The ABDOMEN of bees has many shapes, its form being elliptical, cylindrical, subcylindrical, clavate, conical or subconical, and sometimes semicircular, or concavo-convex. It consists of six imbricated plates, called segments, in the female, and of seven in the male; in the latter sex, in several genera, it takes beneath at its base and at its apex, as well as at the extremity of the latter, remarkable forms and armature. It is very variously clothed and coloured, and sometimes extremely gaily and elegantly so; these various markings often giving the insects their specific characteristics; the clothing of the under side of this segment of the body, likewise, furnishes subsidiary generic characters, especially in the artisan bees, in whom it takes the place of the posterior legs as a polliniferous organ. This is possibly because were the supply conveyed _upon_ their posterior legs it would be rubbed away as they entered the narrow apertures of their nests. Nature does nothing in vain, and there is evidently a purpose in this arrangement. If we can trace peculiarities of structure to efficient reasons, differences of form may be rationally concluded as having their cause too, even if it elude our explanatory research. Although the reason of peculiar structure is not always obvious, it must exist, though undetected; as, for instance, why in some bees, as in _Megachile_, _Osmia_, _Chelostoma_, _Anthidium_, etc., the under side of the abdomen should be furnished densely with hairs to carry their provision of pollen home to their nest, when in other bees, as in _Dasypoda_, _Panurgus_, _Eucera_, _Anthophora_, etc. etc., it is conveyed upon the posterior legs, we do not know; we can only surmise that it is either to save the insect, in the former case, the labour of constructing a larger cylinder for nidification, so to prevent the possibility of its being rubbed off from the external surface of the legs, did these carry it, in entering the burrow, it being protected from this abrasion by being placed beneath the venter. In such insects the abdomen is usually truncated at its origin, or even hollowed within its base, thus to meet the projection of the metathorax, enabling it to draw itself closely up together, making the abdomen and metathorax, as it were, cohere. A different form of abdomen occurs in those bees which carry the pollen on their posterior legs. It is then more or less elliptical or lanceolate, which form permits the legs to be drawn up towards the metathorax within the space that kind of form furnishes, which, by this different but equivalent arrangement, meets the same object. The similarity of the adjustment of the abdomen to the metathorax to that of _Megachile_, etc. in _Apis_ and _Bombus_, by which insects the provision is also carried on the posterior legs, results from the totally different economy and habitation of the social bees, to which this structure is necessary for many purposes.