viii. And this leads me to a subject to which I am desirous now to

bespeak your attention,--the _Growth_, I mean, and size of Insects in this state. As to _size_, larvæ differ as much as insects in their perfect state: these last, however, never grow after their exclusion from the pupa, while larvæ increase in bulk in a proportion, and often with a rapidity, almost without a parallel in the other tribes of animals. Thus Lyonnet found, that the caterpillar of the great goat-moth (_Cossus ligniperda_ F.) after having attained its full growth is at least 72,000 times heavier than when it was first excluded from the egg[485]; and of course had increased in size in the same proportion. Connected with the size of larvæ, is the mode in which their accretion takes place. This, with respect to the more solid parts, as the head, legs, &c., is not, as in other animals, by gradual and imperceptible degrees, but suddenly and at stated intervals. Between the assumption of a new skin and the deposition of an old one, no increase of size takes place in these parts, while the rest of the body grows and extends itself, till, becoming too big for these solid parts, nature restores the equilibrium between them by a fresh moult[486], in which the augmentation of bulk, especially in these parts, is so great, that we can scarcely credit the possibility of its being cased in so small an envelope. Malpighi declares, that the head of a silk-worm that has recently cast its skin is four times larger than before the change[487]. It is very probable, also, that when the outer skin becomes rigid, it confines the body of the larva within a smaller compass than it would expand to if unconfined, so that, when this compression is removed, the soft and elastic new integument immediately swells out, and the animal appears all at once much larger than it was before the moult. In fact, the proximate cause of the rupture and rejection of the old skin is the expansion of the included body, which at length becomes so distended as to split its envelope, aided, indeed, as before described, by the contortions of the creature itself.

The larvæ most notorious for the _rapidity_ of their growth are those of _Musca carnaria_ and other flesh-flies: some of which Redi found to become from 140 to more than 200 times heavier in twenty-four hours[488]: an increase of weight and size in so short a time truly prodigious, but essential for the end of their creation--the rapid removal of dead and putrescent animal matter. As the skins of these larvæ are never changed, we may conclude, if the cause of the change of skin in other larvæ above surmised be accurate, that their skins are more contractile and capable of a greater degree of tension than those of larvæ that are subject to moulting. And two peculiarities observable in them confirm this idea: in the first place, their head is not hard and corneous, as that of the others, but capable of being shortened or lengthened; and in the next, their breathing-pores are not in the sides, but at the extremities of the body, while in the _moulting_ larvæ there are two in almost every segment, which must form so many callous points that impede the stretching of the skin to the utmost. The hairs, spines, and tubercles, that are so often found on caterpillars, must also form so many points of resistance that prevent that full extension of the integument which it might otherwise admit.

There is not always that proportion between the size of larvæ and of the insects that proceed from them that might have been supposed, some small larvæ often producing perfect insects larger than some of those proceeding from such as are of greater size.