348 DIPTERA OP NORTH AMERICA. [PART II. 



fourth longitudinal veins of the wings ; it stands in rather close relation 

 to the genus Dolichopus, which belongs to the first group, differs from it, 

 however, not only by the want of bristles on the first joint of the hind 

 tarsi, but also by the smaller size of its species, the shortness of the first 

 joint of the antenuse, and usually by the more distinct pubescence of the 

 arista ; moreover the lamellae of the hypopygium are usually smaller and 

 not jagged at the end. The genus Paraclius is distinguished by a pecti- 

 nate or subpectinate arista of the antennas, by the face, narrowed below 

 even in the female, and by the peculiar course of the fourth longitudinal 

 vein, the end of which forms a curve with its concavity turned backwards. 

 Closely related to Paraclius is Pelastoneurus ; the distinction between 

 both genera is easy, if attention is paid to the essentially different course 

 of the fourth longitudinal vein, to the very much more broad face, which 

 is also more convex upon its lower part, and to the not sessile but peduu- 

 culated hypopygium of the species of Pelastoneurus, which have the 

 feathered arista of the antennae in common wfth the species of Paraclius. 

 The genus Tachytrechus is very easily recognized by the great length 

 of the perpendicular diameter of the very hairy eyes, by the face, which 

 is narrower towards the middle but broader towards the bottom, and reachas 

 entirely as far as the inferior margin of the eyes ; the palpi are also com- 

 paratively small in the female, and the fourth longitudinal vein has before 

 the middle of its last segment a gentle flexure, from which it converges 

 towards the third longitudinal vein ; the figures of the antennae of the 

 male of T. moechus, which remind of the structure of the antennae of 

 Haltericerus, are an exception to the rule ; however a similar structure 

 occurs by way of exception in the genus Dolichopus. The genus Ortho- 

 chile is so much distinguished by the structure of the proboscis and 

 of the palpi as also by the distance of the tips of the third and fourth 

 longitudinal veins from the tip of the wing, that it cannot be mistaken 

 for any other genus. The genus Hercostomus, to which is to be added 

 a large portion of those species which in my previous works I have com- 

 prised in the genus Gymnopternus, embraces a variety of organizations and 

 is evidently capable of a subdivision into several genera ; in its whole 

 structure it shows the greatest resemblance to Gymnopternus, but in all 

 the species the third and fourth longitudinal veins, towards their ends, are 

 rather strongly converging, while in the species of Gymnopternus they are 

 either entirely parallel, or show but a very slight trace of convergency ; 

 the hypopygium has, in its structure, a great similarity with that of Doli- 

 chopus, but in some species the internal appendages are elongated, brush- 

 like and hairy, like those of Hypophyllus and Haltericerus, which, how- 

 ever, is also the case with some few species of Gymnopternus. The genus 

 Sybistroma, which hitherto remained confined only to a few species, is 

 nearest to the genus Hercostomus, differs, however, by the scutellum which 

 is nmch clothed with hair, by the peculiar structure of the face of the female 

 which is much protruding below, and by the peculiar structure of the 

 arista of the male*. The species of Hypophyllus are easily distinguished 



