STATES OF INSECTS. (Larva.) 201 
-ment 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 larvee most notorious for the rapzdity of their 
growth are those of Sarcophaga carnaria and other flesh- 
flies: some of which Redi found to become from 140 to 
more than 200 times heavier in twenty-four hours?: an 
increase of weight and size in so short a time truly pro-- 
digious, but essential for the end of their creation—the 
rapid removal of dead and putrescent animal matter. As 
the skins of these larvee are never changed, we may con- 
clude, if the cause of the change of skin in other larve 
above surmised be accurate, that their skins are more con- 
tractile and capable of a greater degree of tension than 
those of larvee that are subject to moulting. And two 
peculiarities observable in them confirm this idea: in 
the first place, their real head is excessively minute, so 
as to be no obstacle at all in the process; and in the 
next, their breathing-pores are not in the sides, but 
at the extremities of the body, while in the moulting 
larvee 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 tuber- 
cles, 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. 
* Opusc. i. 27. 
