We know very little about the division of sexes in the other social Hymenoptera, the Meliponas, the bumblebees, and the ants. On the other hand, this subject has been admirably studied by Fabre with numerous solitary nest-making Hymenoptera, especially those which provision their cells with honey and pollen and which constitute the group of solitary bees (Osmia, Chalicodoma and Anthidium), aside from the predatory species which give their progeny paralyzed victims (snch as the bee-eating Philanthus, Sphex, Solenius, etc.). With all these insects, whether they are honey-bearers or predators, the cell is closed after the egg-laying and the provisioning. With all of them, in addition, the male is smaller than the female, except with Anthidium sept end entatum and A. bellicosum, where it is larger.
The female determines the sex of the egg.
Fabre has stated that with the species in which the female is larger than the male the cells are of two kinds, some spacious and well filled, which give out females exclusively; the others smaller and less well provisioned, which always give out males.
Is it to the food, richer or poorer, that we must attribute this exact distribution of the sexes? Experiments will teach us. The large cells of Osmia tricoma are three times as large and as fully provisioned as the others. In a reed in which a row of cells of this bee was found, Fabre took away the large cells and added to the small ones a portion of the provision, and he found that on issuing the large cells still gave out females and the smaller ones males, the former of small size and the latter larger than usual. "This suffices," observes Fabre, "to do away with the very improbable supposition that the determination of the sex depends upon the quantity of food." We shall see later, in another way, that it is no more dependent upon the dimension of the cells. And from this it is proper to conclude that the egg at the moment of being laid has a determined sex,1 and that the role of the female consists in placing it in a proper cell.
The eggs have their sex at the moment of the laying, but are they equally sexed when they leave the ovarian tube to enter the oviduct (Figure 16, O, d) ? If this were so, the female should place her large and small cells in the order of the laying and would not be able to change, but it is just the contrary, as experiment shows. Fabre gave to his Osmia tricoma a composite glass tube of which one half was small and the other large, and the bee made in the first part little male cells and in the second large female cells. To another egg-layer he gave the mason cell abandoned by a Cha-licodoma, contracting certain cells and leaving the others intact, and the latter received female eggs and the former male eggs. What would be the laying if the insect could find only narrow cells? He offered to his Osmias a snail "whose shell, looking like a little ammonite, enlarged by slow degrees and bad at its mouth, in the usable part, a diameter hardly greater than that occupied by a male cell. '1 In order to make its full laying, a female occupied successively seven shells and put in them fourteen cells of which twelve were for males and two only for females. Another used eleven shells, and, being very fertile, placed in them twenty-six cells. "In this exceptional case twenty-five males were found, and one female,-one only, and occupying shell No. 17," which happened to be a slightly larger shell.
1 The observation also shows that the larvae of insects on issuing from the egg are already males or females and this sex can be recognized by certain characters.
Thus the female, aware of the sex of the egg, can, on account of the diameter of the cells, produce only eggs of males. Surely, says Fabre, "when it comes from the ovary the egg has not a determined sex. It is at the moment of laying, or a little before, that it has received the final touch which settles the sex."