the wintering pupae are more darkly coloured than the summer brood, but if the pupae producing the latter be subjected artificially to cold conditions, the winter form of imago results. It is usually impossible, however, to produce the summer form of the species from wintering pupae by artificial heat. From this A. Weismann argued that the more stable winter form must be regarded as representing the ancestral race of the species. Further examples of this " seasonal dimorphism " are afforded by many tropical butterflies which possess a darker " wet-season " and a brighter " dry-season " generation. So different in appearance are often these two seasonal forms that before their true relationship was worked out they had been naturally regarded as independent species. The darkening of wing-patterns in many species of Lepidoptera has been carefully studied in our own British fauna. Melanic or melanochroic varieties are specially characteristic of western and hilly regions, and some remarkable dark races (fig. 43) of certain geometrid moths have arisen and become perpetuated in the manufacturing districts of the north of England. The production of these melanic forms is explained by J. W. Tutt and others as largely due to the action of natural selection, the damp and sooty conditions of the districts where they occur rendering unusually dark the surfaces—such as rocks, tree-trunks and palings—on which moths habitually rest and so favouring the survival of dark, and the elimination of pale varieties, as the latterwould be conspicuous to their enemies. Breeding experiments have shown that these melanic races are sometimes " dominant " to their parent-stock. An evidently adaptive connexion can be frequently traced between the resting situation and attitude of the insect and the colour and pattern of its wings. Moths that rest with the hindwings concealed beneath the forewings (fig. 34, f) often have the latter dull and mottled, while the former are sometimes highly coloured. Butterflies whose normal resting attitude is with the wings closed vertically over the back (fig. 63) so that the under surface is exposed to view, often have this under surface mottled and inconspicuous although the upper surface may be bright with flashing colours. Various degrees of such " protective resemblance " can be traced, culminating in the wonderful " imitation " of its surroundings shown by the tropical " leaf-butterflies " (Kallima), the under surfaces of whose wings, though varying greatly, yet form in every case a perfect representation of a leaf in some stage or other of decay, the butterfly at the same time disposing of the rest of its body so as to bear out the deception. How this is effected is best told by A. R. Wallace, who was the first to observe it, in his work The Malay Archipelago: " The habit of the species is always to rest on a twig and among dead or dried leaves, and in this position, witli the wings closely pressed together, their outline is exactly that of a moderately sized leaf slightly curved or shrivelled. The tail of the hindwings forms a perfect stalk and touches the stick, while the insect is supported by the middle pair of legs, which are not noticed among the twigs and fibres that surround it. The head and antennae are drawn back between the wings so as to be quite concealed, and there is a little notch hollowed out at the very base of the wings, which allows the head to be retracted sufficiently." But the British Vanessids often rest on a bare patch of ground with the brightly coloured upper surface of their wings fully exposed to view, and even make themselves still more conspicuous by fanning their wings up and down. Some genera and families of Lepidoptera, believed to secrete noxious juices that render them distasteful, are adorned with the staring contrasts of colour usually regarded as " warning," while other genera, belonging to harmless families sought for as food by birds and lizards, are believed to obtain complete or partial immunity by their likeness to the conspicuous noxious groups. (See
End of Article: LEPIDOPTERA (Gr. ?emir, a scale or husk, and 7rrepov, a wing)
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