1910] Melander — The Genus Tachydromia 51 



arista is terminal and the palpi are long and narrow. The front of the 

 head is very narrow, its sides almost i)arallel. The emargination of 

 the eyes at the level of the antennae is less deep, and all the facets 

 are of nearly uniform size. The pectus is pruinose, the coating extend- 

 ing backwards to form a conspicuous glistening white spot over the 

 front coxae and under the humeri. The hypopygium is also somewhat 

 smaller than with the other members of the genus. The first basal cell 

 is generally very long. It is to this group that arrogans and connexa 

 belong. 



The species of the second group differ in having a shorter and broader 

 thorax, with the humeri not so pronounced. They lack the pruinosity 

 above the front coxae. The arista is subterminal and the palpi are 

 usually broader. The front of the head is broader, with its sides 

 diverging above. The eyes are more deeply emarginate, and the 

 lower facets are conspicuously larger than the upper. The wings are 

 shorter in proportion to the body, and are not fasciate; the two basal 

 cells are more nearly etjual in size, and the marginal cell is usually 

 shorter. 



Although the genus separates into two definite groups whose char- 

 acters may seem to be of generic value, I hesitate about placing together 

 the species of group two as a restricted genus, for they appear to repre- 

 sent several phyletic lines. The basic points of difference between 

 these species are the following: 



1. simplicior. Wings as in Drapetis: palpi narrow: thorax glistening,, 

 devoid of bristles: humeri prominent. 



2. maculipennis, colva. Palpi narrow: thorax narrow, glistening black,, 

 devoid of bristles, humeri prominent. 



3. insularis. Thorax shorter, somewhat glaucous, humeri smaller: palpi 

 long and narrow. 



4. agens, universalis. Thorax somewhat glaucous, shorter, with bristles; 

 humeri smaller: palpi broader. 



The table following is given for the determination of the American 

 species. Several other species have been referred to this genus by one 

 writer or another. The accompanying notes will explain their status. 



Tachydromia lata Co(juillett ^ is omitted from the tables as it probably 

 is a Drapetis. Since the description states that the mesonotum is 

 broader than long, the legs are provided with bristles and the first 



1 Proc. Ent. Soc. Wash. V. p. 266 (190.3). 



