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.