12 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. 62. 
sheath. The thorax is rather heavy. Each abdominal segment has 
a circlet of bristles or thorns which are directed backward; the spir- 
acles are situated on the proturberances along the anterolateral line 
and there are spines around them; the apical segment ends in two 
long slender thorns which are touching at the base, or in what might 
be termed a bifid spine. 
The writer had the opportunity of examining the early stages of 
Psilocephala frontalis, new species, P. haemorrhoidalis Macquart, 
P. argentifrons, new species, and P. limata Coquillett. Descriptions 
of these are given following the account of the adults and they are 
illustrated on the plate 13. 
GEOGRAPHICAL DISTRIBUTION. 
Counting the species described in the present paper, there are 496 
species and 43 genera in the Therevidae. Only Therera and Psilo- 
cephala are spread over practically all countries, the other genera 
having a more or less limited distribution. Kréber lists 9 genera 
as Palearctic only, 11 as Indo-Australian only, 4 as North American 
only, 2 peculiar io Africa, and 1 to South America. To the peculiarly 
North American genera must be added the new genera described in 
the following pages: Caenotus, Chromolepida, and Pherocera. As in 
other groups, we find the simultaneous appearance of the same or allied 
genera in Australia, South America, and South Africa. 
No European species are known from North America at the present 
time; the European species Thereva annulata Fabricius is very close 
to Thereva vialis Osten Sacken, the male being scarcely distinguish- 
able, but Maj. E. E. Austin has kindly compared the two species and 
has found the females to be quite distinct, although closely allied. 
We know scarcely anything of the Mexican species or of those from 
Central America; Bellardi and Bigot described a few species and later 
Osten Sacken and Williston, but their material was very meager. 
Most of the Therevidae do not penetrate far into northern lati- 
tudes, as they are flies that prefer bright sunshine, but a few species 
go north into Canada and some appear to be truly Boreal in their 
distribution. Some species of Psilocephala have a rather wide distri- 
bution in this country, but most are local and only a few Boreal species 
are known to occur clear across the continent. Psilocephala aldricha 
has been recorded from the east to the west coast, but the eastern 
and western forms are quite distinct species. Thereva brunnea, new 
species, T. senex, T. albifrons, and T. frontalis are Boreal species and 
Psilocephala munda appears to be confined to northern latitudes or to 
high altitudes and occurs from the east to the west coast, as does T’he- 
reva frontalis. Psilocephala haemorrhoidalis and P. frontalis, new 
species, are two common eastern species which are so nearly alike that 
