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.