On this fundamental foam, so to call it, which is soiled with earthy matters, we see here and there masses of a beautiful white foam, in which the bubbles are much smaller.

A process of selection results from variations in density, and here and there we see foam white as snow resting on the dirty foam from which it is produced.

Something of the kind occurs when the Mantis builds her nest.

The two appendices whip the viscous secretion of the glands into foam.

The lightest portion, whose bubbles are of the greatest tenuity, which is white on account of its finer porosity, rises to the surface, where the caudal filaments sweep it up and gather it into the snowy ribbon which runs along the summit of the nest.
So far, with a little patience, observation is possible and yields a satisfactory result.