TAXONOMY OF MUSCOIDEAN FLIES — TOWNSEND 41 



daria. The bristles of certain of the thoracic plates are here used as 

 main atavic characters for separating the Muscoidea from the An- 

 thomyioidea, as will appear later on under Synopses, accessory sup- 

 porting atavic characters being furnished by the type of venation. 



An extra-tactile function is no doubt discharged by certain of the 

 cephalic bristles in the Muscoidea. The orbital bristles (middle 

 fronto-orbital especially) of the females, which are usually wanting, 

 or of less number, in the males, have probably arisen in those forms 

 where present for the purpose of enabling the males to recognize the 

 opposite sex. They are especially conspicuous in profile, when the 

 strongly proclinate middle fronto-orbital are prominently contrasted 

 with the reclinate upper fronto-orbital bristles. A front view would 

 reveal the female in the wider front in most of the forms. The fact 

 that in some forms the males as well have the orbital bristles does 

 not militate against this view, but is explained by a transference of 

 the female character to the male through heredity. The breast nip- 

 ples of male mammals furnish an example of such hereditary trans- 

 fer of a female character to the male, with absolutely no functional 

 cause. 



The bristles of the facialia and the frontal bristles possibly serve 

 for the recognition of forms among themselves. They are most 

 developed in the more inconspicuously colored forms, which run 

 closely together in general habitus. Further confirmatory evidence 

 is found in the fact that conspicuously colored and otherwise striking 

 species often have the cephalic bristles but little developed. It is to 

 be noted, however, that certain of the latter lack abdominal macro- 

 chsetse as well. An absence of abdominal bristles is usually cor- 

 related with a weakness of cephalic bristles, doubtless due in these 

 cases to the marked development of an aerial life-habit. 



Secondary Sexual Characters. — -These should be accorded generic 

 rank when they can be correlated with equally constant characters in 

 the opposite sex. The secondary sexual characters in the Muscoidea 

 are to be found in the comparative width of front, presence or ab- 

 sence of orbital bristles, size and length of third antennal joint, some- 

 times form of latter, varying degrees of holopticism or dichopticism, 

 comparative length of claws, ventral carina, and certain anal pro- 

 cesses of abdomen ; also often in the shade of coloration and distri- 

 bution of pollen, especially on the parafrontals and parafacials, less 

 often on the thorax, and sometimes in the distribution of ground 

 color and even of the pollen of the abdomen. 



