When it attains the adult stage it requires a certain amplitude of lodging, which the other weevils do not require in the same degree.

A pea provides it with a sufficiently spacious cell; nevertheless, the cohabitation of two in one pea would be impossible; there would be no room, even were the two to put up with a certain discomfort.

Hence the necessity of an inevitable decimation, which will suppress all the competitors save one.
Now the superior volume of the broad bean, which is almost as much beloved by the weevil as the pea, can lodge a considerable community, and the solitary can live as a cenobite.

Without encroaching on the domain of their neighbours, five or six or more can find room in the one bean.
Moreover, each grub can find its infant diet; that is, that layer which, remote from the surface, hardens only gradually and remains full of sap until a comparatively late period.

This inner layer represents the crumb of a loaf, the rest of the bean being the crust.
In the pea, a sphere of much less capacity, it occupies the central portion; a limited point at which the grub develops, and lacking which it perishes; but in the bean it lines the wide adjoining faces of the two flattened cotyledons.