The typical Lepidopteran pupa, or “chrysalis,” as shown in the higher families, is an obtect pupa (fig. 11) with no trace of mandibles, the appendages being glued to the body by an exudation, and 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. 1, d) which serve to anchor the pupa to its cocoon or to suspend butterfly-pupae from their pad of silk (fig. 11). In the lower families the pupa (fig. 1, 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, forward 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 the air 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 (Apatura 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 kühniella) 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 9), 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 caterpillars 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 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 clearwings (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 caterpillars 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 (Lemna) 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.

Fig. 12.—Larva of Hepialus humuli (ghost moth).Fig. 13.—Hydrocampa 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 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.

After Marlatt (after Riley), Bull. 4, Div. Ent. U.S. Dept. Agr.
Fig. 14.—Clothes Moth (Tinea pellionella), with larva in and out of its case. Magnified.

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 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.

Fig. 15.—Larva of Orgyia gonostigma. Europe.

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].