Friday, February 19, 2016

Bite of the Black Fly

It happened at the zoo. I was parked innocently opposite the African Elephant yard, with the window rolled down on this nice late afternoon of September 19, 2015, waiting for Heidi to get off work. I noticed a tiny fly had flown into the car, but was truly shocked to see it was a female black fly, likely a species of Simulium....and she wanted my blood.

She got it, too, because I so rarely see members of the family Simuliidae, let alone adult females. I was able to get some respectable images of the 3-4 millimeter vampire precisely because she was occupied pumping blood out of my knuckle. What was she doing here, though?

I was under the mistaken impression that black flies, also known as "buffalo gnats" and, here in Colorado at least, "turkey gnats," seldom venture far from fast-flowing streams and rivers. That is because they spend the egg, larva, and pupa stage of their lives in aquatic torrents. It occurred to me that the elephant yard does have a waterfall, and I would bet that black fly larvae live on the very edge of that artificial cataract.

Well, I have since learned that female black flies are perfectly capable of flying miles in search of a blood meal. There are some records in Canada of black flies migrating over ninety (90) miles from where they grew up as larvae.

Only the female black fly bites. Like mosquitoes, she needs the protein for the proper development of her eggs; and she can lay several hundred eggs in her two- or three-week adult lifespan. Unlike mosquitoes, black flies do not have beak-like mouthparts to extract blood from our capillaries. Black flies slice you open with knife-like mouthparts, then lap up what spills out.

Depending on the species, the black flies lay their eggs on vegetation or other objects in the water, under the water, or scatter them on the water's surface. Interestingly, freshly-laid eggs apparently produce a pheromone (scent) that attracts other adult female flies of the same species, and stimulates them to lay their eggs in the vicinity.

The larvae that hatch spin silken pads on the surface of stones or vegetation in the middle of flowing water. They then anchor themselves to the silk pads with special hooks on the rear of the abdomen. The larvae feed in a unique manner, by deploying a pair of "cephalic fans" that intercept organic particulates from the current. Larvae molt 7-11 times.

Black fly larvae in Arizona

At the end of its larval life, the creature spins a silken bag in which it will pupate. The tapered rear of the bag points upstream into the current, while the wider, open portion projects downstream. The larva molts one final time to reveal a resting stage with branching gills that may reach beyond the lip of the silken bag.

Black fly pupae in Arizona © Tony Palmer

The adult fly emerges in a few days or so, rising to the water surface in an air bubble and floating to an emergent object it can climb onto and finish expanding its wings and hard its exoskeleton. The total time from egg to adult takes roughly 3-4 weeks and is heavily influenced by water temperature. The colder the water, the longer the life cycle. There can be three or four generations per year; winter is typically spent in a dormant larva stage.

Male black fly (note huge eyes meeting at top of head)

There are about forty species of black flies in Colorado, in three genera: Simulium, Prosimulium, and Metacnephia. Different species live at different elevations, and on different sides of the Continental Divide. Surprisingly, the majority are not pests of people or livestock, preferring to feed on birds and other wildlife.

The species that do afflict horses, cattle, poultry, and people can cause severe distress, and may carry diseases. I will spare you the agonizing details of suffering incurred by victims of black fly attacks, as there is no end to the resources where you can learn such information if you are so inclined.

Suffice it to say that it pays to be prepared with an excellent insect repellent if you plan to be in black fly territory. Prevention is always the best tactic for battling *any* bloodsuckers. Take care.

Female black fly on Feb. 24, 2014

Sources: Bechinski, Edward John, and Marc J. Klowden. 2005. Black Flies - Biology and Control. Division of Entomology, University of Idaho (available online as a PDF).
Cranshaw, W.S., F.B. Pearis, and B. Kondratieff. 2013. Biting Flies. Colorado State University Extension. Fact Sheet 5.582.
Kuhn, Dwight. "Black Flies - Life Cycle," Kuhn Photo.


  1. Boy, I wish *I* rarely saw members of the family Simuliidae. Unfortunately, they breed in the creek and swamp right next to our house, and they mob anyone who goes into the yard for most of May, June, and part of July. I think I have a solution to them, though: if you coat a hard-hat with Tanglefoot insect-trapping compound, they will get stuck to it before being much of an annoyance because they tend to zero in on a person's head first. It works great on the local deer flies, too. After writing that previous post, I made another hat that I wore last year, and probably caught on the order of 1000 black flies with it.

  2. Last week I was at a water park in the middle of a desert area. A black fly landed on my shoulder and I felt a sting. I swatted and killed it, but it left a splatter of blood on me. I now assume this is what it was. My arm hurt a little for about an hour or so.

  3. I live in Midwest Wisconsin - we have a 30 ft pool and in the past couple of years we have had small, what I thought were small black bees. They land in our pool and if you try to skim the top of the pool to get them out thru the skimmer, there is a good chance you will get bit. They itch something terrible and leave big welts on me. This year they are terrible - I skimmed at least 200 off yesterday while cleaning the pool. What are these miserable things?

    1. I suggest you take intact specimens to an entomologist at University of Wisconsin, or a museum, or even the public health department (vector control division will have at least one staff entomologist). Do not rely on a pest control company to accurately identify any insect. Good luck.


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