18 INTRODUCTION. 



selves in the winged state, deposit their eggs among plant-lice, 

 upon the blood of which their young afterwards subsist. Many 

 (Conopida, excluding Stomoxys, Tackince, Ocyptertz, Phorce, &c.) 

 lay their eggs on caterpillars, and on various other larvae, within 

 the bodies of which the maggots hatched from these eggs live till 

 they destroy their victims. And finally others (Anihracidce and 

 Volucettce) drop their eggs in the nests of insects, whose offspring 

 are starved to death, by being robbed of their food by the off- 

 spring of these cuckoo-flies. Besides performing their various 

 appointed tasks in the economy of nature, flies, and other insects, 

 subserve another highly important purpose, for which an all-wise 

 Providence has designed them, namely, that of furnishing food 

 to numerous other animals. Not to mention the various kinds of 

 insect-eating quadrupeds, such as bats, moles, and the like, many 

 birds live partly or entirely on insects. The finest song-birds, 

 nightingales and thrushes, feast with the highest relish on maggots 

 of all kinds, as well as on flies and other insects, while the warblers, 

 vireos, and especially the fly-catchers and swallows, devour these 

 two-winged insects in great numbers. 



' The seven foregoing orders constitute very natural groups, 

 relatively of nearly equal importance, and sufficiently distinct 

 from each other, but connected at different points by various 

 resemblances. It is impossible to show the mutual relations 

 of these orders, when they are arranged in a continuous se- 

 ries, but these can be better expressed and understood by 

 grouping the orders together in a cluster, so that each order 

 shall come in contact with several others. 



Besides these seven orders, there are several smaller 

 groups, which some naturalists have thought proper to raise 

 to the rank of independent orders. Upon the principal of 

 these a few remarks will now be made. 



The little order STREPSIPTERA of Kirby, or RHIPIPTERA of 

 Latreille, consists of certain minute insects, which undergo 

 their transformations within the bodies of bees and wasps. 

 One of them, the Xenos Peckii, was discovered by Professor 

 Peck in the common brown wasp (Foliates fuscato) of this 



