1910] Melander — The Genus Tachydromia ol 
arista is terminal and the palpi are long and narrow. ‘The front of the 
head is very narrow, its sides almost parallel. ‘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 equal 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, calva. 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 Coquillett * 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 (1903). 
