Posted by: Ross Gardner | April 19, 2020

Is it a Fly? Is it a Bee?

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Bee-fly (Bombylius major)

Actually, it’s a Bee-fly, which makes it the former.  They are though, excellent bumblebee mimics and fine example of Batesian mimicry, whereby a relatively harmless animal (in this case the fly) has evolved to resemble a relatively harmful animal (in this case a bee with a sting).  Thus the unarmed fly is conferred the benefits of the caution usually afforded to the potentially stinging bee, without going to the trouble of evolving a sting for itself.  This is just one of a great many examples of such trickery in nature.

They seem to be particularly numerous this spring, as common a sight in the garden as in the woods or field edges, wherever, in fact, there are ample, nectar-filled flowers for them to insert that conspicuosly brandished proboscis into.   Another thing required to attract the presence of a bee-fly is a corresponding abundance of solitary bees and wasps.  This is because that potentially cuddly, bumblebee-esque persona conceals a darker, parasitic side, although I really should know better than to attach human sensibilities – good and bad, cuddly and dark – onto other species.  As unpalatable as it might seem, this is just the bee-fly way, just the same as we have ours.

Anyway, I digress somewhat.  Bee-flies parasitise these other insect by scattering their eggs nearby their nest holes, leaving the resulting grubs to find their way inside and to find and consume the host grub before pupating themselves.  I have watched the adults about the congregated nests of mining bees perforating the bare soil of a sunny bank, flicking their abdomens so as to scatter their eggs.  They will produce many eggs, but very few of the hatching grubs with find their way inside a bee burrow and successfully carry its life-cycle.  Thus the balance of life between host and parasite is always maintained.

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A Bee-fly nectaring on Crab Apple (Malus sylvestris) blossom.


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