8 DIPTERA OF NORTH AMERICA. [PART II. 



margins of the abdomen, is likewise generally more extended and 

 much denser in the males than in the females. 



The sexual differences in the coloring may be described as fol- 

 lows : The eyes of the male are frequently of a different color than 

 those of the female, particularly so, for instance, in the genus 

 Diaphorus. The enlargement of the antennal arista in the male 

 often has a different coloring, usually white. The color of the face 

 in the male is generally of a purer, often a brighter shade. In 

 many species, the halteres of both sexes are colored differently — 

 for instance, in an entire group of species of Psilopus, they are 

 black in the male and yellow in the female. Even the cilia of the 

 tegulse have in a few instances (as in some species of Dolichopus) 

 a different coloring in the two sexes. Differences in the coloring 

 of the feet are not rare ; they are especially striking in the genus 

 Psilopus, some species of which have altogether yellow feet in the 

 female, and black femora in the male ; in others, the pale coloring 

 of the feet is more extended in the female than in the male. The 

 ornaments of the feet, peculiar to the males, also differ in their 

 coloring from the corresponding parts of the feet of the other sex, 

 being usually black, sometimes whitish, or with a handsome silvery 

 reflection. Even without displaying any peculiarity of structure, 

 the feet of the male have sometimes white or silvery spots, which 

 are wanting in the female. The wings are in some cases pictured 

 in the male and not in the female. Such are some species of 

 Dolichopus, Tachytrechus and Systenus, the males of which have 

 a black or white spot at the tip of the wing or in its proximity, 

 whereas the female does not show any trace of such a spot. The 

 same peculiarity occurs also in some other genera. 



These, often so conspicuous differences between the sexes of the 

 same species, sometimes render the recognition of their specific 

 identity somewhat difficult. In order to proceed in such cases 

 with some degree of certainty, it is necessary to pay a particular 

 attention to those characters which are usually common to both 

 sexes. The most reliable characters of this kind are: the hairs 

 on the antenna?, especially on the first joint ; the shape of the 

 second antennal joint ; the position of the arista ; the color of the 

 cilia on the inferior orbit; that of the cilia of the tegulse, notwith- 

 standing some exceptional cases of its diversity in the two sexes. 

 In most genera, to these characters may be added the coloring of 

 the feet and of the halteres as well as the nenration of the wings. 



