Monday, September 8, 2014

Horseflies

This has been a great year for Horseflies.  By that I mean there are a lot of them around.  I realize that many people would prefer this particular nuisance disappear entirely and to them I imagine this would be a bad year for Horseflies.  I have always been fascinated by these large flies and enjoy having them around.


My favorite species has always been the Black Horsefly, Tabanus atratus.  This beauty with its soot black finish appears to have been assembled by fine craftsmen.  Female Horseflies feed on fresh blood, primarily taken from large mammals.  Males feed on nectar and pollen and I suspect sweat, since I’ve been buzzed by male Horseflies that land on my sweaty shirt, but do not bite.


The key to sexing Horseflies is the space between the eyes.  The eyes of the male meet on the top of the head.


In the female, there is a spacer that separates the eyes.


I believe this is a female Tabanus calens.  I’m quite impressed with the brightly colored antennae.  Their shape reminds me of some Klingon weaponry, but these are sensory organs, not defensive structures.


If there is any weaponry involved with the Horsefly, it would be the cutting mouthparts protected by the two light brown sheaths located below the antennae.  The cutting mouthparts pierce the skin to allow the blood to flow.  The blood is then taken up by the labella, that dark sack-like structure located below the sheaths.  Human blood is readily taken by Horseflies, much to the discomfort of the donor.  Fortunately, the larger species of Horseflies are quite noisy in flight and can be heard long before they settle down to bite.  I’ve noticed that if you wave and swat at a circling Horsefly, it just widens its circle until you calm down.  If you catch the Horsefly in your hand, hold it for a moment and then release it, it will fly straight away and not come back.  That bit of information probably won’t help many people cope with their Horsefly encounters, but it is interesting.


Most literature states that Horseflies lay their eggs in or near water on plants and rocks near the shoreline.  I’m used to watching Horseflies cruise above small pools and suddenly drop down to smack the water and fly quickly away.  They have been making daily assaults on my Water Garden for the past several weeks.  The image above is the closest I could come to capturing this activity.  I have always assumed that these flies were females depositing eggs, but I’ve never read anything that affirms that assumption.


Maybe smacking the water is some type of cleaning activity intended to rid the flies of external parasites.  Carrying along a mite of two seems to be all the rage this season.  I can’t remember when I’ve seen so many of these red riders on the bodies of large flying insects.  Some may be permanent residents and others just hitching a ride.  Whatever they’re up to, they always make their host a bit more interesting.

6 comments:

  1. I have never examined a horse fly closely enough to determine his/her sex. Fascinating. :)

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  2. Hi, Lois. Horsefly sexing is a skill for which there is little demand, but now you're prepared should the need arise.

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  3. Hi. Thank you for this post. I was lucky to see one of these giants in my garden this summer. It was at least an inch long.

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    1. Hi, Patty. I'm glad you enjoyed this post.

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  4. We just had one of these huge guys fly into our house! Luckily for it, we are not into unnecessary Carnage, so my husband caught it and put it back outside.

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    1. Hi. It sounds like you had an interesting encounter.

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