GREGARIOUS NESTING OF THE MASON BEE HOPLITIS ANTHOCOPOIDES AND THE EVOLUTION OF PARASITISM AND SOCIALITY AMONG MEGACHILID BEES.

As E. 0. Wilson (1971) has shown in his modern classic "The Insect Societies," social insects are among the great achievements of organic evolution. Recent attempts to provide a theoretical basis for the evolution of insect sociality (e.g., Hamilton, 1964, 1972; Lin and Michener, 1972; Wilson, 1971) have relied heavily upon the bees (Apoidea), among which sociality has arisen repeatedly and all intermediate stages of social behavior occur (Michener, 1969). A second interesting evolutionary development has repeatedly arisen among the bees, that of cleptoparasitism or "cuckoo" behavior, in which species of bees lay their eggs in the nests of other species. Field studies of incipient sociality and cleptoparasitism, based upon populations of individually marked bees, are necessary for an accurate understanding of the evolution of these phenomena. Hoplitis (Ho plitis) anthocopoides (Schenck), a European mason bee (Hymenoptera: Megachilidae) that has recently emigrated to New York State (Eickwort, 1971), is normally a solitary nester-each female makes her own nest cells, provisions them, lays her eggs in them, and does not again contact her offspring. This behavior is typical of most species of bees. However, when nests are closely aggregated, the females exhibit two tendencies: to form communal associations in which several females share a single nest, and to usurp the cells made by other females. This species thus provides insights into the evolution of both sociality and cleptoparasitism. Most species in the family Megachilidae conceal their nests in cavities such as hollow twigs. The mason bees, however, build freestanding, mortar nests on the exposed surfaces of rocks and are thus excellent subjects for behavioral study. Fabre (1914) has made famous two species of the mason bee genus Chalicodoma, C. parietina (called C. muraria by Fabre) and C. pyrenaica, with his experimental studies of their nesting behavior. Fabre observed communal and parasitic behavior in C. pyrenaica and his observations will be compared with mine on Hoplitis anthocopoides in the following account. Although the nests of Hoplitis s.s. are superficially similar to those of Chalicodoma s.s., the genera are not closely related within the Megachilinae. Hoplitis is related to the predominantly cavitydwelling Osmia while Chalicodoma is related to the leafcutter-bee genus Megachile. I have previously given a general account of the biology of H. anthocopoides in New York (Eickwort, 1973) and described its nest-building behavior (Eickwort, 1974). A manuscript is in preparation on male territorial behavior. In connection with these studies, I individually marked all the mason bees that were nesting gregariously on a bridge foundation and I observed their behavior almost daily for their entire flight period in 1970. The following account describes communal and parasitic interactions and their importance on the population level. These interactions provide the basis for a discussion of how cleptoparasitism might have evolved in the Megachilidae and why true societies have not evolved in this family as they have in three other families of bees.