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What's Eating You? — July - August 2001

Minnesota’s biting flies

magazine photo of biting bug

By Larry Weber

Bugs that bite get our attention, even though they make up only a small portion of the large group of animals called insects. By far, most insects live completely separate from us. Many prey on other insects and not on people. Thousands of insects eat only plants. Many adult insects don’t even have a mouth, because they don’t feed at this stage of life.

There are six kinds of Minnesota flies that find us attractive. These lesser-loved insects of summer all belong to the order Diptera. While most insects have four wings, all members of Diptera have only two wings. Most famous in Minnesota are mosquitoes. Their bloodthirsty relatives include black flies, deer flies, horse flies, stable flies, and tiny biting midges, called no-see-ums.


Mosquitoes are Minnesota’s best-known biters because they are most abundant. They belong to the family Culicidae. About 170 species live in North America. Of roughly 50 species of mosquitoes in Minnesota, at least 28 bite humans. Only a few hang around in large numbers. Some species prefer to bite other animals.

A typical Minnesota mosquito is about 3/8- to 1/2-inch long with slender legs and scaly wings, which fold over its narrow body. The male and female both have antennae, but the male’s is more feathery. The mosquito has a tubelike mouth called a proboscis.

Life cycle

Mosquitoes begin life as eggs laid on water. Eggs are the overwintering stage of most Minnesota species. They hatch in early spring. The young insect, the larva, is long and wormlike. To breathe, some larvae poke a tube, called a siphon, above the water surface like a snorkel. One species pokes its siphon into a soft plant stem, which provides oxygen and shelter.

Next comes the pupa stage, also in water. The comma-shaped pupa swims on the surface and sinks when disturbed.

Emerging from the pupa stage as adults, mosquitoes mate. Males go off to feed on plant nectar. Although females also get nutrition from plant nectar or sap, they need a meal of blood to help develop their fertile eggs. Different species choose different sources of blood: Amphibians, reptiles, birds, and mammals all donate to the cause.

When does it bite?

Usually starting in May, females go out at dusk to find a blood meal. Mosquitoes detect heat we give off and carbon dioxide we exhale and quickly fly in for a bite.

Early summer is the most active feeding time, but new generations of mosquitoes keep emerging and feeding until frost. One species, the house mosquito (Culex pipiens) overwinters as an adult in buildings, in woodpiles, or behind tree bark. We might encounter this critter in winter if it warms up and becomes active.

How does it bite?

The female’s proboscis has six piercing parts. Four with serrated edges (like a bread knife) cut through the skin to the blood vessels. A fifth part injects saliva, which contains an anesthetic to kill pain and an anticoagulant to keep the blood from clotting and closing the wound. A sixth part is a trough-like tube that pumps the blood into her empty gut. A full meal can double her weight, and often she cannot fly after eating.

A single meal can nourish 100 eggs or more. During a typical adult lifetime of two weeks to one month (adults of some species live six months or more), a mosquito bites one to three times.

Good news, bad news

Unlike mosquitoes in other parts of the world, those in Minnesota do not carry yellow fever or malaria. In our state, mosquito bites have caused a few cases of a serious disease called encephalitis. Some bites have spread heartworm to dogs.

Black Flies

Black flies are in the family Simuliidae. The black fly is tiny, less than 1/4-inch long, with a dark body, humped thorax, and short head, antennae, and legs. Like mosquitoes, both male and female black flies get nutrition from plant nectar. Males are content with that food, but females use their cutting mouthparts to take blood from animals.

Life cycle

Adult black flies are often found near the fast-flowing waters where they lay their eggs. Larvae look like dark worms. They attach to stones and logs underwater and feed on tiny organic particles, which they filter from the stream using fanlike structures. Pupae also live underwater. When adults emerge in spring, they float to the surface, mate, and begin feeding.

When does it bite?

Because ice melts from moving waters before it melts from ponds and lakes, black flies emerge and bite earlier than other insects. The swarms begin feeding in late May. Fortunately, black flies produce only one generation each year. Their numbers peak in June.

How does it bite?

