STATES OF INSECTS. (Larva.) 203 
for its life as a pupa, we may conclude that it had existed 
as a larva at least half the above period. ‘The grubs of 
the species of the genus Cynips attain their full size in a 
short time; but they afterwards remain five or six months 
in the gall before they become pupz*. 
With few exceptions it may be laid down, that those 
larvze which live on dead animals, in fungi, in dung, and 
in similar substances, are of the shortest duration in this 
state; and that those which live under the earth, on the 
roots of grass, &c. and in wood, the longest: the former 
becoming pupz in a few days or weeks, the latter requi- 
ring several months, or even years, to bring them to ma- 
turity. The larvee which live on the leaves of plants 
seem to attain a middle term between the one and the 
other,—seldom shorter than a few weeks, and rarely 
longer than seven or eight months. Aquatic larvee ap- 
pear to be subject to no general rule: some, as the larvze 
of Gnats, becoming pupze in two or three weeks; and 
others, as those of the Ephemera, which are thus com- 
pensated for their short life as flies, in as many years °. 
The cause of all these differences is obviously dependent 
on the nature of the food, and the purposes in the eco- 
nomy of creation to which the larvee are destined. 
x. The last part of the history of larvze relates to their - 
Preparations for assuming the pupa state. When they 
have acquired their full size, after having ceased to take 
food, by a copious evacuation they empty the intestinal 
canal, even rejecting the membrane that lines it and the 
* N. Dict. d Hist. Nat. vii. 129. ; 
> As the larvee of Ephemere usually live in the submerged part cf 
the banks of rivers, perhaps they may be regarded as following the 
economy of subterranean éerrestrial larve. 
