202 STATES OF INSECTS. 



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 larvae 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 a : an increase 

 of weight and size in so short a time truly prodigious, 

 but essential for the end of their creation — the rapid re- 

 moval of dead and putrescent animal matter. As the 

 skins of these larvae are never changed, we may conclude, 

 if the cause of the change of skin in other larvae above 

 surmised be accurate, that their skins are more contrac- 

 tile and capable of a greater degree of tension than those 

 of larvae that are subject to moulting. And two peculi- 

 arities 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 

 larvae 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. 



3 Opiisc, i. 27- 



