Cuckoo bees

The term cuckoo bee is used for a variety of different bee lineages which have evolved the kleptoparasitic behaviour of laying their eggs in the nests of other bees, reminiscent of the behavior of cuckoo birds. The name is perhaps best applied to the apid subfamily Nomadinae, but is commonly used in Europe to mean bumblebees Bombus subgenus Psithyrus. Females of cuckoo bees can be easily recognized in almost all cases, as they lack pollen collecting structures (the scopa) and do not construct their own nests. They often have reduced body hair, abnormally thick and/or heavily sculptured exoskeleton, and saber-like mandibles, although this is not universally true; other less visible changes are also common.

They typically enter the nests of pollen-collecting species, and lay their eggs in cells provisioned by the host bee. When the cuckoo bee larva hatches it consumes the host larva's pollen ball, and, if the female kleptoparasite has not already done so, kills and eats the host larva. In a few cases in which the hosts are social species (e.g., the subgenus Psithyrus of the genus Bombus, which are parasitic bumble bees, and infiltrate nests of non-parasitic species of Bombus), the kleptoparasite remains in the host nest and lays many eggs, sometimes even killing the host queen and replacing her - such species are often called social parasites, although a few of them are also what are referred to as "brood parasites."

Many cuckoo bees are closely related to their hosts, and may bear similarities in appearance reflecting this relationship. This common pattern gave rise to the ecological principle known as "Emery's Rule". Others parasitize bees in families different from their own, like Townsendiella, a nomadine apid, one species of which is a kleptoparasite of the dasypodaid genus Hesperapis, whereas the other species in the same genus attack halictid bees.

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