Horseflies

July 13, 2021

When I got into my car yesterday morning there was a huge horsefly sitting on the hood of my vehicle.  While horseflies were a constant pest in the agricultural areas where I lived in Kansas, I have not noticed many in the urban area where we live in Arkansas.  Horseflies mostly occur in warm areas with suitable moist locations for breeding, but they can occupy a wide range of habitats from deserts to alpine meadows.  They are also adaptable to altitude and range from sea level to at least 10,800 feet (3,300 m).  I believe this was the first horsefly I have seen since we moved here three years ago.  It was huge.

Horseflies are true flies in the family Tabanidae in the insect order Diptera.  They are often large and agile in flight.  They avoid dark and shady areas and prefer to fly in sunlight and are inactive at night.  Horseflies are found all over the world, except for some islands and polar regions (Hawaii, Greenland, Iceland).  Both Horseflies and Botflies (Oestridae) are also called gadflies.  Adult horseflies feed on nectar and plant discharge, and the females bite animals (humans) to obtain blood.  While males have weak mouthparts, the females use specialized mouth parts that allows them to bite and then obtain enough protein from blood to produce eggs.  Female mouthparts have a stout stabbing organ with two pairs of sharp cutting blades.  They then use their spongelike “tongue” to lap up the blood that flows from the wound.  Horseflies act like tiny vampire bats.

Since female horseflies bite their hosts, they can transfer blood-borne diseases from one animal to another.  In areas where diseases occur, they have been known to carry equine infectious anemia virus, some trypanosomes, the filarial worm Loa loa, anthrax (among cattle and sheep), and tularemia. They can reduce growth rates in cattle and lower the milk output of cows if suitable shelters are not provided.  I have watched as horseflies have relentlessly aggravated both horses and cattle.  They seem to prefer the unprotected areas around the eyes and the animal’s backs, both outside the range of the animals’ tails.  Horseflies have appeared in literature since Aeschylus in Ancient Greece mentioned them driving people to “madness” through their persistent pursuit.  Shakespeare also included horseflies in three of his plays (gad flies) where they aggravated the madness of the character.  The do the same to horses.

Thoughts:  While some attribute the horseflies’ name to their size (as big as a horse, at least compared to a house fly), it comes from the fly’s persistence in attacking Equines.  Equine Infectious Anemia (EIA) is a viral disease affecting only members of the Equidae family (horses, ponies, zebras, mules, and donkeys).  There is no vaccine or treatment for the disease.  There is no evidence that EIA is a threat to human health.  While EIA is carried by horseflies, many believe covid-19 was originally transported by a bat.  Horses infected by EIA either die or become a lifelong carrier.  Regardless of where it started, humans do have a vaccine against covid-19.  The problem is getting people to receive the shot.  It seems some would rather risk death.  Follow the science.  Change is coming and it starts with you.

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