The bees are here divided into the following six families: Colletidae, Andrenidae, Halictidae, Melittidae, Megachilidae and Apidae. Although the Apidae includes all the basically social species, there are other species in other families which are also primitively social. Like the wasp species, female bees build and provision their nests in a variety of situations; these may be subterranean or aerial, or even in empty snail shells. The nest may consist of one or several cells. The cell walls are often of the surrounding material, e.g. soil or the in situ plant material, but may be lined with collected plant material, resin, mud or glandular secretions which harden to thin, cellophane-like membranes. The larval food consists of nectar and pollen, sometimes with plant oils, and the cells are mass provisioned. The pollen is normally carried on specialised hairs on the propodeum, ventral gaster or hind legs, but sometimes in the crop of the female. Nectar is always carried in the crop. Some species collect pollen from a limited number of related plant species - these are called oligolectic bees. Others collect pollen from many plant species and are called polylectic species. When each cell is fully provisioned, an egg is laid and the cell sealed. There may be one or more generations each year. Several species are cleptoparasitic on other species of solitary bees. Some halictides are primitively eusocial. The queen rears a first brood, some of which, as adults, remain in the nest as workers and help the queen to rear a second brood of future queens and males.