Abstract
The structure and composition of nests of Osmia rufa have been investigated. The weight of the septa between the cells increases regularly from the first to the last cell, whereas the length of cells and weight of the pollen loaf decrease. The difference of weight between cells of males and those of females is sharper. The number of cells and weight of cocoons with adults decrease in the series of nests built by the same female during the nesting season, and the sex ratio shifts to males. Abundant food resources facilitate an increase in the average weight of the offspring, the number of cells, and shift of the sex ratio toward females. The mechanism of the sex ratio regulation in O. rufa nests includes three consecutive elements. The key element is the presence of a marker of the switching to making low-weight pollen loafs. This marker is the moment of the “overloading” of the last cell in a row of cells intended for future females. Daughters of large females are smaller, and those of small females, larger than their mothers. The probable cause of this discrepancy is the allometric relation between the female’s weight and its working capacity.
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