Sweat Bees

For many people, the common domestic honey bee may be the only bee species that they are aware of. In fact, bees are incredibly diverse, with well over 17,000 species known worldwide (and counting). Not all bees live in social hives like honey bees: the majority are solitary, with individual females each constructing their own nest and stocking it with food stores for their young. One particularly diverse group of bees is the Halictinae.

Foraging Lasioglossum, copyright Beatriz Moisset.

Halictines are mostly small bees, sometimes referred to as 'sweat bees' owing to the predilection of many species for lapping up sweat from the skin of hot humans and other animals (a habit that, while generally harmless, can be rather annoying). They can be distinguished from other bees by a distinctive curve at the base of the basal vein in the forewing. Michener (2007) recognised two tribes within the Halictinae, the cosmopolitan Halictini and the strictly Western Hemisphere Augochlorini. Augochlorins are often bright metallic in coloration; Halictini are less commonly so. Even among bee specialists, halictines can be notorious for the difficulties involved in trying to make sense of them. For instance, the cosmopolitan genus Lasioglossum alone comprises over 1300 known species, and having spent my own time attempting to identify bee specimens back in Australia I can confirm that there are times when it feels like all Lasioglossum, all the time. The majority of halictines construct their nests in burrows in soil; some species build in rotting wood.

Female Augochlora pura mosieri, copyright Bob Peterson.

The Halictinae are a particularly interesting group for studies of bee evolution because they include both solitary and social species. Indeed, some species may be either depending on circumstances. The most common nest type in Halictinae involves a long central tunnel with radiating side branches leading to globular brood cells. In most Augochlorini and species of the genus Halictus, however, the cells are arranged in a single cluster that is suspended within an underground cavity, held in place by earthen struts or by the rootlets of plants. The cells are lined with a protective waxy membrane rich in lactones, secreted by the builder from a gland near the base of the sting. Some species may be communal, with more than one female sharing a single burrow but each building and laying in its own cells (such communality is not necessarily a step on the road towards true sociality but may be a response to a shortage of good nesting opportunities). In social species, the queen is commonly not that different in appearance from associated workers, and if the queen dies the workers may begin producing eggs of their own (if, indeed, they were not already doing so while the queen was alive). Some species, though, may exhibit development of a distinct soldier or major class among the workers with massively enlarged heads and mandibles. In the Australian species Lasioglossum hemichalceum, there may be similarly large-headed males. These big-headed males also have reduced wings, rendering them flightless and bound to the nest. No more than one major male may be present in a colony; if another such male is present, the two will fight to the death. Unlike honey bees, halictine colonies do not often live for more than one season; instead, males and reproductive females usually mate near the end of the growing season, followed by the death of the males. The females hibernate over winter before beginning construction of their own nests the following spring.

Sphecodes albilabris, copyright Fritz Geller-Grimm.

In contrast, a number of halictine species, such as members of the genus Sphecodes, do not construct their own nests but instead lay their eggs in the nests of other bees. This behaviour, known as kleptoparasitism, has arisen in many bee lineages and is usually associated with a recurring set of evolutionary trends. Many kleptoparasites are closely related to their hosts: most kleptoparasitic halictines attack the nests of other halictines though some Sphecodes species mooch off bees in more distant subfamilies and families. Kleptoparasitic bees are commonly less hairy than their self-sufficient relatives, as they have little or no need of the pollen-carrying hairs used by other bees. Many kleptoparasites are more heavily armoured than other bees, to protect them against host resistance. Female Sphecodes have blunt spines on the outside of the hind tibia that may help them push into a host nest. Females of most kleptoparasitic halictines destroy the host egg in a nest cell before laying their own egg; in contrast, bees of other kleptoparasitic lineages usually leave the host egg undisturbed and it is the parasitic larva that executes the host. In most cases, the kleptoparasitic female abandons the nest once she has laid there, but in some species parasitising social hosts, the kleptoparasite may remain in the nest and inveigle herself into society there, continuing to enjoy the fruit's of her hosts' labours.

REFERENCE

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.