Dark-edged Bee-fly

In Britain, there are only four species of Bee-fly in the genus Bombylius. These are: B. canescens, B. minor, B. discolor and B. major. Of these 4, B. canescens and B. minor are the rarest, with the former mainly confined to the open, flowery habitats of south-west England and south Wales, and the latter only recorded in Dorset and the Isle of Man in recent times. B. discolor is more common than the previous pair, however it is still largely confined to southern England and the south Wales coast, though records from Warwickshire indicate that it is spreading north.

Bombylius major is the most common of the 4 British Bombylius species and is easily distinguished from the others by the solid dark band on the leading edge on the wings. This feature has also inspired its imaginative name of the 'Dark-edged Bee-fly'. Its distribution covers practically all of England and Wales, and the some parts of Scotland. It is also the only species of Bombylius found on the UEA campus.

Bombylius major distribution map © NBN Atlas

Bee-flies are named as such because they are flies which mimic and parasitise bees. They are fairly large for a fly, with the body length of B. major measuring 6-12.5mm (not including the extended proboscis). Bombylius species generally inhabit open areas, such as gardens, grasslands and open woodland clearings, although the Western and Heath Bee-flies (B. canescens and B. minor) require more specialist habitats.

Dark-edged Bee-fly (Bombylius major) at rest on the UEA campus, showing
off its ridiculously long proboscis!

Bombylius species have a very interesting life cycle as they lag their eggs in the nests of solitary mining bees. The bees species that they parasitise varies between bee-flies. Andrena species of bee are the main hosts of B. major and B. discolor, with the latter particularly favouring Andrena flavipes and Andrena cineraria. B. canescens favour the Lasioglossum and Halictus species, while B. minor chooses species of Colletes mining bees.

In order to parasitise their chosen species, the adult female bee-flies collect sand or dust at the tip of their abdomen and use it to coat their eggs. It is thought that they do this in order to camouflage them and to make the eggs heavier. Increasing the weight of the eggs makes the next stage easier, whereby the female locates a nest entrance of the chosen mining bee, hovers above the hole, and proceeds to flick her eggs into the burrow - the additional weight on the eggs makes this task marginally easier to accomplish!

Once the bee-fly's larva hatch, it crawls further down into the burrow and wait until the host bee's own larvae are almost full-sized. It's at this point that the bee-fly larva attacks the bee larva, feeding on its bodily fluids and eventually killing it. Obviously this behaviour is bad news for the bees, however bees and bee-flies have coexisted for millennia, with no evidence that bee-flies cause declines in their host bee populations.

Here is an incredible video showing a female B. major flicking her eggs into the nest of her host bee species:




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