173 



the order is comparatively meager ; and that I realize the possibilities 

 of error that beset me in the present attempt. 



SCOPE OF WORK 



In order to keep the size of this paper within a reasonable com- 

 pass I have given generic keys to imagines of those families only that 

 are not correctly dealt with in Williston's "Manual of North American 

 Diptera". 



I have covered the larval and pupal stages of all the groups avail- 

 able to me, depending but rarely upon printed descriptions for differ- 

 entiating characters. In the Orthorrhapha I have succeeded in obtain- 

 ing larvae or pupae, or both, of all the families except Macroceridae, 

 Orphnephilidae, Nemestrinidae, and Apioceridae. The Cyclorrhapha 

 are not so fully represented, but I have a large quantity of material 

 on hand which will serve as a basis for an extensive paper on this 

 group, and this will be published as Part II of the present article. 



A bibliography of general papers on Diptera will be given at the 

 conclusion of Part II, and papers upon single families will be listed 

 in connection with the text dealing with such families. 



Characters of the Larvae 



Suborder ORTHORRHAPHA 

 Division Nematocera 



Broadly speaking, the dipterous larvae of Brauer's eucephalous 

 group of the Nematocera may be distinguished from those of other 

 Diptera and also from other orders, by the following characters. 



Head with opposed, usually toothed, mandibles, often, in the 

 aquatic or semiaquatic forms, with conspicuous brushes on their ex- 

 ternal surfaces ; variously constructed antennae which may be barely 

 distinguishable elevations (Mycetophilidae, Sciaridae) or elongated, 

 and consisting of from I to 6 joints and occasionally retractile within 

 the head (Tanypinae) ; well-developed maxillae with i- or 2-jointed 

 palpi ; no well-developed labial palpi ; a more or less chitinized labial 

 plate, or submentum, which is frequently dentate upon its anterior 

 margin ; eyes, when present, indicated by a single or double pigmented 

 area on each side. True legs never present ; prothoracic segment occa- 

 sionally with a pair of pseudopods which may be entirely or partly 

 fused (Chironomidae, Simuliidae) ; anal segment in some cases with 

 a pair of more or less elongated pseudopods which are sometimes 

 armed with curved retractile claws in 2 or more concentric series; 



