Dec, 1918] Nearctic Species of the Genus Laphria 145 



ogy, nevertheless it must be admitted that the significance of 

 these characters is not thoroughly understood. Sexual poly- 

 morphism is well known in the Lepidoptera. In another group 

 of Arthropoda, the Crustacea, and particularly in crawfishes, 

 the same phenomenon occurs, and in this case differences in 

 type extend to the male genitalia. 



We do not know whether comprehensive life-history investi- 

 gations, especially rearing prolonged to several generations, will 

 vitiate some of the classification based on genital characters, 

 but until such researches have been made we can only proceed 

 upon the basis of observed differences. Meanwhile confidence 

 in the method is inspired by discovery of correlations of other 

 characters with those of the genitalia, and especially by the 

 disclosure among specimens sorted on the basis of genital 

 structure, of previously unnoticed differences in other significant 

 details. 



For the sake of ease of observation only the external structure 

 of the male hypopygia has been used in the present study. 

 When it is necessary to refine the classification of the nearctic 

 Laphria, undoubtedly additional useful characters may be 

 found in the hooks, claspers and other more hidden details of 

 the genitalia.* 



The present revision comprises 23 species of which 10 are 

 described as new and 9 varieties of which 5 are here first char- 

 acterized. On the basis of this experience the writer would 

 expect new forms among every considerable collection of flies 

 on the genus Laphria. 



In the following arrangement, groups of species are separated 

 by spaces; looseness of relationship within groups is indicated 

 by bracketing individually the comparatively less related 

 species, and closer kinship by bracketing in pairs the nearer 

 relatives. 



* The terminolog}' employed for the parts of the hypopygia is that learned 

 by the writer in the study of homoptera, especially the Psyllidae. That portion 

 of the hypopygium bearing the forceps is the genital valve; the opposed portion, 

 the anal valve. For details of these structures see an article by R. E. Snodgrass, 

 entitled "The inverted hypopygium of Da.syllis and Laphria." (Psyche. 9, Oct., 

 1902, pp. 399-400, PI. 5). It may be said the "inversion" in Laphria is of rather 

 fortuitous occurrence. Recently emerged specimens have the hypopygium in 

 normal position, with genital valve opposed to lower surface of abdomen, as seen 

 in Dolichopodidae, etc. All degrees of rotation of the hypopygium occur, and 

 apparently, copulation, ending in a tail to tail position of the flies, has much to 

 do with the inversion. 



