With respect to those larvæ which imitate slugs by the viscid covering that besmears them and issues from their pores, we learn from Professor Peck that this exudation takes place as soon as they are hatched; that the animal retains its humidity although exposed to the fiercest heat of the sun, and that at the last moult the skin becomes quite clean, and free from all viscidity[452]. It is probable that the other limaciform larvæ are similarly circumstanced. Madame Merian has figured an _onisciform_ one, the legs of which, she says, are covered with a viscid skin: this produced a _Noctua_. Those of _Papilio Anchises_ also are slimy, and adhere to each other[453].
v. Amongst other qualities which attach to larvæ, we must not omit to say something concerning their _Colour_. For though those which live in darkness, in the earth, in wood, in fruits, &c. are, with few exceptions[454], of an uniform whitish colour, yet such as are exposed to the influence of the light are usually adorned with a vast variety of tints, sometimes the most vivid that can be imagined. That the white colour of the former may be attributed to the absence of light is proved by an experiment of M. Dorthes, who having forced some to live under glasses, exposed to the light, found that they gradually became brown[455]. To attempt any classification of coloured larvæ would be in vain, since they are tinged with almost every possible shade that can be conceived, of many of which it would be difficult to find examples elsewhere; and infinitely diversified as to the arrangement and figure of their multiform markings and spots. A few general remarks, therefore, are all that you will expect on this head. Many are of one uniform colour; while a variety of tints, very different, and very vivid and distinct, ornament others. Sometimes they are distributed in longitudinal rays or bands, at others in transverse ones. Sometimes they are waved or spotted, regularly or irregularly; at others they are sprinkled in dots, or minute streaks, in every possible way. Various larvæ are of the colour of the plant on which they feed, whence they are with difficulty discovered by their enemies. Thus, a large proportion of _Lepidoptera_ are green of different shades, sometimes beautifully contrasted with black bands; a circumstance which renders the caterpillars of two of our finest insects of this order as lovely as the fly: I mean that of _Papilio Machaon_ and _Saturnia Pavonia_. Very frequently the larvæ of quite different species resemble each other so exactly, in colour as well as shape, as scarcely to be distinguishable: this sometimes takes place even where they belong to different genera, as in those of _Bombyx versicolor_ a moth, and _Smerinthus Populi_ a hawk-moth. And it sometimes happens, very fortunately for distinguishing allied species, that where the perfect insects very nearly resemble each other, the larvæ are altogether dissimilar. Thus, the female of _Pieris Rapæ_ is so much like the same sex of _Pieris Brassicæ_, that it might be taken for a variety of it, did not the green caterpillar of the one, and the spotted one of the other, evince the complete distinction of these butterflies. _Noctua Lactuca_, _N. umbratica_, and several other species of the same tribe, which includes _N. Absinthii_, _Verbasci_, _Chamomillæ_, _Abrotani_, are so extremely alike, that the most practised eye can scarcely discover a shade of difference between them, though their larvæ in colour and markings are constantly distinct[456]. The markings of species belonging to the same family are usually different; but in some cases the latter may be prejudged from the former. The larvæ of many of the genus _Sphinx_ L., for example, have their sides marked by oblique streaks running from the back in a direction towards the head; and by this last circumstance they are distinguished from those of _Bombyx versicolor_, _Attacus Tau_, and others of the same tribe, which have also lateral oblique striæ, but running from the back towards the tail[457]. The colours of individual larvæ of the same species are usually alike, but in _Sphinx Elpenor_ and some others they vary exceedingly. Many, like those of _Lasiocampa Rubi_, _Saturnia minor_, &c., are of one colour when first disclosed, and assume others quite different in riper age. Just previously to _changing their skin_, the tints of most larvæ become as dull and obscure, as they are fresh and vivid when the change has fully taken place; and in some instances the new skin is quite differently marked from the old one. This is remarkably the case with the last skin of some of the larvæ of the genus _Tenthredo_ L., which is entirely different from all the preceding ones.