FOOD OF INSECTS. 387 



merable swarms of gnats which abound in swampy places, 

 particularly in regions which but for them would be lost 

 to sensitive existence, should ever taste blood, it seems 

 clear that they are usually contented with vegetable ali- 

 ment. Indeed the males, as well as those of the horse- 

 fly of which even the females readily imbibed the sugared 

 fluid offered to them by Reaumur a , never suck blood at 

 all ; so that they must either feed on vegetable matter, 

 which in fact I have observed them to do, or fast during 

 their whole existence in the perfect state. 



Though insects, generally considered, have thus a 

 much more extensive bill of fare than the larger animals, 

 each individual species is commonly limited to a more 

 restricted diet. Many both of animal and vegetable 

 feeders are absolutely confined to one kind of food, and 

 cannot exist upon any other. The larva of Gasterophilus 

 Equi can subsist no where but in the stomach of the 

 horse or ass, which animals therefore this insect might 

 boast with some show of reason to have been created for 

 its use rather than for ours, being to us useful only, 

 but to it indispensable. The larvae of Syrphus Pyrastri 

 according to De Geer eat no other Aphis but that of the 

 rose b . Most Ichneumons and Sphecina prey each upon 

 a single species of insect only, which therefore they 

 would seem to have been formed for the express purpose 

 of keeping within due limits. Reaumur mentions having 

 once found in a parcel of decaying wood the nests of six 

 different kinds of the latter tribe, each of which was filled 

 with flies of a distinct species . Cerceris ^ auritus and 

 PhilantJms Icetus in the larva state feed solely on the 

 weevil tribe of Coleoptera, the latter being restricted even 



8 Reaura. iv. 280 " De Geer, vi. 112. ' Reaum. vi, 271 . 



2 c 2 



