354 



DIPTERA OF NORTH AMERICA. 



[PART II. 



a distinct convexity of the wing. The males are usually distinguished by 

 striking ornaments on their feet, especially on the middle feet. The species 

 of Plagioneurus are altogether distinct on account of the very diagonal 

 position of the posterior transverse vein ; otherwise their neuration is very 

 near to that of Pelastoneurus, while the glabrousness of the first joint of 

 the antennas renders it utterly impossible to mistake them for any species 

 of the latter genus. 



To the second sub-group, embracing those genera the transverse vein of 

 which is strikingly approximated to the posterior margin of the wing, 

 belong the following genera: Liancalus (35), Scellus (37), and Hydro- 

 phorus (38). Liancalus is easily distinguished from Scellus and Hydro- 

 phorus by all the femora being slender and unarmed. In the species of 

 Scellus the fore-femora are beset on the under side with rather strong 

 bristles', catching into a row of similar bristles of the fore tibias, and the 

 third and fourth longitudinal veins are rather strongly convergent toward 

 their ends, while the species of Hydrophorus have only on the under 

 side of the fore-femora, towards the basis, a few elongated thorn-like 

 bristles ; the under side, however, not only of the fore-femora, but also of 

 the fore tibiae, is beset only with short bristles, and the third and fourth 

 longitudinal veins are parallel towards their ends, or almost so. 



To the next following genus, Achalcus, I have assigned this position on 

 account of the general structure of its body, which led me to suppose the 

 existence of a relationship between it and the following genera ; I also 

 presume, therefore, that the structure of the dorsum of the thorax, which 

 I have not been able to ascertain positively in any of the few specimens 

 belonging to me, will, when found out, justify the location I have assigned 

 to the genus. In general the species of Achalcus may be easily dis- 

 tinguished, as the total absence of the sixth longitudinal vein is a cha- 

 racter peculiar to them ; the species hitherto made known are of a non- 

 metallic color, and of a very small size. 



The genera belonging to the second group, the thorax of which has on 

 its posterior part a more or less concave, sloping surface, are the following : 

 Medeterus (39), Chrysotimus (41), Xanthochlorus (42), and Saucropus 

 (43). Medeterus differs essentially from the other three genera by the 

 apical position of the arista, the very large size of the proboscis, and the 

 totally disengaged hypopygium. The genera Chrysotimus and Xantho- 

 chlorus contain only small species, and are easily distinguished by their 

 coloring, which is either entirely yellow, or in part yellow, in part metallic- 

 green ; Chrysotimus is easily distinguished from Xanthochlorus, by the 

 subapical position of the arista and by the much smaller and imbedded 

 hypopygium ; with Xanthochlorus the position of the arista of the antennas 

 is dorsal, and the swollen and rather disengaged hypopygium is directed 

 backward so as to assume the appearance of a prolongation of the abdomen 

 of the male. The species of Saucropus are of a more considerable size 

 than the species of Chrysotimus and Xanthochlorus, and their feet are 

 comparatively longer ; the second joint of the antennas somewhat differs 



