Horse flies harass horses, humans and toads

By Jeff Mitton

When I was 12, my friends and I sought relief from sweltering heat and oppressive humidity by slipping into a small lake near Pompton Plains, N.J.

We swam out to a raft and hung on, enjoying the cool water. But then I noticed leeches on the barrels that supported the raft, and I warned my friends. We climbed up on the raft to check for and remove leeches.

That is when the horse flies found us. Our respite from the heat had turned into a bloody fiasco. I still remember it well.

The fly family Tabanidae has 4,500 species, and it includes horse flies (genus Tabanus) and deer flies (genus Chrysops). About 1,000 species of horse flies are described. Like many other flies, adult horse flies eat nectar and pollen to get the energy needed to fly. But horse flies have several additional food items.

Aphids suck juices from plants and produce an excess of sugars that they exude from glands on their upper surfaces. Ants tend and manage aphids, essentially farming them, to harvest the excess sugars, known as honeydew. But ants don't get it all, so the leaves of some plants become speckled with honeydew. Several species of salt marsh horse flies take spilled honeydew from the leaves of marsh elder, Iva frutescens, supplementing the sugars they harvest from flowers.

The larvae of horse flies live in moist environments, such as moist soil, rotting logs, and the mud in marshes and around ponds and lakes. The larvae eat virtually everything they come across, mostly worms and small insects. But in the ephemeral pools left by thunderstorms in the arid Southwest, they prey upon another food source.

New Mexico spadefoot toads, Scaphiopus multiplicatus, live in burrows in sandy soils and use ephemeral pools for breeding. The eggs quickly hatch to turn into tadpoles that quickly transform into miniature adults, ½ to 3/4 inches long.

Spadefoot toads have very fast development so they can finish development before the water evaporates, but there is another incentive to get out. Tadpoles are often seen resting on the muddy bottom at the edges of the pools, but many soon become fixed in place, struggling but unable to escape.

Larval horse flies, as large as the young toads, grip them with their mouthparts, inject venom and pull them into the mud while draining their fluids. Horse flies can take a substantial proportion of the new generation of toads before they have a chance to emerge.

Like mosquitoes, adult female horse flies need a blood meal to reproduce. Females have the mouthparts to cut or jab, depending on the species, but males, lacking these mouthparts, are unable to bite. Horse flies favor horses for blood meals and can takeas much as 300 ml of blood (two-thirds of a pint) per day from a horse.

Last summer, I was working with my graduate student Scott Ferrenberg in the forest at CU's Mountain Research Station when a horse fly started circling. They are easy to spot, for they are large, noisy and have conspicuous, menacing eyes. Their eyes, like those of all flies, are compound, but they have horizontal, iridescent ridges that seem to glow with bright colors. I decided to take the opportunity to get a photo.

As the fly landed on my right hand, I passed the camera to Scott and sat down to steady my hand. I saw widely spaced white stilettos, the cutting mandibles, sink into the skin and then felt a dull ache. Scott, it seemed to me, took his sweet time getting some photos, and seemed generally amused by the task.

Now that I have had the opportunity to look into the literature, I appreciate that it was not prudent to let a horse fly bite. They transmit parasites such as trypanosomes and can transmit tularemia from rabbits to humans. My hand itched for several days, and a lump formed at the bite site and lasted for several weeks. The itching and swelling were probably side effects of antigenic responses, perhaps to anticoagulants injected by the fly.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder. This column originally appeared in the Boulder Camera.

November 2012