The mason wasps are narrow-bodied insects. being coloured black with yellow stripes and spots. They are smaller than the more familiar social wasps. The adults emerge in May, or more usually in Yorkshire, in June. After mating the female searches for a nest site Yorkshire species either excavate cells in the ground or use hollowed-out plant stems or old beetle borings in wood. After a cell is built an egg is laid attached to the upper wall from which it usually hangs by a short thread. The cell is then filled with a variable number of paralysed insect larvae, usually the caterpillars of the Lepidoptera but sometimes the larvae of Coleoptera. The cell is then sealed. Each nest of cells is built and provisioned by a single female although several females may nest close together in a favourable nesting area On hatching from the egg, the larva eats the provided prey in one to two weeks and then usually enters a pre-pupal resting stage to pass the winter, pupation occurring some three weeks before the adults emerge. The pupal period of the female is longer than that of the male, so enabling the males to emerge first. The adults' main sources of food are the nectaries of flowers and the honeydew of aphids, although they probably take insect body fluid when malaxating the prey. The adults are particularly associated with the plant families Umbelliferae, Euphorbiaceae and Rosaceae. Fuller details of the biology of mason wasps are given by Spradbery (1973).