Cuckoo bee
Image:Cuckoo bee.jpg|300px|thumb|upright=2|A cuckoo bee from the genus Nomada, sleeping.
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 sometimes used in Europe to mean bumblebees in the subgenus Psithyrus. Females of cuckoo bees are easy to recognize in almost all cases, as they lack pollen-collecting structures 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.
The number of times kleptoparasitic behavior has independently evolved within the bees is remarkable; Charles Duncan Michener lists 16 lineages in which parasitism of social species has evolved, and 31 lineages that parasitize solitary hosts, collectively representing several thousand species, and therefore a very large proportion of overall bee diversity. There are no cuckoo bees in the families Andrenidae, Melittidae, or Stenotritidae, and possibly the Colletidae.
Cuckoo bees 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, 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 melittid genus Hesperapis, whereas the other species in the same genus attack halictid bees.