Any member of two genera constituting the insect tribe Bombini (family Apidae, order Hymenoptera), found almost worldwide but most common in temperate climates. Bumblebees are robust and hairy, average about 0.6–1 in. (1.5–2.5 cm) in length, and are usually black with broad yellow or orange bands. Bombus species are nest builders, often nesting in the ground, commonly in deserted bird or mouse nests. They live in organized groups, with a queen, drones, and workers (seecaste). Psithyrus species are social parasites (seeparasitism); they lay their eggs in Bombus nests, where the eggs and larvae are cared for by Bombus workers.