AMERICAN DIPTERA. 67 



shining black without the white pollinose vittae of the normal female, 

 or the bi'assy pubescence of the male, only white pollinose on humeri 

 and pleurtie. The sides of front also are more black, with less silvery. 

 I thought to consider this a case of dimorphism, rather than to con- 

 sider as distinct the latter form, which approaches the male in general 

 habitus. The capture of pairs in coitu will be needed to confirm or 

 refute the supposition. 



However, Mr. C'harles Robertson, of Illinois, in correspondence 

 with me on the above point, writes that he can differentiate the males, 

 and believes, therefore, that there are two distinct species to be dis- 

 tinguished in both sexes. This may be so, and the fact that indi- 

 viduals of the sanie sex, in two nearly allied species, are sometimes 

 hardly to be separated favors this view. I quote Mr. Robertson's 

 letter in full so far as it pertains to this subject, and confess that his 

 views seem very probable, 



" Put in a row all males having a longitudinal fuscous stripe on the abdomen- 

 In a parallel row put all males in which this fuscons stripe is wanting. Then in 

 line with the first set put the ' normal' females and in line with the second set 

 put the ' dimorphic' females. It will be noticed tirst that the fuscous stripe also 

 bears a pollinose stripe and indicates a connection with the normal females which 

 have a pollinose stripe down the abdomen. Then it will be noticed that the first 

 set of males runs larger than the second, which goes to show that they are dis- 

 tinct, and that they belong to the ' normal' females which also run larger than 

 the ' dimoiijhic' females. In the large males it will be seen that the first posterior 

 cell varies from closed to petiolate as in the normal females, while the small males 

 have the cell more petiolate, as in the ' dimorphic' females. Then it will be ob. 

 served that the bend of the fourth vein is more rounded in the small males, and 

 generally more angulate in the large ones. Comparing the abdomens of the two 

 sets of males with those of the corresponding females will show that they agree 

 in the distribution of pollen, which I think is very importaiit. Probably other 

 points will be found, but if the specimens are arranged as I have suggested, and 

 each specimen in the one row compared with the one standing opposite in the 

 other row, I am satisfied the conclusion will be drawn that there are two distinct 

 species which can be separated easily, though the males are a little harder to 

 separate than the females, a very common thing in closely allied species. 



" I have considered that black specimens of the 'dimorphic' females were the 

 same as C. palladi Towns., but the small species is really a redder one than the 

 large." 



In case it is shown that there are two species in the above forms, 

 Macquart's name will still stand for the species which possesses the 

 " normal" females, and Walker's name will still remain a synonym 

 because he described the " normal" female. Loew's name, however, 

 can be applied to the species possessing the " dimorphit;" females, 

 which will thus be known as C. divisa. 



TKANS. AM. KNT. SOC. XXII. MARCH, 1895. 



