Halictinae
Within the insect order Hymenoptera, the Halictinae are the largest, most diverse, and most recently diverged of the four halictid subfamilies. They comprise over 2400 bee species belonging to the five taxonomic tribes Augochlorini, Thrinchostomini, Caenohalictini, Sphecodini, and Halictini, which some entomologists alternatively organize into the two tribes Augochlorini and Halictini.
The subfamily Halictinae also belongs to the hymenopteran monophyletic clade Aculeata, whose members are characterized by the possession of a modified ovipositor in the form of a venomous sting for predator and prey defense. Including all eusocial and cleptoparasitic Halictidae taxa, these small bees are pollen feeders who mass provision their young and exhibit a broad spectrum of behavioral social polymorphies, ranging from solitary nesting to obligate eusociality. Estimated from the fossil record, eusociality in this subfamily evolved about 20 to 22 million years ago, which is relatively recent in comparison with other inferred eusociality origins. Thus, the Halictinae are believed to model the primitive eusociality of advanced eusocial hymenopterans. Because of their polymorphic sociality and recently evolved eusociality, the Halictinae are valuable to the study of social evolution.
Tribes
Augochlorini
The roughly 250 species belonging to the tribe Augochlorini exist only in the New World, mainly inhabiting the Neotropics and some areas of North America. Augochlorini sociality, though not well understood, is significantly polymorphic across its range, as well as between and within species and genera. Facultative eusociality has been observed in genera such as Augochloropsis and Megalopta, and cleptoparasitism has recently developed separately in the three augochlorine genera and subgenera Temnosoma, Megalopta , and Megommation .Thrinchostomini
The two genera Thrinchostoma and Parathrincostoma comprise the tribe Thrinchostomini. These insects are large, nonmetallic bees residing in Madagascar and the African and Asia tropics. Twelve of the 56 Thrinchostoma species are native to Madagascar and exhibit some host-plant specificity. Parathrincostoma species, two of which are native to Madagascar, are likely cleptoparasites, indicated by a lack of pollen-collecting structures in their female morphology. Though evidence of sociality and nesting biology is limited, observed populations in southern Africa are believed to be solitary.Caenohalictini
Species of the tribe Caenohalictini inhabit areas in only the New World and are similar in physical appearance to Augochlorini. Caenohalictine species practice either solitary or communal nesting. Some genera are nocturnal.Sphecodini
The tribe Sphecodini contains five cleptoparasitic genera, Austrosphecodes, Eupetersia, Microsphecodes, Ptilocleptis, and Sphecodes. Species of the tribe oviposit their eggs onto or near the pollen stores of their hosts' nests. These cleptoparasitic bees can be host generalists or specialists. Some species exhibit especially aggressive parasitism, attacking and sometimes killing solitary or social nest host female before ovipositing eggs into pollen-provisioned host cells. Such parasites inhabit every continent. Some taxonomists regard it as part of the tribe Halictini.Image:Lasioglossum2843.06.w.jpg|thumb|left|Lasioglossum bee on mountain mint
[Halictini]
Assigned more than 2000 described species, the Halictini are the largest tribe of halictid bees, including considerable behavioral diversity. Lasioglossum, Mexalictus, and Patellapis sensu lato are notable genera. Most species belong to the genus Lasioglossum, which encompasses a variety of nocturnal and diurnal, socially parasitic, solitary, eusocial, and communal bees. Lasioglossum species are distinguished by a weakened outer wing venation, while species of Mexalictus resemble Lasioglossum in body shape, but possess strong wing venation. Mexalictus includes six described species of rare bees observed in humid areas of high elevation ranging from southeast Arizona to northern Guatemala. The social behavior of Mexalictus species is unknown. Recent observational data of Patellapis s. l. suggest the genus practices communal nesting, with as many as eight females sharing a nest. Most Patellapis s. l. species inhabit southern Africa and Madagascar, though species are also found in tropical Asia and northern Australia.Social diversity
A great diversity of social systems exists between and within halictine species. These discrepancies in social phenologies occur both locally and across geographic locations. The variety of colony organizations expressed by halictine species is represented along a gradient ranging from solitary nesting to obligate eusociality.Eusociality
Eusocial behavior is associated with cooperative nesting and brood care, an overlap in adult generations, and a division of social roles, marked by intracolony reproductive hierarchies. The social roles of a eusocial colony are distributed into castes that include the reproductive queen, female workers which forage for pollen and care for the brood and nest, and brood composed of potential workers and reproductive males and gyne. In the Halictinae, one or more females found a colony site, and initiate its development, first producing brood of workers to forage for resources and care for future offspring followed by brood of males and gynes to mate and propagate the nest's genes. A mated gyne is a potential queen that will either disperse to a new nest, succeed to the position of the former nest queen, or suffer subordination or harm by the current queen.In a mating season, a eusocial halictine queen usually lays multiple broods, with earlier broods composed of dominantly female workers and later broods of reproductive males and gynes. A eusocial queen monopolizes the reproductive capacity within a nest, preventing reproduction of colony workers. Overlapping of adult generations importantly enables worker daughters to contribute to the rearing of later broods. However, eusociality also exists within generations with no adult overlap. As a result of the heavy work load requirement of nest building and repeated food provisioning in eusocial colonies, nest sharing also saves time, energy, and natural resources.
When multiple females found a colony, a dominant queen can subordinate another foundress. However, such foundresses may remain in the colony, likely because of the potential to succeed to the queen caste if the current queen dies or loses her position. Also, subordinate females may find an opportunity to lay their own eggs in the nest.
The large colony size of eusocial insects elicits both costs and benefits. Increased group size is often correlated with increased predator attraction. However, in appropriate situations, aggregations of individuals can provide more effective predator defense. Under situations of low parasite and predator threat as well as plentiful territory and resource availability, the risk of attracting a predator from aggregating is greater than the possible benefits of collaborative nesting and reproductive rearing. When parasite and predator threats are high and territory and resources are limited, a greater number of individual workers may improve parental care of offspring. For example, increased numbers of workers in a colony increase the effectiveness of nest defenses such as stinging enemies and blocking nest entrances. Possession of venomous stinger in the Halictinae were likely beneficial in the subfamily's evolution of eusociality by providing a thwart to the increased predator attention caused by group living. Also a benefit of cooperative nesting, the requirement for foraging away from the nest does not necessitate temporary nest abandonment when a surplus of females are available to stay behind in the nest, reducing risks of brood predation.