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ecology : Out of the strong came forth sweetness

HENRY GEE                  Wednesday 24 May 2000

"The mimicry between droneflies and honeybees has been fooling humans for
over 2000 years," say Y. C. Golding and M. Edmunds of the University of
Central Lancaster, Preston, UK, who have just worked out why.

Ancient Egyptians and Greeks thought that bees were born from the rotting
carcasses of cattle. It even got into the Bible (Judges, chapter 14, verse
8). Strongman Samson tore a lion in two while on the way to woo a
prospective bride among the Philistines. On his return, "there was a swarm
of bees" in the lion's broken body.

Not until 1898 did entomologists recognize that these bees born of decay and
death are, in fact, droneflies -- insects more closely related to houseflies
and bluebottles than bees. Droneflies lay their eggs in ditches and sewers,
and adults 'mimic' bees to fool potential predators.

A harmless fly that looks sufficiently like a bee causes a hungry bird to
hesitate for long enough to allow the fly to flee. Indeed, wherever bees go,
they are followed by a gaggle of 'false bees' -- droneflies, and other
insect impostors -- hoping to cash in on the bees' stinging reputation.

But to the trained human eye, droneflies and bees are different. And some
birds, with their far sharper vision, can tell a bee from a dronefly ten
metres away, so who do the droneflies think they're fooling?

Golding and Edmunds report in Proceedings of the Royal Society of London1
that what droneflies lack in appearance they make up for in behaviour: they
may not talk the talk, but they can walk the walk.

The researchers watched bees, droneflies and other insects about their
business in a variety of summertime settings, from a glade by a river bank,
to plantings in city-centre parks. Unlike other insects, droneflies spend as
much time in flowers, and the same amount of time flying between flowers, as
bees. This bee-like behaviour could be enough to prompt second thoughts in
the minds of predators.

There is, however, a fly in the ointment.

For this kind of mimicry (known as 'Batesian' mimicry) to work, the theory
goes that the mimic must be substantially rarer than the model. The
rationale is easy: a bird won't risk pecking a stripy insect if it is likely
to be a bee, so the mimic's disguise remains intact. However, if harmless
mimics outnumber the models, birds will soon rumble the deception and pluck
up courage to attack -- safe in the knowledge that the prey is more likely
to be harmless.

At several of the sites examined by the researchers, droneflies
substantially outnumbered real bees, and yet the droneflies still seemed to
be benefiting from looking, and behaving, like bees. The researchers suspect
that what we are seeing is an unstable situation, a result of human
disturbance.

Droneflies, like rats, cockroaches, tuberculosis and other familiars of
corruption, are camp-followers of humanity. The recent spread of human
beings (and their nasty habits) across the globe has generated unprecedented
opportunities for droneflies to breed. So although mimics now mob bee
models, predators, it seems, have yet to twig.

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Golding, Y. C. & Edmunds, M. Behavioural mimicry of honeybees (Apis
mellifera) by droneflies (Diptera: Syrphidae: Eristalis spp.). Proceedings
of the Royal Society of London 267, 903-910

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