175 



are often readily distinguishable. The head is usually almost fully 

 retractile within the thorax, being permanently exserted in Xylophag- 

 idae, Coenomyiidae, and Stratiomyiidae. Respiration in this group 

 is normally carried on by means of prothoracic and posterior spiracles, 

 the latter being situated upon the ultimate ( Stratiomyiidae, Leptidae, 

 etc.), penultimate (Asilidae, Mydaidae, Bombyliidae), or antepenul- 

 timate (Therevidae, Scenopinidae) segments. Some species of perip- 

 neustic Dolichopodidae are an exception to this rule. Many species 

 of Brachycera have on their ventral surface locomotor organs, which 

 may consist of mere transverse irregular swellings (Asilidae, in part, 

 Mydaidae, Bombyliidae, etc.), paired pseudopods (Leptidae, part), or 

 transverse series of spinules (Xylophagidae). Many of the families 

 have conspicuous bristles on the body surface, especially on the ventral 

 surface of the thoracic segments and the dorsal and apical surfaces 

 of the anal segment (Asilidae, Mydaidae), or on all of the thoracic 

 and abdominal segments (Xylophagidae, Stratiomyiidae). 



The normal number of segments in this group is 12, exclusive of 

 the head; but in several families — Tabanidae, Stratiomyiidae, Lep- 

 tidae — only 1 1 are distinguishable. Except in the Stratiomyiidae the 

 pupa is free, that is to say, not enclosed in the indurated larval skin. 

 Rarely the pupa is enclosed in a loose cocoon {Medctcrus) or in a very 

 compact one (Drape tis). 



Suborder CYCLOREHAPHA 

 Division Acroptera 



This group includes but one family, Lonchopteridae. The sys- 

 tematic position of the group has long been in doubt, but the most 

 recent and comprehensive work upon all stages of the species clearly 

 points to their much closer affinity with the Cyclorrhapha than with 

 the Orthorrhapha. The larvae are distinguishable from those of other 

 Diptera by the fact that thev have but 9 well-defined segments in addi- 

 tion to the head. 



A full discussion of the characters of this group appears under 

 the family heading on a subsequent page. 



Division Aschiza 



This group consists of Syrphidae, Pipunculidae, Platypezidae, 

 and Phoridae, according to Brauer. I have before me larvae and 

 puparia of all but Pipunculidae. The cephalopharyngeal skeleton is 

 better developed in this group than in Schizophora, but it is less per- 

 fect than in the most specialized orthorrhaphid larva. In tiie aphi- 



