Gigantic black horse fly

An unwelcome guest at any family picnic…

female black horse fly

A gigantic black horse fly, with scissor-like mouth parts ready to slice into tender skin.  About 1.5 inches in length, and a solid black color everywhere, this is a fearsome beast.  A space between the wrap-around eyes means this is a female — the sex that bites and draws blood from man and beast.

female black horse fly mouth parts

Within the vertically oriented sheath of mouthparts below the eyes are scissor blades that slice and shred their way into the skin of tough cow or horse hide with ease.  A sponge-like structure on the end of the proboscis then mops up the blood that pools on the skin.  

Female horse and deer flies require a blood meal to obtain sufficient protein to complete egg development. Males lack the cutting mouthparts and feed only on nectar and pollen.  These biting flies are serious pests of livestock, not only because of their annoying buzzing near sensitive spots on the face, but because they take such large quantities of their hosts’ blood (30 flies can suck up 1/3 pint of blood in less than 6 hours).

Following their blood feast, female Black horse flies deposit their eggs on or near aquatic vegetation where the larvae will develop over the next couple of years, feeding voraciously on aquatic insects or snails.

female black horse fly

A fly to reckon with…

23 thoughts on “Gigantic black horse fly

  1. Great macro shots, Sue, of a horse fly that looks big enough to put a saddle on and ride. In general, I like insects, but I have a whole lot less fondness for those that will administer painful bites to me. Those mouth parts are scary-looking! Ouch,

  2. Great pictures. I did not realise that they could be so debilitating for livestock nor that they did not have piercing mouthparts. That explains a lot of the “ouch” factor. Amelia

  3. Why have these hateful vampires become so active everywhere in the last 10 days? They are everywhere, attacking everything, including my dog. Is there anything I can put out to discourage them?

    • Hopefully they don’t land on you. Another blogger on the east coast also reported giant horseflies recently, so this must be the time of year for the, to appear.

  4. My grandson and I were out in the pool this evening. One of these females appeared and stayed for quite awhile. After reading about it we think it was looking for a place to lay eggs. It was huge and we were scared. It did not try to hurt us but we kept careful eye on it. We are in the city in Oklahoma in the town square according to Jake. The nearest horse or cow is surely 7 or 8 miles at the very least. It sure looked like a predator to us.

  5. I just found one today in Eastern PA. She was laying dead on my patio. I also found a dead Monarch in the yard today. One of my neighbors also found a dead Monarch today. Do you think they may have died due to pesticide sprayed by a neighboring farm?

    • Yes, that would be the likely cause for finding dead insects. That’s too bad, Monarch butterflies have a tough time trying to survive in our hostile chemical environment. Thanks for your comment.

  6. Thanks for pointing out the REAL difference between the male and female of the species. The other day I was able to take a few photos of I believe it to be the male Gigantic Black Horse Fly. Close to about 2″ in length and about an 1″ ~ 1.25″ closed wingspan.
    Wouldn’t mind sharing with you to assist in differentiating the male and female perspective.

    • Killed one in my garage, this evening. The little demon actually was diving at me until I got lucky and caught it resting on the ceiling. It is everything I read here. I spend the last hour examining it under a magnifying glass and researching it on the net. I am familiar with horeseflies and green heads and have been the recipient of numerous bite over my life. I am glad this was one didn’t latch on to me. I can’t imagine what that would feel like. I live in NC. Save it for future reference.

      • wow! Yes, they are not to be messed with. I am not around horse stables where they might occur, and this one (the only one I’ve seen) was found about a mile from where horses were pastured. I feel sorry for their intended victims. Thanks for your comment.

  7. In the last month on Lake Mead in Nevada we’ve had a couple of occasions where the black horseflies were diving at us and aggressively trying to attack us on the beach and then actually followed us out into the boat that we were on and landed on my boyfriends back and bit through his T-shirt three times leaving welts on his back. Another time, on a different part of the lake, a horsefly was harassing and buzzing everyone on the beach. These flies are scary and aggressive!!!

    • Yes, they are — scary and aggressive. I have gotten quite a few comments testifying to just what you described. They really are a pest. I can see why horses would start running furiously when bitten by one of these flies!

  8. Wish I could post a picture of the biggest, blackest fly we’ve ever seen, taken today in our Orange County, New York back yard. We’ve lived on this 20 acre wooded property with 2 acre groundwater wetland pond, so we’re accustomed to seeing various bugs, but never had anything like this before! Looks like the giant fly in this article, like a giant housefly, black as night and almost 2 inches long.

    • Yes, first ever sighting at our Houston home. Huge body with big “wrap-around sunglasses” type eyes. Beautiful and menacing like a fighter jet.

      • Aren’t they amazing?! And scary. I can just imagine how much it hurts to get bitten by one of these — it certainly makes horses jump.

  9. We live in Southwest FL and every year I’ve seen at least one of these huge black flies near the pool. I had no idea they could actually BITE! Thankfully they’ve never been aggressive and we don’t bother them just observe since they’re quite a curiosity. Glad to have more information about them we will certainly keeping our distance!

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