The laying is numbered automatically; each cocoon tells us its respective age by the place which it occupies.
A number of eggs bordering on fifteen represents the entire family of an Osmia, and my observations enable me to state that the distribution of the sexes is not governed by any rule.

All that I can say in general is that the complete series begins with females and nearly always ends with males.

The incomplete series--those which the insect has laid in various places--can teach us nothing in this respect, for they are only fragments starting we know not whence; and it is impossible to tell whether they should be ascribed to the beginning, to the end, or to an intermediate period of the laying.

To sum up: in the laying of the Three-pronged Osmia, no order governs the succession of the sexes; only, the series has a marked tendency to begin with females and to finish with males.
The mother occupies herself at the start with the stronger sex, the more necessary, the better-gifted, the female sex, to which she devotes the first flush of her laying and the fullness of her vigour; later, when she is perhaps already at the end of her strength, she bestows what remains of her maternal solicitude upon the weaker sex, the less-gifted, almost negligible male sex.

There are, however, other species where this law becomes absolute, constant and regular.
In order to go more deeply into this curious question I installed some hives of a new kind on the sunniest walls of my enclosure.