In a few weeks your garden will be buzzing with bees, hoverflies and butterflies, all feverishly collecting pollen and nectar from your flowers. Bumble bees are some of the most conspicuous garden visitors, and easily recognisable by their coloration and hairy bodies. However, if you take a close look at the bumble bees busying themselves with your flowers, you may spot an impersonator among them: the large narcissus fly (also known as the narcissus bulb fly), Merodon equestris.
By resembling a bumble bee, this hoverfly has a better chance of being left alone by predators on the lookout for a juicy morsel but not willing to risk being stung. You may say that you could easily tell the hoverfly in the film from a bumble bee. However, perfect resemblance is not necessary; a slight hesitation or moment of doubt from the predator diverting its attention to another prey, apparently less dangerous, gives the fly the chance to buzz away.
Evolving to look dangerous is a defence mechanism first described by the English naturalist Henry Walter Bates (1825-1892) while studying butterflies in the Brazilian Amazon rainforest. Therefore this powerful example of natural selection in action is now known as Batesian mimicry: it happens when a harmless species disguises itself as a dangerous one thereby gaining protection from predators. Many other hoverflies developed this strategy by acquiring the colouration and hairiness of bees and wasps. Even more remarkably, the large narcissus fly not only mimics bumble bees, but has different forms, each resembling a different colour group of bumble bees.
Merodon equestris is not welcomed in gardens because its larvae feed on bulbous plants such as daffodils, narcissus and bluebells, and can cause considerable damage. However, as often is the case in nature, there is another side. Hoverflies in general are frequent visitors to a wide range of wild flowers and many agricultural crops, and are considered the most important group of pollinators after bees. Together with other flies, hoverflies are believed to be particularly important for certain arctic and alpine flowers. They have received much less attention than bees and butterflies, but are effective pollinators in natural and agricultural ecosystems.
These websites have more information on the ecological roles of hoverflies:
All about hoverflies