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The Secret World of Robber Flies

One of the many rules of the outdoors is that if you keep your eyes open long enough something will take your breath away. Take flies, for instance. Do you appreciate them? Don’t feel bad if you don’t. I find myself not appreciating flies almost every day. They buzz in my ears, try to take bites out of my arm, and drown in my beverages. Overlooking flies is a mistake though. What we envision when we think of the average fly is just a tiny slice of what’s out there in “fly-world”. Want a fly to take your breath away? I present to you, The robber fly.

Robber fly - Photo by Juan Emilio

 

Before we get to that, let’s back up for a second. What is a fly? We throw around “fly” to describe a lot of insects, but the true flies are a distinct evolutionary group (dragonflies, butterflies, fireflies, damselflies, and a number of other insects called flies are not actual flies). And flies are not a small group. There are over 15,000 discovered and scientifically described species of flies in North America. Globally, that number climbs to 150,000. That doesn’t include flies unknown to science. The best guess by entomologists is that there are over one million unique species of flies in the world. That’s a lot of flies.

For perspective there are about 5,000 total species of mammals worldwide and maybe 32,000 different fish. There are more flies than there are vertebrates. Think on that.

Robber flies (family Asilidae) aren’t uncommon. You have almost certainly seen one before, but, as it zoomed past your head with a loud humming buzz, you probably assumed it was a small dragonfly (again, not an actual fly) or large wasp. The first challenge of spotting robber flies? They’re fast. The second is that they aren’t big on hovering. The robber fly hides on the underside of a big leaf, or tucks itself behind a thick plant stem, and waits, and waits. Until “shoooom!” they whizz by your head, snatch a wasp or a butterfly out of the air, and disappear as fast as they came.

 

Efferia With Prey - Photo by Katja Schulz

 

Whenever I am lucky enough to catch a glimpse of a robber fly, my first thought is that they are all head. In reality what looks like a giant head is actually a big bulky thorax, on the front of which sit oversized eyes, a pointy mouthpart, and often large tufts of setae (insect hair). Though there are physical variations in the nearly 1,000 species of robber flies in North America I have never seen one without long, stilt-like legs. With those long legs keeping their wings well elevated above whatever perch they are on, robber flies seem perpetually ready for lift off. They are the attack helicopters of the insect world.

Robber flies are one of my very favorite insect groups. But for me there’s more to the story than their cool looks and hunting habits. I had never heard of robber flies until three years ago. I took an entomology class, and the robber fly was shown in a slideshow of locally common insects. In the slideshow the robber fly was pictured eating a dragonfly. I was astonished. What kind of insect eats a dragonfly!? There I was, at age 27, thinking I had a good handle on my local wildlife, and before me is this great, burly, dragonfly-eating bulldog of an insect. I thought, “You’re telling me these things have been flying around since I was a kid?” Of course they’ve been flying around for quite a bit longer than that.

I mentioned, at the start, the tendency for the outdoors to take your breath away if you just pay attention long enough. That’s why I love robber flies. I discovered, in my own hometown, robber flies at the age of 27. How phenomenal is that?

Robber fly - Photo by Bob Peterson

 

This excitement came bubbling to the surface for me back in June of this year. I was working with the other Parklands gardeners near our Industrial Agriculture Garden at the Silo Center. Above our heads came a loud buzzing. Horse fly? Bumblebee? We all turned apprehensively. After a moment of searching I saw the familiar, big “head” silhouette on the backlit underside of a corn leaf. For a moment I was a little kid again.

The gardening team has been noticing robber flies all over the park ever since. Once, near the Louisville Loop bridge south of Boulder Pond, we were startled by a voracious buzzing near us. The source? A large wasp, maybe a queen yellow jacket … with a robber fly latched onto its back. It made my afternoon.

Robber flies prefer to live and hunt in open fields, so there are plenty of places to find them in The Parklands. Sky Meadow in Turkey Run Park is particularly excellent habitat. To catch a glimpse of these elusive predators, I suggest focusing on insects in flight. Try to follow them with your eyes until they land. Then creep up close enough to try and make an identification. Once you’ve seen a few in person, you might start to recognize some on the wing.

“But wait!” you say. “This hunter of dragonflies, subduer of wasps, snatcher of butterflies, you’re suggesting we go in search of this insect!?” It would seem, if internet reports can be trusted, that the robber flies can deliver a nasty bite if handled roughly. I don’t know anyone who has ever been bit by one though. I promise. I’ve only had the privilege of one landing on me once. Just don’t go grabbing them with bare hands and the robber fly will happily let you remain an awed observer.

For more information on the life and natural history of the robber flies: http://entnemdept.ufl.edu/creatures/beneficial/flies/robber_flies.htm

For a short selection of common Kentucky Robber Flies: http://www.uky.edu/Ag/CritterFiles/casefile/insects/flies/robber/robber.htm#bearded

Chris Erickson

As a Parklands Zone Gardener, Chris Erickson supports the planning and maintenance of gardens throughout Turkey Run and Broad Run Parks. Chris joined The Parklands team in the spring of 2016 after spending time as a teacher in the northeast.

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