Western Horse Fly

Large Fly
Location:  Death Valley, California
August 27, 2010 4:33 am
We encountered some large – about 20cm long – flies in death valley, california whilst on holiday there. We were walking along a trail next to a dry creek. The flies were black and light grey and one landed on my back and penetrated my shirt so that I felt a pinch. Just curious as to what they are.

Western Horse Fly

Hi Andy,
You may compare your image of a Western Horse Fly,
Tabanus punctifer, to images posted to BugGuide, but there is no information on the specifics of the species.  You may, however, read about Horse Flies in general on the genus page of BugGuide.  Only the female Horse Flies bite and suck blood from warm blooded animals.  Charles Hogue, in his excellent book Insects of the Los Angeles Basis, writes extensively about the Western Horse Fly.  He observes:  “The adults are large robust flies nearly 3/4 inch (20mm) in body length.  The male possesses very large eyes, which meet on the midline of the head, making it appear to be nearly all eye;  the back of the thorax is black except for a fringe of white hairs along the side and rear borders.  The female differs in that the eyes are separated and the back of the thorax is all white or pale cream.”  Your photo is that of a female, hence the bite through your shirt.  The larvae of the Western Horse Fly develop in water, so even though your email indicates this sighting was in Death Valley, we suspect it may have been close to either Salt Creek or Devil’s Hole.  Hogue has additional information:  “Because they possess a voraious appetite for the blood of horses and cattle, the female flies may be extremely bothersome, especially when numerous.  They have been observed biting rhinoceroses, tapirs, and hippopotamuses at the Los Angeles Zoo.  They occasionally bite humans, with painful results.  Natural saccharine fluids, such as fruit juices and nectar from flowers nourish the nonbiting males and also serve as a diet supplement for the females.”

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BugMan aka Daniel Marlos has been identifying bugs since 1999. WhatsThatbug.com is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

1 thought on “Western Horse Fly”

  1. I lived in Death Valley for a couple years. The locals call them Bombers due to the loud sound of them in flight. I did some looking around online back then, and as far as I could tell they’re very localized to the area. They bite right thru fabric (as you know), it hurts, and they’re pretty aggressive. They can survey a solid hit from a fly swatter, so the locals will literally carry tennis/badminton rackets in self defense. If you have work to do outside anywhere from May to September, those things are the bane of your existence.


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