296 | Capt. F. Balfour-Browne on 
1. The Thoraz.—To some extent the form of the thorax is 
of use as a discriminating character, although by itself it 
is of little value. 
Comparing the males :—In the northern species the sides 
of the thorax tend to diverge, so that it is wider behind than 
in front, the greatest width being almost at the posterior 
angles. In the common species the sides of the thorax, 
although curved, run more or less parallel, the greatest width 
being some distance in front of the posterior angles. 
Comparing the females :—In the northern species the sides 
of the thorax are more or less parallel, so that it is about as 
wide, or very nearly as wide, posteriorly as anteriorly. Also 
the length in the median line from the anterior to the poste- 
rior edge is usually rather more than half the breadth at the 
broadest part. In the common species the sides of the thorax 
contract, so that it is distinctly narrower behind than in 
front, and the length in the median line is not quite half the 
breadth at the broadest part. 
The form of the thorax is, however, not an entirely reliable 
character, as some males of the northern species are more 
parallel-sided, and some males of the common species, espe- 
cially perhaps from some of the northern localities, have the 
thorax with at least a suggestion of greater width behind. 
In the females this character is even less reliable, which is 
specially unfortunate, since it is the best I can find, the 
female sexual armatures being quite similar. I will go so 
far as to say that the variation and overlapping of the two 
species in the form of the thorax in the female are such that I 
do not feel confident in determining the species from female 
specimens only. In three cases in which I had only females 
I named them tentatively. In two of these cases I later 
found males, and I happened to have been right in my deter- 
mination, but I still feel that the female character is not 
reliable. 
2. The anterior tarsal claws of the males form a useful 
character for distinguishing the two species. In the northern 
one these elaws are usually much longer and the inner claw 
is much more strongly curved than in the common species. 
In both cases the curve is nearer the apex than the base, but 
the claw is more bent in the northern species, while in the 
common one the curve is gentle and regular. Whereas the 
outer (posterior) claw in the northern species is often slightly 
shorter than the inner (anterior) one, in the common species 
the two claws are more usually equal in length. The claw- 
