Health & Fitness
Robber Flies: The Thief in Your Backyard --- by Rachel deFino --- Muhlenberg College Class of 2016
Did you know that robbers and thieves may be frolicking in your back yard?
Everyone knows about those classic insects you can find close to home: ladybugs, butterflies, cicadas, bees, and lightning bugs. But, there’s one that’s probably been hiding under your nose that you’ve never noticed called Robber flies, or Assassin Flies and Bee Killers. They could be living in your garden without you knowing.
There are an estimated 7000 different species of robber flies in the world. Their size can range from as small as half the depth of an iPhone 5 to as long as a fun sized snickers bar (two of my favorite things). Their bodies can be completely bare to covered in hair. But two things all robber flies have in common is a large indent and a tuft of hair called a “beard” between their eyes. These are great ways to identify them because these master killers are also masters of disguise. Like all true flies Diptera, they only have two wings.
Some species of robber flies mimic bees and wasps. This misidentification can lead to kids running away in fear of a fly that is virtually harmless (to humans, that is). These mimics use their disguise to get close to their prey, bees and wasps, without raising suspicion. The most common mimic genera found in Pennsylvania are the Hanging-thieves (wasp mimics), and Bee-like Robber Flies (you guessed it, bee mimics). Hanging-thieves are usually red, orange, or brown in color, while Bee-like Robber Flies are fuzzy with black bodies with yellow hair and yellow-striped abdomens. There are many more kinds of wasp and bee mimicking robber flies, but you most likely won’t find them around here. I’m sure some of you are thinking, “Why should I care about a bug that kills other bugs?” Well, while robber flies are considered non-beneficial when feeding on honeybees and other pollinators, these flies feed on such a wide variety of prey that many pests and nuisances are kept in check through their feeding. Robber flies feed on yellow jackets, mosquitoes, Japanese Beetles, grasshoppers, and other garden pests, as well as butterflies, moths, and spiders; unlike most kids who will only eat chicken fingers for 15 years, these guys are not picky eaters. This makes them very beneficial for your garden.What’s really impressive is not what they feed on, but how they catch their prey. Robber flies kill with patience, speed, strength, and toxins. The robber fly will perch on a twig, a flower, or tall grass and lie in wait for prey to come flying by, turning its head side to side to increase its visibility. (This perching and head moving behavior are another way to identify a mimic.) Robber flies will then push off with their strong legs, pursue their prey, and catch it midflight, one of few insects to do this. The robber fly grabs hold of the insect with its long, strong legs and uses its piercing, hard proboscis to inject neurotoxins, which paralyze its prey, and proteases, which liquefy its insides. The robber fly carries its prey back to the perch and enjoys its insect smoothie. The Hanging-thieves are so called because they will hold onto the perch with one or two legs and eat while hanging from the perch.
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There is a down-side to robber flies being so good at their job. Since most species don’t have courtship rituals, males simply approach a female. A female spotting a male flying in her vicinity will attack with its quick reflexes leading to unintended cannibalism and, unfortunately, no baby robber flies. Not exactly the type of night the male robber fly was looking forward to!
Although rarely noticed, robber flies are found worldwide, mostly living in hot and arid climates. Those found in Pennsylvania live in open fields and meadows. You’re most likely to find the Giant Robber Fly, one of the largest and fiercest robber flies, Efferia, Hanging-thieves, and Bee-like Robber Flies around the Lehigh Valley.
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The next time you’re out in your garden, a field, or a meadow, look out for a perching robber fly hiding in plain sight. Over the summer, take your kids outside to learn about the unsung hero in your backyard who kills the scary spiders and buzzing mosquitoes, because these awesome insects deserve more recognition then they’re given.
Robber Flies: The Thief in Your Backyard by Rachel deFino is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.