bumblebee

bumblebee, (genus Bombus), genus of over 250 species of large bees. Bumblebees occur over much of the world but are most common in temperate climates. They are absent from most of Africa and the lowlands of India and have been introduced to Australia and New Zealand to aid in the pollination of various flowering plants.

Authorities have long recognized two genera: Bombus, the nest-building bumblebees, and Psithyrus, the parasitic cuckoo bumblebees. In the first quarter of the 21st century the group underwent a major taxonomic revision, and Psithyrus is now nested as a subgenus within Bombus.

Physical description

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Like all insects, bumblebees have a chitinous exoskeleton, and their bodies are divided into three segments: the head, thorax, and abdomen. Bumblebees are robust and hairy, average about 1.5 to 2.5 cm (about 0.6 to 1 inch) in length, and are usually black with broad yellow or orange bands. Two large compound eyes and three simple eyes, or ocelli, are located on top of the head. Their keen eyesight, which includes the ability to perceive ultraviolet light, is complemented by two sensitive odor-detecting antennae. Workers are equipped with a specialized pollen-collecting structure on the fourth segment of both of the hind legs, known as a pollen basket, or corbicula.

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Like honeybees, bumblebee workers are equipped with a venomous stinger at the tip of their abdomen. Although they can sting multiple times, most are not aggressive and only sting defensively if provoked.

Natural history

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Most Bombus species are social bees; i.e., they live in organized groups. Each nest has a fertile queen, drones (males), and infertile female workers. They often nest in the ground, commonly in deserted bird or mouse nests.

The nest-building Bombus queen lays her eggs in the nest after spending the winter in hibernation. The first brood generally develops into four to eight worker bees, all of whom are female. Shortly after emerging as adults these workers take over from the queen the duties of collecting pollen and caring for the hive. The queen then retires to a life of egg laying. For a while only worker progeny are produced, and the colony grows until it contains 50 to 600 bees. In late summer, with the large population of workers bringing in abundant food, males and new queens are produced. Although some males develop from unfertilized eggs laid by the queen, most hatch from eggs laid by workers. In early fall the original queen stops laying eggs, and the colony, including the queen, gradually dies out. During this period the larvae of certain moths and beetles prey on the remaining eggs and larvae in the nest.

The next season’s queens have developed from eggs laid late in the season. The future queens, when fully grown, leave the nest, mate, and find a sheltered place in which to hibernate for the winter. The lone queens then start new nests in the following spring.

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Cuckoo bumblebees

The former Psithyrus species are brood parasites. They are often called cuckoo bumblebees because they lay eggs in the nests of social bumblebees in a manner analogous to some cuckoos (bird order Cuculiformes), which lay eggs in the nests of other birds. Cuckoo bumblebees, having no worker caste, enter the nests of nest-building Bombus species to lay their eggs, which are then cared for by duped workers. The resemblance between a parasitic species and the species it parasitizes is often remarkable. The British species B. vestalis sometimes stings the nest-building Bombus queen to death. There are then no nest-building larvae produced to compete with the parasitic larvae for the attention of the workers. See also cuckoo bee.