Using four slashing teeth, the female fly cuts a shallow well in the victim’s skin. As blood seeps into the wound, the female laps it up. Her saliva contains chemicals to partially numb the nerves and an anticoagulant to keep the blood from clotting. These chemicals cause the skin around the bite to swell and itch.

The female uses temperature-sensitive cells in the tip of her antennae to help find victims. She also uses sight, and seems to be attracted to the color blue, such as a blue jacket, shirt, or jeans.

Because flies follow each other in swarms, we are likely to get several bites at a time. Unlike mosquitoes, black flies feed exclusively during the day; their feeding frenzies end after dark.

Good news

While most of us do not appreciate black flies, it is good to keep two things in mind. First, black fly larvae are a sign of unpolluted waters. Second, adults pollinate our beloved blueberries.

Horse Flies

The biggest of our aerial attackers, the horse fly may reach 1 inch in length. It belongs to the family Tabanidae, which has about 350 species in North America. The name horse fly refers to its attraction to horses and other large animals, including people.

The horse fly’s stout body is mostly hairless and often black, gray, or brown with a broad band on its abdomen. The fly has short antennae and large, iridescent green and purple eyes.

When and how does it bite?

The male horse fly stays among plants and feeds on nectar and pollen. The female searches for a meal of blood. Her bladelike mouthparts cut skin. She usually goes for legs or wet skin. Flying fast, she can hit hard. Female horse flies often patrol lakeshores in summer. Many a northland swimmer has emerged from a dip only to be greeted by hungry flies.

Life cycle

The female horse fly deposits her eggs on plants near ponds and lakes. Larvae, which look like thick worms, develop in this aquatic world. The predacious larvae spend two winters there before pupating in spring. Adults emerge and feed mostly in July or early August.

Deer Flies

Close cousins of horse flies, deer flies also belong to the family Tabanidae. The name deer fly refers to its habit of feeding on white-tailed deer.

The average deer fly is about 1/2-inch long. Take a close look at a deer fly and you will see that it is quite beautiful, with its combination of transparent, patterned wings and metallic green-gold eyes with a spotted pattern. This light-colored fly is triangular. Its small head is rounded like a dome with short antennae. Some species have a black V-shaped mark on the abdomen.

When and how does it bite?

Deer flies are those pesky bugs that buzz around our head (even getting caught in our hair) on hot summer days, usually in July. Even though they appear to fly fast, they’re slow and easy to swat compared with other insects. Like black flies and horse flies, deer flies retire for the night.

A hunting female fly often waits patiently in shrubs at the edge of a forest, marsh, field, or roadside until a potential food source passes by. Seeking her blood meal, the female fly makes an incision that causes blood to flow.

Life cycle

After eating, she lays her eggs on plants sticking up out of wetlands. When the larvae hatch, they fall into the water. Larvae live as predacious worms in the water where they overwinter. Pupae develop along the water’s edge in early summer. The adult stage lasts through much of June and July. Only one generation develops each year.

Bad news

Both horse flies and deer flies carry tularemia and other diseases that can harm humans and other animals.

Stable flies

A lesser-known, but still quite disturbing, biter is the stable fly. Named for its habit of hanging out around farms, the stable fly is a member of the house-fly family, Muscidae. Like the house fly, it originally came from Europe, most likely with cattle.

At 1/4- to 1/2-inch long, it is about the size of a house fly. It has clear wings, black legs, and a gray body with four stripes on its thorax and black spots on its abdomen.

When and how does it bite?

The fly’s proboscis permanently protrudes from its face and has teeth at its tip to cut into skin. Both males and females bite to feed on blood.

Like horse flies, stable flies hang around lakes. They are sometimes called storm flies, because some people say they bite more during rain. Perhaps people just get more bites when they take shelter under a tree or on a porch and wait for the rain to end.

Life cycle

Eggs develop in cowpies, barn straw, piles of cut grass, and waterweed cast up on lakeshores. Here they overwinter as larvae or pupae. The pupa lives in a hardened larval skin, called a puparium. Adults emerge in summer. Several generations may develop each year.

