NESTING BIOLOGY OF THE BEE, OSMIA DIMIDIATA MORAWITZ, 1871 (HYMENOPTERA, MEGACHILIDAE) IN THE CRIMEA
Nest structure and composition have been studied based on the material of 133 nests of Osmia dimidiata taken from trap-nests placed in five localities of Crimea. The nests contain from 1 to 9 cells (mean, 2.2). Partitions between the cells in the nests are made from masticated leaves. Most of the cells have massive hemispherical bottom and thin flat lid. Final plug has the structure similar to the bottoms of the cells. The cells are placed in the nest cavity solely or arranged in rows of 2 or 3 cells separated by the galleries. Diameter of the occupied cavities positively correlates with the rate of the females in the posterity and with the mass of the cocoons with imagoes and negatively correlates with the lengths of the cells. The cells with females are 1.5 times as long as cells with males. The mass of the cocoons with females is twice that of the cocoons with males. Peculiar characters of O. dimidiata nest structure may mean that the natural nesting substrates of this species in the nature are abandoned nest cells of other bee and wasp species. The characteristic mechanism of the estimation of the mass of the pollen loaf stored in the cells and of the determination of the sex ratio in the posterity had evolved in the O. dimidiata due to its tenantry. This mechanism is based on two constants: the sex-linked length of the cells, and fixed length of the distance between the pollen loaf and the cell lid. The choice of the nest cavities by the female has the special emphasis: she prefers broader and longer tubes for the cells with females and narrower and shorter ones for the cells with males. General sex ratio of the posterity in the nests is male-biased (1 ♀ :2.5 ♂) and depends on the food supply abundance. Imago flight of O. dimidiata lasts in Crimea from the last ten days of May until the end of July. Hibernation occurs at the adult stage in the cocoons. Females collect pollen from flowers of the plants of the family Asteraceae or, rarely, Lamiaceae. The nests are disrupted by Melittobia acasta, Monodontomerus aereus, M. obscurus, Sapyga quinquepunctata, Stelis phaeoptera and Trichodes apiarius; 21 % of the posterity dies via these enemies, and 22 %, from other affects. Explanations why Pseudosmia taurica Radoszkowski, 1874 (nec Osmia taurica Radoszkowski, 1887) is not a synonym of O. dimidiata are given.