DEVELOPMENT OF INSECTS. 423 



There are some insects whose imago-life has but a very short 

 duration, the performance of the generative act being appar- 

 ently the only object of this state of their existence : and such 

 for the most part take no food whatever after their final emer- 

 sion, their vital activity being maintained, for the short period 

 it endures, by the material assimilated during their larva 

 state.* But those whose period of activity is prolonged, and 

 upon whose energy there are extraordinary demands, are 

 scarcely less voracious in their imago than in their larva- 

 condition ; the food they consume not being applied to the 

 increase of their bodies, which grow very little after the as- 

 sumption of the imago-state, but chiefly to their maintenance ; 

 no inconsiderable portion of it, however, being appropriated 

 in the female to the production of ova, the entire mass of 

 which deposited by a single individual is sometimes enormous. 

 That the performance of the generative act involves not 

 merely a consumption of material, but a special expenditure 

 of force, appears from a fact 'to be presently stated, corre- 

 sponding to that already noticed in regard to plants. 



Now if we look for the source of the various forms of 

 vital force which may be distinguished as constructive, 

 sensori-motor, and generative that are manifested in the dif- 

 ferent stages of the life of an insect, we find them to lie, on 

 the one hand, in the heat with which the organism is sup- 

 plied from external sources, and, on the other, in the food 

 provided for it. The agency of heat, as the moving power 

 of the constructive operations, is even more distinctly shown 

 in the development of the larva within the egg, and in the 

 development of the imago within its pupa-case, than it is in 



* It is not a little curious that in the tribe of Rotifera, or Wheel-ani- 

 malcules, all the males yet discovered are entirely destitute of digestive 

 apparatus, and are thus incapable of taking any food whatever ; so that not 

 only the whole of their development within the egg, but the whole of their 

 active life after their emersion from it, is carried on at the expense of the 

 store of yolk provided by the parent. 



