We shall have occasion to remark on some of the habits of _Dioxys cincta_ when considering the history of the mason-bee (_Chalicodoma_), but one very curious point in its economy must here be noticed. The _Dioxys_, which is a much smaller bee than the _Chalicodoma_, lays an egg in a cell of the latter, and the resulting larva frequently has more food in the cell than it can consume; there is, however, another bee, _Osmia cyanoxantha_, that frequently takes advantage of an unoccupied cell in the nest of the _Chalicodoma_, and establishes its own offspring therein. The _Dioxys_, it seems, cannot, or at any rate does not, distinguish whether a cell is occupied by _Chalicodoma_ or by _Osmia_, and sometimes lays its egg in the nest of the _Osmia_, though this bee is small, and therefore provides very little food for its young. It might be supposed that under these conditions the _Dioxys_ larva would be starved to death; but this is not so; it has the power of accommodating its appetite, or its capacity for metamorphosis, to the quantity of food it finds at its disposal, and the egg laid in the _Osmia_ cell actually produces a tiny specimen of _Dioxys_, only about half the natural size. Both sexes of these dwarf _Dioxys_ are produced, offering another example of the fact that the quantity of food ingested during the lifetime of the larva does not influence the sex of the resulting imago. The highly endowed bees that remain to be considered are by some writers united in a group called Apidae, in distinction from Andrenidae. For the purposes of this work we shall adopt three divisions, Scopulipedes, Dasygastres, Sociales.