204 STATES OF INSECTS. 



more than twenty years. Ample allowance being made 

 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 L. attain their full size 

 in a short time ; but they afterwards remain five or six 

 months in the gall before they become pupae a . 



With few exceptions it may be laid down, that those 

 larvae 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 pupae in a few days or weeks, the latter requir- 

 ing several months, or even years, to bring them to ma- 

 turity. The larvae 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 larvae ap- 

 pear to be subject to no general rule: some, as the larvae 

 of Gnats, becoming pupae in two or three weeks; and 

 others, as those of the Ephemerce, which are thus com- 

 pensated for their short life as flies, in as many years b . 

 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 larvae are destined. 



x. The last part of the history of larvae relates to their 

 Preparations for assuming the pupa state. When they 

 have acquired their full size, after having ceased to take 



a N. Diet. d'Hist. Nat. vii. 129. 



b As the larvae of Ephemeras usually live in the submerged part of 

 the banks of rivers, perhaps they may be regarded as following the 

 economy of subterranean terrestrial larvae. 



