350 



DIPTERA OF NORTH AMERICA. 



[PART II. 



The SECOND PRINCIPAL DIVISION of the Dolichopodida? is 

 divided, according to the shape of the third joint of the antennae, into two 

 sub-divisions ; the genera of the Jirst sub-division (No. 17-23) have this 

 joint, either in both sexes or at least in the male, pointed and provided with 

 an apical arista ; in the genera of the second sub-division (No. 24-44), it is 

 short in both sexes, and if it should be somewhat elongated in the male, 

 it is not pointed, and the arista is either dorsal, or, at the utmost, sub- 

 apical. 



The genera of the First Sub-division are divided into hvo groups. To 

 the first group belong those genera, the posterior transverse vein of which is 

 distant from the margin of the wing. They are the following : Synarthrus 

 (17), Systenus (18), Rhaphium (19), Xiphandrium (20), Porphyrops (21), 

 and Smiliotus (23). Synarthrus shares with Syntormon not only the 

 peculiar structure of the second joint of the antennae, but also the remain- 

 ing characters, with the single exception of the first joint of the antennae, 

 which is glabrous. Systenus strikingly differs from all the other genera 

 of the group by the pointed abdomen and the very pedunculated hypo- 

 pygium of the male ; to its distinguishing characters may also be reckoned 

 the distinct pubescence of the third joint of the antennae, and the remark- 

 ably sharp dividing line between the lower part of the face and the upper 

 one. Rhaphium stands in close relation to the two following genera, is 

 however distinguished from them by the very elongated antennae, even in 

 females, and by the still more considerable stoutness of the female pro- 

 boscis ; moreover from Xiphandrium it is distinguished by a less slender 

 structure of the body, a much stronger pubescence, and a stronger flexure 

 of the fourth longitudinal vein. The species of Xiphandrium have, like 

 the species of Porphyrops, only in the males an elongated third joint of the 

 antennae, in the females it is short ; their difference consists in a greater 

 slenderness, less pubescence, and less flexure of the fourth longitudinal 

 vein ; moreover the largest species of Xiphandrium are scarcely equal in 

 the length of their body to the smallest species of Porphyrops. The dis- 

 tinguishing characters of the genus Porphyrops become manifest from 

 what has just been stated about the two preceding genera. The genus 

 Smiliotus has the antennae of equal size and form in both sexes ; they 

 have on the under side, from the tip almost to the base, a distinct ex- 

 cision ; the face of the male is broad, and its palpi are as large as those 

 of the female ; finally, the abdomen shows externally one segment less 

 than the related genera ; the feet are comparatively short and rather vigor- 

 ous ; the small hypopygium is deeply imbedded and its appendages are 

 often difficult to perceive. 



To the second group, which is characterized by a close proximity of the 

 transverse vein to the posterior margin of the wing, belongs the only genus 

 Aphrosylus (23) ; the third joint of its antennae is but little elongated, 

 but pointed ; the pendant palpi are larger in the male than in the female ; 

 the proboscis is bent towards the chest, the face is narrower above, and 



