28 NORTH AMERICAN DIPTERA. 



which is at or before the tip in Psychoda, and distinctly be- 

 hind it in Pericoma. In their study, one should use prefer- 

 ably a compound microscope, and it may be necessary either 

 to mount a wing, or at least to remove the scales. Attention 

 should be paid to the shape of the antennae and palpi, as well 

 as the genitalia, legs and wings. Psychoda alternata Say is 

 our most common species, apparently extending over all the 

 United States. 



8. 



Rather small, slender, nearly bare species. Proboscis some- 

 what projecting; palpi four-jointed ; antennae long, the basal 

 joints thick, those of the flagellum hair-like, and the joints 

 indistinctly distinguishable. Eyes round, dichoptic ; no ocelli. 

 Thorax strongly convex, without transverse suture ; scutellum 

 transverse ; metanotum arched. Abdomen long and slender, 

 composed of seven or eight segments, thickened posteriorly in 

 the male, pointed in the female. Legs long and slender; 

 coxae somewhat elongated ; tibias without terminal spurs. 

 Wings comparatively large ; auxiliary vein present, termina- 

 ting in the costa before the middle of the wing ; the second 

 vein arises from the first near the middle of the wing and 

 appears to be the beginning of the third vein, which continues 

 its direction while the second arches suddenly forward at the 

 anterior cross-vein and is furcate ; fourth vein furcate ; four 

 posterior cells present; the two basal cells very large; the 

 anterior cross-vein is placed at the beginning of the third 

 vein, where the second vein curves forward. 



The family Dixidse comprises about a score of known species 

 belonging to the single genus Dixa. It has been placed among 

 the Tipulidae and Culicidae, but seems best isolated into a sep- 

 arate family. The larvae are aquatic, resembling those of the 

 mosquitoes. The flies are found in bushy, moist places about 

 forests, and have been observed by Winnertz dancing in the 

 air in swarms. 



