Moths (Sphingidae).  Such an arrangement tends to make the insect less easily seen than were it to display a continuous area of the same colour.  The ‘looper’ caterpillars mentioned above afford remarkable examples of ‘protective’ resemblance, for many of them show a marvellous likeness to the twigs of their food-plant, tubercles on the insect’s body resembling closely the little outgrowths of the plant’s cortex.  It has been shown by E.B.  Poulton (1892) that many caterpillars are, in their early stages, directly responsive to their surroundings as regards colour.  Usually green when hatched, they remain green if kept among leaves or young shoots of plants, while they turn red, brown, or blackish if placed among twigs of these respective hues.  This effect appears to be due to a direct response of the subcutaneous tissue to the rays of light reflected from the surrounding objects.  The sensitiveness dies away as the caterpillar grows older, since little or no change of hue in response to a change of environment could be induced after the penultimate moult.

[7] The ‘hairs’ of an insect are not in the least comparable to the hairs of mammals, being in truth, modified portions of the cuticle, secreted by special cells.

Among those families of the Lepidoptera which are usually regarded as low in the scale of organisation, caterpillars are very generally protected by the habit of feeding in some concealed situation.  For example, the great larvae of the Goat Moth (Cossus) and the whitish caterpillars of the Clearwing Moths (Sesiidae) burrow through the wood of trees, eating the timber as they go.  The little irritable caterpillars of the Bell Moths (Tortricidae) roll leaves, fastening the edges together with silk, and thus make for themselves a shelter; or they bore their way into seeds or fruits, like the larva of the Codling Moth that is the cause of ‘worm-eaten’ apples, too well-known to orchard-keepers.  Very many small caterpillars mine between the two skins of a leaf, eating out the soft green tissue, and giving rise to a characteristic blister in form of a spreading patch or a narrow sinuous track through the leaf.  The caterpillars of the Clothes-moths (Tineidae) make for themselves garments out of their own excrement, the particles fastened together by silk.  In such curious cylindrical cases they wander over the wool or fur, feeding and indirectly supplying themselves with clothing at the same time.

The case-forming habit of the Clothes-moth caterpillars leads us naturally to consider the similar habit adopted by their allies the Caddis-larvae which live in the waters of ponds and streams, for the Caddis-flies (Trichoptera) have much in common with the more primitive Lepidoptera.  The caddis-larva is as a rule of the eruciform type, but with well-developed thoracic legs, and with hook-like tail-appendages; by means of the latter it anchors itself to the extremity of its curious ‘house.’  It is of interest