Now if we consider in which species these four kinds of marking are of general occurrence, not only in the small group of the Sphingidæ but in the whole order Lepidoptera, we shall arrive at the following results:—
1. Complete absence of marking, so common in the larvæ of other insects, such as the Coleoptera, is but seldom found among Lepidopterous caterpillars.
To this category belong all the species of Sesiidæ (the genera Sesia, Trochilia, Sciapteron, Bembecia, &c.), the larvæ of which, without exception, are of a whitish or yellowish colour, and live partly in the wood of trees and shrubs and partly in the shoots of herbaceous plants. Subterranean larvæ also, living at the roots of plants, such as Hepialus Humuli at the roots of hop, and H. Lupulinus at those of Triticum Repens, possess neither colour nor marking. These, like the foregoing, are yellowish-white, evidently because they are deprived of the influence of light.[142] The larvæ of certain small moths, such as Tortrix Arbutana and Pomonana, which live in fruit, and many case-bearing Tineina, are likewise without marking and devoid of bright colour, being generally whitish. Many of the small caterpillars which feed exteriorly are also—so far as my experience extends—without definite markings, these being among the most minute, such as the greenish leaf-mining species of Nepticula. It is among the larger species that we first meet with longitudinal and oblique stripes. Eye-spots do not occur in any of these larvæ, a circumstance of the greatest importance for the biological significance of this character, as will be shown subsequently. The small size of the caterpillars cannot be the sole cause of the absence of such eye-spots, since in young Smerinthus caterpillars one centimeter long, the oblique stripes are beautifully developed, and the larvæ of many of the smaller moths considerably exceed this size. The surface of these caterpillars therefore, i.e., the field on which markings are displayed, is not absolutely too small for the development of such a character.
Besides the larvæ of the Micro-lepidoptera and of those species living in the dark, there is also a complete absence of marking in the young stages of many caterpillars. Thus, all the Sphingidæ of which I have been able to observe the development, show no markings immediately after emergence from the egg; in many they appear very soon, even before the first moult, and, in other species, after this period.
2. The second category of markings, longitudinal stripes, is very widely distributed among the most diverse families. This character is found among the larvæ of butterflies, Sphingidæ, Noctuæ, Micro-lepidoptera, &c., but in all these groups it is absent in many species. This last fact is opposed to the view that this character is purely morphological, and leads to the supposition that it may have a biological value, being of service for the preservation of the individual, and therefore of the species.
I find that such marking is of service, stripes extending longitudinally along the upper surface of the caterpillar generally making the latter less conspicuous. This, of course, does not hold good under all circumstances, since there are many species with very striking colours which possess longitudinal stripes. Let us consider, however, a case of adaptive colouring, such as a green caterpillar, which, on this account only, is difficult to see, since it accords with the colour of the plant on which it lives. If it is a small caterpillar, i.e., if its length and thickness do not considerably exceed that of the parts of its food-plant, it can scarcely be better concealed—stripes would hardly confer any special advantage unless the parts of the plant were also striped. But the case is quite different if the caterpillar is considerably larger than the parts of the plant (leaves, stalks, &c.). The most perfect adaptive colouring would not now prevent it from standing out conspicuously as a larger body, among the surrounding parts of the plants. It must be distinctly advantageous therefore to such a caterpillar to be striped, since these markings to a certain extent divide the large body into several longitudinal portions—they no longer permit it to be seen as a whole, and thus act more effectively than mere assimilative colouring in causing it to escape detection. This protection would be the more efficacious if the stripes resembled the parts of the plant in colour and size, such, for instance, as the lines of light and shadow produced by stalks or by long and sharp-edged leaves.
If this view be correct, we should expect longitudinal stripes to be absent in the smallest caterpillars, and to be present more especially in those species which live on plants with their parts similarly disposed, i.e., on plants with numerous thin, closely-growing stalks and grass-like leaves, or on plants with needle-shaped leaves.
It has already been mentioned that the smallest species are devoid of longitudinal striping. The larvæ of the Micro-lepidoptera show no such marking, even when they do not live in the dark, but feed either on the surface or in superficial galleries of the leaves (Nepticula, &c.), in which they must be exposed to almost as much light as when living on the surface. The fact that the subdorsal line sometimes appears in very young Sphinx-larvæ is explained, as has already been shown, by the gradual backward transference of adaptational characters acquired in the last stage of development.
It can easily be demonstrated that longitudinally striped caterpillars mostly live on plants, of which the general appearance gives the impression of a striped arrangement. We have only to consider in connection with their mode of life, any large group of adaptively coloured species marked in this manner. Thus, among the butterflies, nearly all the Satyrinæ possess larvæ conspicuously striped—a fact which is readily explicable, because all these caterpillars live on grasses. This is the case with the genera Melanargia, Erebia, Satyrus, Pararge, Epinephele, and Cænonympha, no species of which, so far as the larvæ are known, is without longitudinal stripes, and all of which feed on grasses. It is interesting that here also, as in certain Sphingidæ, some species are brown, i.e., adapted to the soil, whilst the majority are green, and are therefore adapted to living grass. Just as in the case of the Sphingidæ also, the brown species conceal themselves by day on the earth, whilst some of the green species have likewise acquired this habit. I have already shown how this habit originates from the increasing size of the growing larva, which would otherwise become too conspicuous, in spite of adaptive colour and marking. A beautiful confirmation of this view is found in the circumstance that only the largest species of Satyrus, such as S. Proserpinus, Hermione, Phædrus, &c., possess brown caterpillars. I should not be surprised if a more exact investigation of these species, which have hitherto been but seldom observed, revealed in some cases a dimorphism similar to that of the Sphingidæ; and I believe that I may venture to predict that the young stages of all these brown larvæ—at present quite unknown—are, as in the last-named group, green.
Besides the Satyrinæ, most of the larvæ of the Pierinæ and Hesperidæ possess longitudinal stripes, which are generally less strongly pronounced than in the former subfamily. Some of the Pierinæ live on Cruciferæ, of which the narrow leaves and thin leaf- and flower-stalks present nothing but a linear arrangement; other species of this group, however, feed on Leguminosæ (Lathyrus, Lotus, Coronilla, Vicia), and some few on broad-leaved bushes (Rhamnus). This last fact may appear to be opposed to the theory; but light lateral stripes, such for example, as those possessed by Gonepteryx Rhamni, can never be disadvantageous, and may be of use, even on large leaves, so that if we consider them as an inherited character, there is no reason for natural selection to eliminate them. In the case of caterpillars living on vetch, clover, and other Leguminosæ, it must not be forgotten that, although their food-plants do not present any longitudinal arrangement of parts, they always grow among grasses, the species feeding on such plants always resting between grass stems, and very frequently on the grass itself, so that they can have no better protective marking than longitudinal stripes. The striping of the Hesperidæ larvæ, which partly feed on grasses but mostly on species of Leguminosæ, can be explained in a similar manner.