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Adult hover fly. Photo: <a href="http://www.flickr.com/people/malcolm_nq/">Malcolm Tattersall</a> cc, <a href="http://creativecommons.org/licenses/by-nc-sa/2.0/deed.en">some rights reserved</a>
Adult hover fly. Photo: Malcolm Tattersall cc, some rights reserved

Natural Selections: Hover Flies

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A common invasive species, the hover fly, or drone fly, looks remarkably like a honeybee. But in its youth, it carries the loathsome monicker "rat-tailed maggot". Martha Foley and Dr. Curt Stager discuss Batesian mimicry: innocuous creatures who imitate more dangerous species.

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The hover fly is an invasive species that lives almost all over the world. They originally come from Europe and Asia, and have spread to North America, Australia and South Africa. Though in its larval stage it is called the rat-tailed maggot, in its adult form it is a harmless organism.

“The adults have several names, none of which are as disgusting as the baby name. They call them hover flies or drone flies or flower flies,” said Stager. “Probably a lot of people have seen them and didn’t even know it because they look just like honeybees. They’re the same size, they buzz like them, they hover just like them, more than other flies would do.”

According to Stager, hover flies are fuzzy and brown in color. They travel from flower to flower, collecting nectar, but they don’t make honey. These flies are also completely harmless.

“If you look close, you can actually tell that they’re not a honey bee. For one thing, they’ve got two wings instead of four like a bee would have,” said Stager. “And also if you do get close enough and you’re not nervous about it, look really close, look at their face and it’s a fly face; they have these gigantic eyes. In fact, the males have eyes that are so big, they meet in the middle of their forehead.”

These insects have attracted the attention of biologists around the globe. Research has been conducted on their physical and behavioral mimicry of bees. Scientists have looked at how long the flies hover and the way in which they fly from flower to flower. This data has been compared to similar data collected about bees.

“It’s called Batesian mimicry, when you mimic something that’s toxic or can defend itself and you’re helpless and you just want to look like the dangerous thing. So that deters predators,” explained Stager. For example, most flies will leave flowers very quickly in case a predator is lurking nearby. But hover flies will fly away slowly because it adds to the deception that they are bees.

The hover flies act like they have stingers even though they don’t. Stager said, “So it’s to their benefit, despite the possible risk that a predator could catch them. It’s kind of like the assumption that the predator won’t try because they look more dangerous than they are.”

Hover fly larvae live in stagnant water or in carcasses. Stager said, “They do some pretty interesting things. One thing that really surprised me was when the flies lay their eggs, let’s say on a carcass, and they hatch out into little larvae, then the larvae grow for a while and then they give birth to more larvae before growing up.”

This is called paedogenesis, or creation of babies from babies, according to Stager. This is a rare phenomenon in the animal world. This is similar to some insects such as aphids, who will have wingless offspring when there is no pressing need to disperse.  Stager said, “They just pop them out, actually, without even mating. So it’s kind of the same, though it’s coming from adults in that case. And then later on, as the food runs out or it’s time to mix up your genes and things, then a generation is born with wings. So in a way you could say that this is sort of a modified version of that, where you just make copies of yourself and then make a generation that can grow up and fly and spread around to other places.”

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