The Osmia's repugnance to big cylinders is quite justified. The work in fact is longer and more costly when the tubes are wide. An inspection of a nest constructed under these conditions is enough to convince us. It now consists not of a string of chambers obtained by simple transverse partitions, but of a confused heap of clumsy, many-sided compartments, standing back to back, with a tendency to group themselves in storeys without succeeding in doing so, because any regular arrangement would mean that the ceilings possessed a span which it is not in the builder's power to achieve. The edifice is not a geometrical masterpiece and it is even less satisfactory from the point of view of economy. In the previous constructions, the sides of the reed supplied the greater part of the walls and the work was limited to one partition for each cell. Here, except at the actual periphery, where the tube itself supplies a foundation, everything has to be obtained by sheer building: the floor, the ceiling, the walls of the many-sided compartment are one and all made of mortar. The structure is almost as costly in materials as that of the Chalicodoma or the Pelopaeus. It must be pretty difficult, too, when one thinks of its irregularity. Fitting as best she can the projecting angles of the new cell into the recessed corners of the cell already built, the Osmia runs up walls more or less curved, upright or slanting, which intersect one another at various points, so that each compartment requires a new and complicated plan of construction, which is very different from the circular-partition style of architecture, with its row of parallel dividing-disks. Moreover, in this composite arrangement, the size of the recesses left available by the earlier work to some extent decides the assessment of the sexes, for, according to the dimensions of those recesses, the walls erected take in now a larger space, the home of a female, and now a smaller space, the home of a male. Roomy quarters therefore have a double drawback for the Osmia: they greatly increase the outlay in materials; and also they establish in the lower layers, among the females, males who, because of their earlier hatching, would be much better placed near the mouth of the nest. I am convinced of it: if the Osmia refuses big reeds and accepts them only in the last resort, when there are no others, it is because she objects to additional labour and to the mixture of the sexes.