296 Capt. F. Bal four-Browne on 



1. The Thorax. — To some extent the form of the thorax is 

 of use as a discriminating character, although by itsell: 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 daws of the males form a useful 

 character for distinguishing the two species. In the northern 

 one these claws 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- 



