230 DIPTERA OF NORTH AMERICA. [PART II. 



are species, where the two sexes do not agree in these characters. 

 Mr. Bigot divided the genus Psilopus into the following twelve 

 smaller genera : Megistostylus, Mesoblepharus, Agonosoma, Mar- 

 garitostylus, Oarislylus, Condylostylus, Eurostomerus, Dasypsi- 

 lopus, Heteropsilopus, Psilopus, Sciapus, and (Edipsilopus. In 

 accordance with what I have said above, I cannot adopt these 

 genera, based in part upon differences in the ornamentation of the 

 feet in the male, and upon other distinctions of a similar value. 

 If the genus Psilopus is to be further subdivided, this division 

 will have to be based upon the observation, that the species of 

 Psilopus diverge in two directions in their general habitus ; one 

 of these two sections embraces all our European species, and a 

 number of similar species, mostly from Northern Asia and North 

 America, the other section includes the large majority of the extra- 

 European species. If these two branches are to be raised to in- 

 dependent genera, then such characters must be found, which can 

 distinctly separate them from each other. There is no want of 

 plastic distinctions, which may be used for such a purpose, but 

 they are so manifold and of such an intricate nature that I believe 

 to have found a more useful mark of distinction in the color of the 

 cilia of the tegulas, which in all the species of the first branch 

 known to me are whitish, in those of the second branch however 

 black. These characters have been used by me merely for the 

 establishment of the two subsections of the genus Psilopus. 



Although the species of Psilopus are so numerous and appa- 

 rently resemble each other so much, nevertheless they can be 

 easily distinguished, if only the necessary attention is paid to the 

 plastic differences and not merely to the differences in color, 

 which are often insignificant and more or less inconstant in almost 

 all the species. The males of the different species especially can 

 be easily distinguished, as they are remarkable by peculiarities 

 in the structure of their antenna?, wings and feet, which belong to 

 their sex only ; they offer, besides, useful marks of distinction in the 

 anal appendages ; among the females of the smaller species, how- 

 ever, the distinction becomes, sometimes rather difficult. Of the 

 characters taken from the coloring, the most unreliable are those 

 taken from the coloring of the head, thorax, and abdomen, espe- 

 cially in those species, the metallic coloring of which is shifting 

 between the blue and the green ; a little more available are the 

 characters based upon the picture of the wings, if only we do not 



