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Bumblebees are social insects which form colonies with a single queen. Colonies are smaller than those of honeybees, growing to as few as 50 individuals in a nest. Female bumblebees can sting repeatedly, but generally ignore humans and other animals. Cuckoo bumblebees do not make nests; their queens aggressively invade the nests of other bumblebee species, kill the resident queens and then lay their own eggs which are cared for by the resident workers.

Bumblebees have round bodies covered in soft hair (long, branched setae), called pile, making them appear and feel fuzzy. They have aposematic (warning) coloration, often consisting of contrasting bands of colour, and different species of bumblebee in a region often resemble each other in mutually protective Müllerian mimicry. Harmless insects such as hoverflies often derive protection from resembling bumblebees, in Batesian mimicry, and may be confused with them. Nest-making bumblebees can be distinguished from similarly large, fuzzy cuckoo bees by the form of the female hind leg. In nesting bumblebees, it is modified to form a pollen basket, a bare shiny area surrounded by a fringe of hairs used to transport pollen, whereas in cuckoo bees, the hind leg is hairy all round, and pollen grains are wedged among the hairs for transport.