Eusociality is the phenomenon of reproductive specialisation found in some species of animal, whereby a specialised caste carries out reproduction in a colony of non-reproductive animals.
The most familiar examples are insects such as ants, bees, and wasps (the order Hymenoptera), with reproductive queens and sterile workers. Eusociality may be easier for these species to evolve due to their haplodiploidy, which increases the significance of kin selection; it is thought that eusociality evolved 11 separate times within this order. The Hymenoptera are often referred to as "social insects", but it is now preferred to use the term "eusocial insects" for those species that exhibit eusociality, and not to apply the term "social insects" to other Hymenoptera: it is important to realise that not all the Hymenoptera are eusocial.
Another extremely widespread insect group exhibiting eusociality are the termites (order Isoptera). Eusociality is also known among mammals (several naked mole rat species), crustaceans and other arthropods.