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. As people when they advance far in years usually become more simple in their dress than when they were young, so the larvæ in question change an agreeably variegated skin for one of a uniform and less brilliant colour[458]. Madame Merian has observed with respect to _Attacus Erythrinæ_, that its caterpillar is at first yellowish, with nine black striæ on each side: when arrived at one third of its size, they become orange; the striæ are obliterated, and in their place a round black spot appears on each of the eight intermediate segments[459]. Mr. Sheppard has remarked to me, that the skin of that of _Sphinx Ligustri_, after being under ground four days, was changed from a vivid green to a dull red. Very rarely, however, it becomes of a more brilliant hue just before entering the pupa state: thus, that of another hawk-moth (_Smerinthus Tiliæ_) changes to a bright violet; and the yellow hairs of that of _Laria pudibunda_ then become of a lovely rose colour. And here I may observe, that the hairs and spines also, of larvæ, vary greatly in colour. They are to be met with brown, black, red, yellow, violet, white, &c. De Geer found, that in the larva of _Cimbex nitens_ the two sides of the body were of a different colour, the left being of a deep green, whilst the right side and the rest of the body were paler[460]; but as he saw only a single individual, this was probably an accidental circumstance. Though the caterpillars, as I lately said, of one of the most beautiful butterflies and moths that inhabit Britain contend with the perfect insect in loveliness, yet in general no judgement can be formed of the beauty of the future fly from the colour of the larva; and the young Aurelian must not flatter himself always with the hope, because the caterpillar excites admiration by its colours and their arrangement, that the butterfly or moth it is to produce will do the same; nor ought he to despise and overlook a sombre or plain-coloured individual of the former, under the idea that it will produce one equally plain of the latter, for it often happens that the splendid caterpillar gives a plain butterfly or moth, and _vice versâ_. De Geer, however, gives us two instances of conformity between the colours of the caterpillar and those of the future moth; the one is that of the common currant-moth (_Phalæna G. grossulariata_ L.), the caterpillar of which is white, ornamented with several black spots varying in size. At the two extremities it is yellowish, with a longitudinal ray of the same colour on each side, the head and legs being black. These colours are all to be found in the fly, the ground of its wings being white ornamented with many black spots of different sizes. Its upper wings are traversed by a yellowish band; and towards their base is a spot of the same colour. Its body is yellowish, with black spots; but the head and legs are black[461]. The other is that of a green caterpillar, which gives a green moth, figured by Reaumur (_Pyralis prasinaria_ Fab.)[462]. Sometimes, also, the sex of the future perfect insect may be predicted from the colour it exhibits in its first state: thus, the brown caterpillars of _Noctua Pronuba_ produce males, and the green ones females[463]. The sexes, also, of _N. exoleta_ and _Persicariæ_ differ in that state.

vi. To the full account of the _Food_ of insects given in a former letter[464], which had reference chiefly to their larva state, it is only necessary in this place to add a few particulars not there noticed. Many larvæ when first excluded, as those of _Pieris Cratægi_, &c. devour the shells of the eggs from which they have proceeded[465]; and others (_Cerura Vinula_, _Sphinx Euphorbiæ_, _Noctua Verbasci_), though their usual food is of a vegetable nature, eat with great apparent satisfaction the skins which they cast from time to time, not leaving even the horny legs. This strange repast seems even a stimulating dainty, which speedily restores them to vigour, after the painful operation by which they are supplied with it. Under this head it will not be out of place to mention, that some larvæ of insects, which feed only on the juices of animals, or the nectar and ambrosia of flowers, have no anal passage, and of course no feces. This is said to be the case with the grubs of bees, wasps, the larvæ of _Myrmeleon_, &c.[466]