Good news, bad news

Unlike its infamous cousins the house fly and tsetse fly, which transmit serious human diseases the world over, the stable fly does not spread disease in Minnesota. It does, however, spread animal diseases in other parts of the world.

Biting Midges (no-see-ums)

North country campers may be surprised to be bitten by tiny insects that can hardly be seen. So minute are these midges, called no-see-ums, that they can easily go through window screens and the mesh of some tents.

Also called punkies and sand flies, these 1/10-inch-long critters are the smallest of the northland biters. They belong to the family Ceratopogonidae, which has about 460 North American species. Most feed on nectar. Several midge species will attack other insects. If a mosquito is full of blood, a no-see-um may bite her and steal the blood.

When and how does it bite?

A few species of midges go for human blood. The females need protein-rich blood so their eggs can develop. Like mosquitoes, midges feed at dusk. They puncture skin with a pair of mandibles, which look like scissors blades.

Life cycle

Midge eggs are laid in ponds. The larvae look like thin worms. The pupa floats but does not swim. Adults usually emerge in summer and feed in June and July. Adults do not fly far from their breeding grounds. Some people say you can escape from no-see-ums by simply moving a few feet away.

Good news

Thanks to biting midges we have chocolate. One species pollinates cacao trees, the source of chocolate, in lowland tropical forests.

Anatomy of a Bite

Most female biting flies, from the smallest no-see-um to the biggest horse fly, share a similar kind of feeding organ. It is a complex, tubelike bundle of mouthparts called a proboscis. The proboscis locates, cuts, and pierces blood vessels, and draws a blood meal from many kinds of animals.

Thanks to the work of entomologists with scanning electron microscopes, we can take a very close look at the outer and inner mouthparts of biting flies. Images 2 through 5 are about 3,000 times larger than life-size and images 6 and 7 are about 400 times larger than life-size.

In the case of mosquitoes, black flies, horse flies, deer flies, and biting midges, the covering of the proboscis is made up of two parts: the labrum, or upper lip, and the labium, the lower lip. This pair of lips is so long that it forms a tube. The upper lip cups the lower one in an upside down U-shape. It has receptor tips on the end that help the fly find where to start feeding.

Inside the covering, or lips, is a bundle of five stylets--stiff cutting and piercing tools. A pair of mandibles cuts the skin from side to side with sawlike edges. A pair of maxillae moves up and down and pulls the stylets deeper with backward-facing teeth. A single hypopharynx injects anticoagulant saliva through a duct.

Once the stylets sever a blood vessel, blood is drawn into the food channel formed by a groove in the center of the upper lip and the upper surface of the mandible.

The stable fly has a proboscis quite different from other biting flies. Its proboscis is so rigid, the whole organ can pierce the victim. The sensing receptors on the closed lips help the fly locate a good spot to bite. The fly presses the proboscis into the spot, causing the lips to turn inside out and expose blades to cut skin and bring the proboscis deep into a blood vessel. A salivary duct and a food channel inside the proboscis force anticoagulants into the wound and carry blood into the fly.

- Susan Binkley

Shoo Fly tips

  1. Repellents containing the chemical DEET seem to work best (except for black flies). Choose a product with a small percentage of DEET. Apply to clothing and limit contact with skin. Read the labels carefully for precautions, apply according to directions, and wash off your skin when you go indoors.
  2. Trying to rid an area of mosquitoes with bug bombs or sprays does not work.
  3. Bug zappers kill beneficial insects as well as those that bite. They don’t control mosquitoes and may actually bring them closer to us. Build a bat house instead. Bats love mosquitoes.
  4. No-see-um mesh and screens work great.
  5. Wear a hat, long-sleeved shirt, and long pants. Netting over a hat also helps.
  6. Plan your canoe trip for late summer to avoid many insects.
  7. Put up with biting insects, as the author does.

- Larry Weber

Larry Weber teaches science at The Marshall School in Duluth. He is a naturalist and author of Backyard Almanac.

A complete copy of the article can be found in the July-August 2000 issue of Minnesota Conservation Volunteer, available at Minnesota public libraries.