Tuesday 17 February 2009

Deceitful drone-flies

Honey bee
Today we had a beautiful springy afternoon, a brief respite in this cold winter. I saw a drone-fly, Eristalis tenax, feeding on a clump of snowdrops. Drone-flies are a textbook example of Batesian mimicry: a harmless species resembling a stinging one so that predators are deceived and avoid it. Adult drone-flies are hover-flies that resemble honeybees in color, size and even in their flight behaviour. As adults they also feed on flowers - even the same species - making the similarity very precise. But, is there any evidence that predators actually avoid drone-flies mistaking them for bees? In a clever experiment Golding and colleagues actually investigated this using humans - biology university students and schoolchildren. They tested how good they were telling bees, bumblebees and wasps apart, and from their mimic hover-flies using picture plates and questionnaires. Their results show that humans were generally bad taxonomists, as half of the students could not identify a bee, even when they were biology students!. However, the study concluded that more people thought that the mimics would sting than control flies (not resembling bees, wasps or bumblebees), but fewer people thought the mimics would sting than the actual hymenopteran models. This shows that the mimicry works although is not completely effective. An interesting result was that students who had been previously stung were better at identifying the hymenopterans as stinging and were better at telling wasps, bees and bumblebees apart, indicating that experience is an important factor. The study provides the first experimental evidence that mimicry in drone-flies is effective and might help them avoid predation, not only by humans trying to swat or spray them, but by their natural bird predators.
 The students in the previous study were asked to look at small photos of flies and bees for a short period of time, just like any predator who has just seconds to decide if to snatch the insect or let it go. A close up examination of the drone-fly however, easily reveals its nature. Its antennae are quite short compared with the honey-bee, it has got a single pair of wings and typical large fly eyes, also, it lacks polled baskets on the rear legs.

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