Studies in the Theory of Descent, Volume I

by August Weismann

That simple adaptive colouring prevails widely among caterpillars is shown by the large number of green species.[132] It may be fairly said that all caterpillars which possess no other means of protection or defence are adaptively coloured. These facts are now well known; so also is the explanation of the varied and striking colours of many caterpillars given by Wallace.[133] There is, however, novelty in the proof contained in the foregoing descriptions of larval development, as to the manner in which the di- and polymorphism of caterpillars can be explained from the external phenomena which they present, these phenomena being well adapted for showing the great importance of protective colouring to the larvæ. We have here presented a double adaptation, although not quite of the nature of that which I formerly admitted on hypothetical grounds.[134] In the first place, from the developmental history there results the conclusion that all Sphinx-larvæ which, in the adult state, are di- or polymorphic, are unicolorous when young. Thus, the caterpillars of Chærocampa Elpenor all remain green till the fourth stage, when they mostly become light or dark brown, and only very seldom retain their green colour. Chærocampa Porcellus behaves in a precisely similar manner; as also does Pterogon Œnotheræ, which inhabits the same localities, and is found on the same food-plant, but is not very closely related to the Chærocampa. In this species also (P. Œnotheræ) the brown is more common than the green form in the adult state, both varieties showing a complicated marking. The young larvæ possess only a light green colour, and a pure white subdorsal line as the only marking; they are so well adapted to the leaves of their food-plants, Epilobium Hirsutum, and E. Rosmarinifolium, that they can only be detected with great difficulty. After the third moult they become brown, and can be easily seen when at rest on their food-plant.

Now in all known caterpillars brown colours are adaptive, sometimes causing a resemblance to the soil, and at others to dead leaves or branches. As soon, therefore, as the caterpillars have attained a considerable size, they remain concealed by day.[135] The truth of this observation not only appears from various entomological notes, but I have frequently convinced myself of its accuracy. I well remember from the earliest times that C. Elpenor, especially when the larva is adult, always rests by day among the dead branches and leaves of its shrub-like food-plant, Epilobium Hirsutum; and even when this species lives on the low-growing Epilobium Parviflorum, it conceals itself by day on the ground, among the tangled leaves and branches. I have observed that Sphinx Convolvuli has a precisely similar habit, for which reason it is difficult to obtain, even in localities where it occurs very commonly.

In the neighbourhood of Basle I once found at mid-day a brown caterpillar of Pterogon Œnotheræ on an isolated dead branch of Epilobium Rosmarinifolium, and I was informed by Herr Riggenbach-Stähelin—a collector of great experience who accompanied me—that these caterpillars always rest (by day) on withered plants as soon as they become brown, but before this change they are only to be found on green plants.

Thus, it cannot well be doubted that the change of colour is associated with a change in the habits of life, and the question arises as to which has been the primary change.

If the view here entertained, that the later brown coloration is adaptive, be correct, the species must have first acquired the habit of concealing itself by day on the ground and among dead herbage, before the original green colour could have been changed into brown by natural selection. This must represent the actual facts of the case.

Nearly allied species which at an advanced age are not dimorphic, but are darkly coloured in all individuals, are especially calculated to throw some light on this point. For instance, the caterpillar of Deilephila Vespertilio, which comes under this denomination, is light green when young, and rests both by day and night on the leaves of the plant on which it feeds. As soon as it acquires its dark colour—after the third moult—it changes its habits, concealing itself by day on the ground and feeding only by night. For this reason collectors prefer seeking for it in the evening, or with a lantern by night.

The most instructive case, however, is that of Deilephila Hippophaës, in which no change of colour is associated with age, the caterpillar, throughout its whole life, remaining of a greyish green, which exactly matches the colour of the leaves of its food-plant, Hippophae Rhamnoides. Nevertheless this species also possesses the habit of feeding only at night as soon as it has attained to a considerable size, hiding itself by day at the root of its food-plant. Collectors expressly state that this larva can scarcely be found by day, and recommend that it should be sought for at night with a lantern.

From the foregoing facts and considerations it may fairly be concluded, that the habit of hiding by day, possessed by these and other allied caterpillars, was acquired when they resembled the leaves in colour, and that the adaptation to the colour of the soil, or dead foliage and withered branches, ensued as a secondary consequence.

But why have these caterpillars acquired such a habit, since they appear to be perfectly protected by their resemblance in colour to the green leaves? The answer to this question is easily given when we consider in which species this habit generally occurs.

Does the habit prevail only among the species of the one genus Deilephila, and in all the species of this genus? This is by no means the case, since, on the one hand, many species of Deilephila, such as D. Euphorbiæ, Galii, Nicæa, and Dahlii, do not possess the habit, and, on the other hand, it occurs in species of other genera, such as Macroglossa Stellatarum, Sphinx Convolvuli, and Acherontia Atropos.