( 20 ) 
original hypothesis ; but neither will his new view bear this 
general application, as I have indicated in the paper referred 
to, and which further experience has confirmed. We often 
detect a sexual difference in the form of the tibia, or the 
tarsi, but which is confined chiefly to the anterior pair, and 
to the genus Crabro. A dilatation in the form of a con- 
vexo-concave scale of either the anterior tibize or tarsi is 
found in several males of that genus, and in others we per- 
ceive a peculiarity in the structure of the intermediate tibiz 
and the first joint of their tarsi. The apex of the tibiz is 
always armed with a pair of moveable spines, called cal- 
caria, except the anterior pair, which have but one. One 
is sometimes pectinated, as in Ammophila. Mr. Kirby has 
supposed them to be for the purpose of assisting the in- 
sect in burrowing, but I consider them as designed to sup- 
port the leg, and to prevent its slipping, as they always 
stand off at an angle with the tibia, and they are move- 
able, that they may be adapted to the position of the limb. 
Those of the anterior pair are singularly formed, having a 
curved incision fringed with short hair in the interior, 
which is made to close over a corresponding incision, also 
fringed, at the base of the first joint of the tarsi, and they 
thus together form a circular aperture, through which the 
insect passes its antennz when it wishes to clean them, 
as it were combing them; or may not the friction be sub- 
servient to their receiving atmospheric impressions? for I 
have observed them thus occupied when there was not the 
least appearance of soil upon the antennz, and after long 
confinement in a box. 
The ABDOMEN is composed of seven joints in the male, 
and six in the female:* it is either sessile, subsessile, or 
* The female of Crabro verillatus, according to Vander Linden, forms an ex- 
ception, but I agree with St. Fargeau that Vander Linden has here made a 
mistake, and considered as the female of that species the male of another. 
