Bees are classified according to common character traits
Bees can be classified in many different ways. Their taxonomic classification uses shared characters that are indicative of relatedness between species, and this is best expressed as an evolutionary tree. However, nest architecture, sociality and floral biology can also be used to classify bees. According to taxonomic classification, any species can only belong to one group. Other classification systems cross taxonomic boundaries, and species groups will differ according to the classification criteria. Buzz pollinators, for example, will include some carpenters and some diggers, but neither all carpenter nor all diggers. The different classification systems have their uses.
South African bees are divided into six cosmopolitan families. Four of them (Melittidae, Colletidae, Andrenidae and Halictidae) are short-tongued bees. They may be able to extend their mouthparts a long way into tubular flowers, but then the entire mouthparts will be elongated and not just the tongue. Two families are long-tongued bees (Apidae, Megachilidae). The South African pollen-collecting bees can all quite easily be placed into families using The Bee Genera and Subgenera of sub-Saharan Africa (http://www.abctaxa.be/volumes/vol-7-bees/). This booklet can also be used for identifying bee genera and sub-genera.
Bees construct their nests, and the cells within their nests, in many different ways. Some of these are unique to a taxonomic group: all leafcutter bees belong to the genus Megachile, but not all Megachile species are leafcutters – some are daubers or resin bees. Carder bees belong to several closely related genera. Mining tunnels into the ground occurs in many unrelated genera (and in different families), and boring into wood occurs in both long-tongued bee families. Knowing their nest architecture is useful for specific purposes. Tunnel-nesting bees, for example, only inhabit bee hotels, which are artificially provided tunnels for bees, usually in wood. Bees with similar nests could behave quite differently in them. Some small carpenter bees are social, with very small colonies, whereas others are solitary.
Most bees nest in cavities, while some have exposed nests. The best-known cavity nests are those of honeybees – they live naturally in holes in trees and in the ground, and are kept by beekeepers in hives. Stingless bees inhabit cavities similar to those of honeybees, but smaller, and they don’t dig their own cavities. All other cavity nests are tunnels. Some bore their own tunnels (miner and carpenter bees), while others use existing insect burrows, including those of other bees. They mostly raise their larvae in cells within their nests. Exposed nests are usually made of mud (daubers), resin (resin bees) or plant fibre combed to look like cotton wool (carder bees). Daubers, resin bees and carders have either exposed or cavity nests.
Exposed nests of dauber and resin bees are spherical, comprising several oval cells packed closely together. The whole nest, after all the cells have been constructed and provisioned, is rounded off into a ball with mud and/or resin. Carder bees simply have a ball of ‘cotton wool’ in which cells are constructed.
Among the cavity nesters, honeybees make cells out of wax (honeycomb). They use similar cells to store pollen and honey (made from nectar) that they feed to their larvae. Stingless bees have larval cells and much larger pots, also made from a waxy material, for storing honey and pollen. Only honey and stingless bees store food for their larvae outside brood cells. Other pollen- and nectar-collecting bees provision a brood cell and lay an egg in each cell. The materials used to make brood cells are reflected in their names: sand or mud (masons), resin (resin bees), wood shavings (carpenters), leaf or petal (leaf cutters), cellophane secretions (hyaline and plaster bees) or cottony fibre (carders).
Some bees do not collect pollen, namely males and parasitic bees. They are therefore not important pollinators. Most parasitic bees are cuckoos, meaning they lay their eggs in other bees’ nests. The parasite kills the host’s larva and feeds on its provisions. Social parasites replace, or co-exist with, queens of other social species. The host species’ workers raise the parasites larvae. Parasitic bees therefore do not construct nests.
Why are there so many female bees?
In bees, fertilised eggs develop into females and unfertilised eggs develop into males (therefore females have twice as many chromosomes as males). After mating, females store the sperm in a spermatheca and are able to control which egg cells are fertilised, thus they can determine whether they lay female or male eggs. This enables social bees, including honeybees, to have a hive full of female workers and only a few males – when needed.
Dr C. Eardley is from the Agricultural Research Council, Plant Protection Research Institute, and has worked on the taxonomy of African bees and the conservation of their biodiversity for 40 years. His email address is email@example.com.