Think quick: Bee!
For most of us, a highly social hive of buzzing honey bees come to mind. But this is actually only a tiny sliver of the social structural pie. Here are some (but not all) other types of organization:
Solitary: Most bees are solitary, where a single female makes her nest alone. Solitary bees lay their eggs in small cells on top of a bed of food – the egg later hatches and feeds itself. Adults typically emerge from their cells around the same time, forage, lay their eggs, and then die while larvae/pupae wait underground for the next appropriate ’emergence’ season. This means adult generations do not overlap.
Gregarious nesters: These bees often appear social, as many solitary females will nest individually, but nearby one another, in ‘aggregations’.
Communal nesters: This is when multiple solitary females all share one nest, but lay their own eggs in individual cells within that nest.
Facultatively social: These species can be solitary or social, depending on environmental cues. In one species, Ceratina australensis, two sisters will sometimes form a colony together instead of nesting alone, with one foraging and reproducing and the other acting solely as a guard.
Primitively eusocial: Here, there are reproducing ‘queens’ and nonreproducing (but not sterile) ‘workers’. Queens and workers generally look similar, and workers can sometimes replace queens.
Advanced eusocial: The honey bee colony: reproducing queens, nonreproducing, functionally sterile workers. Workers and queens do not look similar. The workers care for the queen’s young, and there are overlapping generations of adults.
Wikipedia has a great chart (bottom of page) showing the differences between terms used to describe sociality, including: Eusocial, Semisocial, Subsocial,and Quasisocial.
This paper discusses some theory on the evolution of eusociality.
This paper addresses how advanced eusociality may have arisen through other types of sociality.