The egg of this insect when first laid is oval in shape and light green in colour, becoming bronzy a few days before the emergence of the larva. The young larva is very attenuated, with only ten legs. Its colour is pale yellow striped with brownish-pink near the segmental divisions. It is very active, and does not devour the egg-shell after emergence.
The full-grown larva has the body much flattened underneath. In colour it is pale brownish-pink, with numerous irregular darker markings, which in some specimens almost form two broad subdorsal lines. The under surface of the larva is pale green. There is a series of fleshy filaments of a pinkish-brown colour along each side of the insect, and an extra pair of prolegs on the ninth segment.
This caterpillar is, however, very variable, its colouring appearing to depend largely on its surroundings. The favourite food-plants are Leptospermum ericoides and Aristotelia racemosa. The larvæ found on the former plant are usually pale yellowish-brown, whilst those from the latter are much darker brown, often mottled with grey like the stems of the Aristotelia. A specimen I once found on a mountain beech (Fagus cliffortioides), the gnarled stem and branches of which were covered with grey lichens and mosses, was mottled with the most beautiful shades of greenish-grey. These larval varieties are very interesting, and in order to test the direct influence of food on the colouring of the larvæ, I once divided a batch of eggs deposited by a single female into two equal parts, and fed one half on Aristotelia, and the other half on Leptospermum. The differences in colouring between the two lots of larvæ thus treated were, however, of the most trivial description. This somewhat surprised me at first, as I had previously observed quite distinct varieties on each plant, when found in a state of nature. Hence I am now disposed to think that these differences have been brought about gradually, by natural selection acting on larvæ feeding on the same plant for a large number of generations. By this means a sufficient amount of variation might be accumulated, to cause the closest possible approximation in colouring to the stems of the several food-plants. It is also noteworthy that many of these food-plants grow in widely dissimilar localities, so that the free inter-breeding of insects dependent on them would not be likely to occur, and thus the peculiarities of colouring adapted to the stems of each food-plant would not be disturbed by the effects of inter-breeding.
In connection with the foregoing experiment it is also interesting to observe, that the specimens fed on Aristotelia matured much more rapidly than those on Leptospermum; the former plant evidently being the more nourishing food for the larvæ. Also that out of the batch fed on Aristotelia 28 became moths, of which 12 were males and 16 females; whilst out of those fed on Leptospermum only 24 became moths, of which 15 were males and 9 females. In all other respects, excepting food-plant, the two lots of larvæ were subjected to identical treatment.
During the day this larva rests quietly attached to the stem of its food-plant, where it is very difficult to detect, as the filaments so closely embrace the twig or tree-trunk that the whole insect exactly resembles a swelling in the stem.
The pupa of D. floccosa is enclosed in a loose cocoon on the surface of the ground.
The perfect insect appears about September, and continues in more or less abundance until the end of April. There are most likely several broods in a season, and, as we frequently meet with specimens of the moth on mild days in the middle of winter, it probably also hibernates.
This insect is usually observed at rest on fences and tree-trunks, where its grey mottled colouring causes it to closely resemble a patch of lichen.
DECLANA JUNCTILINEA, Feld.
(Plate [X]., fig. 37 ♂, 38 ♀.)