158 DIFTERA OP NORTH AMERICA. [PART II. 



?*us are comparatively larger, and have a different outline, because 

 the anal angle is more protruding. 



If, separating the typical species of Diaphorus from the typical 

 species of Chrysotus, we follow out these characters through a 

 series of those species, which, notwithstanding the separation of 

 their eyes upon the front, are placed into the genus Diaphorus, 

 on account of their general habitus which approaches the typical 

 species of Diaphorus, we find that those among the above men- 

 tioned marks of distinction, which are peculiar only to the males, 

 hold also good among these species ; at least I do not know 

 of any species which, being placed on account of its general 

 habitus among the species of Diaphorus, had not on the pos- 

 terior end of the hypopygium bristles of greater length and 

 remarkable strength, or in which the pulvilli of the fore tarsi of 

 the male were not elongated. 



It is different, however, with those marks of distinction which 

 belong to both sexes, as also with the more slender form of the body, 

 the greater length of the feet, and the more projecting anal angle 

 of the wings of the species of Diaphorus; each of these charac- 

 ters gradually fades away from species to species so that, taken 

 singly, these characters are utterly insufficient to decide whether 

 a species belongs to Diaphorus or to Chrysotus. As it happens, 

 however, that where one character decreases, another one becomes 

 more salient, it follows that in their totality they are sufficient 

 to distinguish the females of both genera, with the exception, 

 perhaps, of a few isolated cases. In doubtful cases it will be well 

 to compare the description of the species of both genera. 



The character of the genus Diap>horus may, therefore, be estab- 

 lished, as follows : Form of the body rather elongated. Eyes 

 of the male never contiguous on the face, in some species sepa- 

 rated upon the front, in others not. Antenna? short, the first 

 joint bare, the second transverse, the third short, distinctly hairy, 

 with a dorsal or subapical, rarely with an apical arista. Wings 

 rather large, with strongly projecting anal angle, and thus usually 

 somewhat broader towards the basis ; the posterior transverse 

 vein is either a little beyond, or in, or before the middle of the 

 wing, never close to the margin of the wing ; the last segment 

 of the fourth longitudinal vein is almost straight, or but gently 

 inflected, seldom interrupted, so that its end is thus more approxi- 

 mated to the third longitudinal vein, without, however, converging 



