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The Natural History of Horse Flies

Chlorotabanus crepuscularis
Chlorotabanus crepuscularis (Diptera: Tabanidae) - (Photo: used with permission)

In this episode of What’s Biting You, entomologist Dr. Art Evans and VPM radio producer Steve Clark discuss the natural history of horse flies, including the only green species that occurs in North America.

Steve Clark:  I'm Steve Clark with Dr. Art Evans, entomologist, and this is What’s Bugging You.   You came in here this morning and you showed me a very interesting horsefly.  I've never seen that, but I have seen some others.  And I've had the experience of being bitten by horseflies.

Art Evans:  Really?  Where were you when you were being attacked?

Clark:  At Fentress Road in Chesapeake.

Evans:  You know exactly where it was. [laughing]

Clark:  On a horse farm. 

Evans:  Oh gosh, yeah.

Clark:  It was a common occurrence.

Evans:  Horse flies breed in wetland areas, so the females will lay an egg mass on plants that are hanging over the edge of a pond, and the larvae drop into the mud, and that's where they complete their development.  And they’re predators.  They feed on other aquatic invertebrates.  There was a story I read years ago about some entomologist on a field trip in Arizona, and they went up to a dried up cattle tank.  It was just mud on the bottom, and here were all these frogs that were splayed out in the middle of the day.  And they picked up the frogs, and there was a great big horse fly larva that had this thing by the guts and was just sort of pulling it down into the mud.  [laughing] But they’re impressive predators. 

Clark:  That's not a real bite or is it?

Evans:  The adult females bite.

Clark:  They bite.

Evans:  Right, their mouth parts are modified for cutting through the skin, cutting through the capillaries, and then sopping up the blood that wells up into the wound.  Mosquitoes, for example, they have piercing sucking mouthparts, and they will puncture your skin right into the capillary, and then they draw blood right out of your circulatory system.  So it's an entirely different system.  Both of these insects incorporate chemicals in their saliva that keep the blood flowing.  The nice thing about mosquitoes is, if you want to look at it this way, is they also add a little anesthetic to it, so you don't know that you're being attacked right away.  Horseflies, not.  [laughing] There's nothing anesthetic about it at all, which I'm sure many of our listeners can attest.  One of the really neat horseflies, if I may, here in eastern United States is the green horsefly.  This is a species that comes out at night.  In fact, its name tells you everything you need to know, Chlorotabanus crepuscularis, the green crepuscular horsefly.  Crepuscular means flying at dusk or at dawn, but this thing flies into the night.  It's attracted to lights.  And the females do bite, although I've never experienced them firsthand. 

Clark:  Are they looking for horses?  What are they doing at night? 

Evans:  Well, they’ll feed on other animals too.  So there are deer out there, and I imagine any large mammal is fair game.  And as you know, horseflies are incredibly persistent.  You can brush them off, shoo them off. [laughing] But back to your story about being along the Chesapeake, it's wetland areas where you see large concentrations of horseflies and deerflies too, their cousins. 

Clark:  Yes.

Evans:  And I've been in situations where these great big black horseflies in Death Valley, out in the middle of nowhere, came at us by the hundreds.  All I could do was have a net and kept swinging the net to try and collect as many as I could before they’d bite me.

Clark:  I also had an experience with these horse flies over on the Eastern Shore, Chincoteague.

Evans:  Oh, perfect.

Clark:  Pony penning.

Evans:  They are strong flyers.  They can go pretty good distances.  They have excellent vision.  The males don't bite at all, but the females are the ones that draw that blood meal so their eggs will mature.  They're really amazing animals. 

Clark:  And you have a nice picture of one, so . . . 

Evans:  Yeah, we'll make sure the picture of Chlorotabanus crepuscularis is up online so people can take a look. 

Clark:  Dr. Art Evans is a research associate at the Virginia Museum of Natural History.  You’ll find photos, audio, and links to the museum and Art’s Facebook page at

Dr. Arthur V. Evans teams up with VPM Radio producer Steve Clark for a weekly feature, “What’s Bugging You?,” which airs during NPR’s Morning Edition. The program takes its name from another of Evans’ books “What’s Bugging You – A Fond Look at the Animals We Love to Hate.”

Tune in each Tuesday at 8:45 a.m. or at 5:44 p.m. on VPM News