present, although in a state of repose frequently concealed 
beneath the clypeus; it is very various in form, in Oxybelus 
it is lunulate; in Pompilus transverse, rounded in front 
and slightly emarginate; in Ceropales triangular; in Phi- 
lanthus it is laterally reflexed, sinuate anteriorly, with alobate 
appendage beneath; in Cerceris and Diodontus distinctly 
emarginate; in Mellinus transversely quadrate; and in 
Pemphredon lugubris bilobate. In the majority it is ante- 
riorly fringed with long hair, and the general rule is, that 
when concealed beneath the clypeus it is membranaceous, 
and.only when fully exposed is it horny. The structure of 
the mandibles will frequently determine their use, as in 
Sapyga, Pemphredon, &c. where they are very broad, and 
armed with several teeth. We thus see their adaptation 
to the specific purpose of forming their cylindrical cells in 
wood. Their analogous conformation in the wood-boring 
bees (Megachile, Saropoda, &c.) corroborates this conclu- 
sion; but still this is not uniformly the case, for in several of 
the Crabronide, which are distinctly wood burrowers, the 
mandibles are merely bidentate at their apex. May not this 
disparity arise from the different nature of the wood they 
are instinctively led to form their nests in? for I have 
always found that the insects with this shaped mandible 
make their cells in sub-putrescent and soft woods, chiefly 
decaying willows; but the object of nature in giving to some 
of the true Fossores such immense arcuate mandibles as we 
perceive in Ammophila, Sphex, &c. is less obvious. The 
only apparent use to which I have observed them applied, 
is, in the female, to assist her in conveying her prey, and in 
the male, to seize the female. I have never observed any of 
the tribe feed with them; in fact their only sustenance, and 
that taken but rarely, appears to consist of the nectar of 
flowers. In the instance of Mutilla there is a great sexual 
