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_.
The habit in question must therefore be the result of certain external conditions of life common to all those species which rest by day. The mode of life common to them all is that they do not live on trees with large leaves or with thick foliage, but on low plants or small-leaved shrubs, such as the Sea Buckthorn.[136] I believe I do not err when I attribute the habit possessed by the adult larvæ, of concealing themselves by day, to the fact that the green colour is protective only so long as they are small--or, more precisely speaking, as long as their size does not considerably exceed that of a leaf or twig of their food-plant. When they become considerably larger, they must become conspicuous in spite of their adaptive colour, so that it would then be advantageous for them to conceal themselves by day, and to feed only by night. This habit they have acquired, and still observe, even when the secondary adaptation to the colour of the soil, &c., has not been brought about. We learn this from _D. Hippophaës_, which remains green throughout its whole larval existence; and no less from the green forms of the adult larvæ of _Sphinx Convolvuli_, _Chærocampa Elpenor_, and _Porcellus_, all of which conceal themselves by day in the same manner as their brown allies.