If the perpendicular face of the dihedral angle be high enough, it sometimes happens that a second row of cells is placed above the first row backing on to that plane; a third row occurs less often. The nest is then one of several storeys. The lower storeys, the older, contain only females; the upper, the more recent storey, contains none but males. It goes without saying that the surface layer, even of the lower storeys, can contain males without invalidating the rule, for this layer may always be looked upon as the Chalicodoma's last work.
Everything therefore contributes to show that, in the Mason-bee, the females take the lead in the order of primogeniture. Theirs is the central and best-protected part of the clay fortress; the outer part, that most exposed to the inclemencies of the weather and to accidents, is for the males.
The males' cells do not differ from the females' only by being placed at the outside of the cluster; they differ also in their capacity, which is much smaller. To estimate the respective capacities of the two sorts of cells, I go to work as follows: I fill the empty cell with very fine sand and pour this sand back into a glass tube measuring 5 millimetres (.195 inch.—Translator's Note.) in diameter. From the height of the column of sand we can estimate the comparative capacity of the two kinds of cells. I will take one at random among my numerous examples of cells thus gauged.
It comprises thirteen cells and occupies a dihedral angle. The female cells give me the following figures, in millimetres, as the height of the columns of sand:
40, 44, 43, 48, 48, 46, 47 (1.56, 1.71, 1.67, 1.87, 1.87, 1.79, 1.83 inches.—Translator's Note.),
averaging 45. (1.75 inches.—Translator's Note.)
The male cells give me:
32, 35, 28, 30, 30, 31 (1.24, 1.36, 1.09, 1.17, 1.17, 1.21 inches.—Translator's Note.),
averaging 31. (1.21 inches.—Translator's Note.)
The ratio of the capacity of the cells for the two sexes is therefore roughly a ratio of 4 to 3. The actual contents of the cell being proportionate to its capacity, the above ratio must also be more or less the ratio of provisions and sizes between females and males. These figures will assist us presently to tell whether an old cell, occupied for a second or third time, belonged originally to a female or a male.