A few days ago I went for a very nice walk around a nearby reservoir in what was unseasonably very warm sunshine. The warmth and light had predictably brought all the insects out to play; butterflies were gliding past me, including Brimstones, Peacocks, Commas and Small Tortoiseshells. A few tiny glinting ground beetles scuttled across the track before me, Honey Bees and Bumblebees were very busy at the newly opened flowers of Milkmaids, Celandines and Cherry blossom. There were quite a few flies too, flesh flies, dung flies and tiny gnats, but most conspicuous were the plump buzzing bodies of numerous Bee-flies which were sallying up and down the path, hovering in the sun or perching on a flower.
I rather like Bee-flies, perhaps because they are one of the select few fly species I can actually identify. They do also have a rather interesting natural history and compared to most flies are one of the more photogenic and approachable. So, Bee-flies of the genus Bombylius are fat-bodied flying insects with a thick coat of yellow-ish hair, they have long dark legs and a very long spear-like proboscis which they use like a straw to suck nectar out of flowers. In flight their wings move incredibly fast (100 beats a second) and they fly in sharp jerky movements with prolonged periods of hovering – such that they appear to be attached to the end of a stick like a character from a cheap 60’s children’s cartoon. Obviously their distinctive appearance is for the objective of mimicking an actual bee, probably as a false-warning to predators.
You can tell that they aren’t a type of bee because they only have one pair of wings (bees have two), also their eyes meet at the top of their head (bee eyes are separated) and their long proboscis mouthparts are quite unlike any bee’s. Bee-flies are usually seen on warm, sunny days in a wide range of habitats from March through to August, they prefer to feed on flowers such as Lesser Celandines, Dandelions and Primroses. In Britain we have four native species, of which Bombylius major – the Dark-edged Bee-fly – is by far the most common and widespread. The other three scarce species are the Dotted Bee-fly (restricted to the south), the Western Bee-fly (restricted to the south-west) and the Heath Bee-fly (restricted to Dorset heaths). Here’s a link to an identification guide if you’re interested – http://bit.ly/bee-flies
But perhaps the most interesting thing about Bee-flies is their reproductive cycle, which is a little more insidious than their cuddly appearance would have you think. So the mother Bee-fly will dab her rear-end on some dusty ground or sand, this is to use the dust later to coat her eggs as they come out – perhaps to give them weight or camouflage. She will then locate the nesting tunnel of a solitary bee (usually an Andrena mining bee), upon finding one she will position herself with utmost skill in a hovering position over the entrance hole and with a swift little flick forward of her buttocks an egg will be flung into it.
No harm done so far, however, when the bee lays her egg (if there wasn’t one already in there) and seals up the tunnel the Bee-fly egg will hatch and the little grub will proceed to eat the food provisions that the bee put in there for its young. Which is a bit naughty. That is not all, for when the Bee-fly larva has grown into a fat, gelatinous grub it goes all Alien on the (real) bee larva and devours it. It then pupates, well fed, and emerges next spring as an adult Bee-fly ready to mate and repeat the process (for that is life).
So a bit of a parasitic cuckoo of a species, these Bee-flies, leaving the hard work of digging and provisioning a tunnel to the industrious Hymenoptera. Just to point out though, there is no evidence that this parasitism harms bee populations in any way and as they have co-existed for goodness-knows how long I would say that there are far worse problems for bees than these sneaky flies (just before you go setting up a petition for Bee-flies to be culled or something). It’s all part of wonderful nature though, I would thoroughly recommend going out on a sunny day and keeping an eye out for these distinctive mimics and cuckoos, they are fairly easy to ID and interesting to observe, see if you can find any of those rarer species too.