ERIOPTERA. 151 



The North American species, contained in the genus Erioptera, 

 as defined above, may be distributed into the following groups : — 



A. The pnefurca ends in the second submarginal cell, which is longer 



than the first ; the inner end of the discal cell (or, when it is open, 



of the cell with which it coalesces) is on the same line with the 



small cross-vein (Tab. I, fig. 16, 17, 18). 



1. The posterior branch of the fourth longitudinal vein is forked (in 



other words, when the discal cell is open, it coalesces with 



the second posterior cell ; when it is closed, the inner end 



of the third posterior cell is nearer the basis of the wing 



than the inner end of the second). 



a. The seventh longitudinal vein is arcuated (converging to- 



wards the sixth) in such a manner, that the axillary 

 cell is broader in the middle than near the margin of 

 the wing (Tab., I, fig. 16) : subgenus Erioptera. 



The six species of this subgenus (E. chlorophylla, straminea, 

 vespertina, sejrfemtrionis, chrysocoma, villosa) form a very natural 

 group ; their venation is exactly the same ; their discal cell is open, 

 coalescent with the second posterior cell ; their third posterior 

 cell is rather long ; their male forceps seems to be built upon the 

 same plan, and consists of two basal pieces, bearing several horny 

 branches each (compare Tab. IT, fig. 20, the forceps of E. vesper- 

 tina) ; their wings are immaculate, their feet without well-marked 

 bands. Although the above named six North American species 

 have the discal cell open, the mere fact of its being closed would 

 not prevent a new species from being included in this group, if 

 the agreement in the other characters was sufficient. The present 

 group almost answers to Dr. Schiner's genus Trichosticha ; but 

 it seems to me that Erioptera is a more appropriate name for it, 

 as it will probably include the majority of the species, as well as 

 the most typical forms, of the genus Erioptera in the sense of 

 Meigen's principal work. 



b. The seventh longitudinal vein is straight, diverging from the 



sixth ; hence the axillary cell is much broader near 

 the margin of the wing than in the middle ; discal cell 

 closed. 

 * The fork of the posterior branch of the fourth longitudi- 

 nal vein (containing the third posterior cell) has 

 the usual structure, that is, consists of two gently 

 arcuated branches (Tab. I, fig. 17) : subgenus 

 Acyphona. 



