WILD BEES
19
burrowing into the soil, thus provisioning four cells in the first nest and two in the second, making a total offspring of six. Occasionally two females occupy a single nest, either simultaneously or one after the other. In the English colonies of A. ferox and A. bucephala, however, all the females share the same entrance hole at the same time. This may be an important clue to the way in which social behaviour was established in the Hymenoptera, and if the females can be shown to co-operate within the nest, would be the first report of social activity in this genus. After the period of flight in early summer the adult Andrenids die, males before females. T h e larvae develop in their cells in the soil during the summer, pupating in the autumn. T h e adults hatch in late autumn, but remain in their cells throughout the winter, burrowing to the surface in the spring, when colonies, sometimes consisting of many thousands of nests may develop. Since there is no known social co-operation between females, the factors determining the build up of such colonies have aroused interest, and in A. flavipes it has been found that females mate and nest only within the nest-site in which they themselves were reared. T h e nature of the soil and the lack of encroaching Vegetation, characteristic of the nest site, must favour this. Since they are strongly attracted by the odour of the site and not by that of the females, males will only seek nubile females within the nesting area, often ignoring those outside. T h e fact that this dependence on odour may be connected with the origins of social life is apparent when we consider how greatly the highly social honeybees depend on scents for their Organisation and communication. T h e origins of social behaviour have been of abiding interest to entomologists, and as the genus Halictus contains some social and semi-social species. as well as solitary species, it has commanded a great deal of attention. Although a number of social species are included among the English halictids, the most illuminating insights into their biology have come from French and Canadian work. T h e typical solitary nesting species, such as H. rubicundus, has a simple life cycle: females emerge from the ground in the spring, construct burrows and provision cells, from which males and females emerge and mate in late summer. T h e males die and the inseminated females burrow into the ground again, from which they may be dug up and put into Observation boxes in the laboratory. By simulating the conditions of spring, they may be induced to begin nesting prematurely. From this solitary nesting life, the origin of social life would theoretically require development along the following lines, each stage illustrated with present day examples:—