34 DIPTERA OF NORTH AMERICA. 



received the name of AnthophilsR. The differences in the organi- 

 zation of the Muscarise calyptratse are much smaller than those of 

 any two families among the first series of Diptera hrachycera, 

 which ends with the Tachydromidae, with the sole exception of the 

 group formed by the Hybotidse, Empidse., and Tachydromidse, in 

 which a similar relationship exists. Consequently the families in 

 question here owe their existence much more to the immense 

 number of species and genera than to a real necessity, based on 

 differences of structural characters. Hence it is much more diffi- 

 cult to define their limits, and one must already be well acquainted 

 with a great number of forms, in order to attempt to point out with 

 certainty the right place for new ones. In the limitation of these 

 families I have made use of what has been said about them in 

 Walker's British Diptera; for however insufficient I may find it, I 

 know of nothing better to be put in its place. 



In the family of Dexidse a number of genera have been already 

 formed ; as they still require considerable sifting and a much 

 sharper limitation than they have at present, it seems useless to 

 enumerate them here. 



The N. A. Dexidse known to me cannot all be placed in the 

 genera hitherto erected. The species about whose position there 

 is no doubt belong to the genera: Prosena St. Farg., Microph- 

 thalma Macq., Dinera Rob. Desv., and Estheria Hob. Desv. 

 There is also no doubt about Trichodura Macq. and Megaproso- 

 pus Macq. occurring in N. A. 



Fam. XXXIV. TACHINIDAE. 



Charact. — Bristle of the antennae bare or with a very short pubescence. 

 Thorax short. First posterior cell closed or only slightly opened. 

 Legs short. 



The immense extent of this family renders the formation of sec- 

 tions indispensable. It is best divided into four sections, which 

 might perhaps be raised into families. The two first of them are 

 the Tachinina and Ocypterina, both of which differ from the two last 

 by their abdomen being beset with long bristles. All Tachinina 

 have an oval abdomen, or when it is nearly cylindrical, its first 

 segment is much shortened. The abdomen of the Ocvpterina is 

 always of a slender cylindrical form, and its first segment elongated. 



