Tour de Fly

By Eric R. Eaton | May 2, 2003
From Missouri Conservationist: May 2003

When you are shooing house flies off your picnic plate, or swatting mosquitoes at the fishing hole, it is hard to imagine that flies of any kind could be good for anything.

There are, however, some species of flies (Diptera) that help us. Pomace flies, for example, are vital research subjects in genetics. Hover flies and bee flies pollinate flowers. Robber flies kill many harmful insects, while tachinid flies are parasites of other pests. Even blow flies help crime scene investigators solve homicides.

You don't even have to leave home to encounter flies. We all know the pesky house fly, Musca domestica, in the family Muscidae. They are known to spread a variety of diseases, but ironically are fastidious groomers. Watch one and you will see it rub its feet, draw a leg over each wing and wipe its eyes. Those eyes are better than ours at perceiving motion, making the fastest fly swatter look like slo-mo. Pads on their feet let house flies cling to slick surfaces, or alight on the ceiling.

All flies go through the same life cycle, beginning life as an egg, which hatches into a larva, or "maggot." The larva feeds and grows, shedding its outer cuticle (exoskeleton) several times. Eventually, the larva molts a final time into a pupa. Inside this capsule-like stage, the insect is re-programmed, and its cellular structure rearranged into that of an adult fly. The time it takes to complete this metamorphosis depends on variables like temperature and humidity.

Adult houseflies live only two or three weeks, but there can be ten to twelve generations in a summer! The cycle from egg to adult can take as little as seven to ten days in warm weather. Horse dung is the favorite breeding medium for house fly larvae.

Besides house flies, you may confront "fruit flies" hovering over your bananas. These are actually pomace flies of the family Drosophilidae. Attracted to fermenting substances, they have an extraordinarily high tolerance for alcohol. Larvae eat mostly the yeasts that infect bruised produce.

Amazingly, these tiny flies have giant chromosomes in their salivary glands, allowing researchers to easily pinpoint specific genes. This unique feature, coupled with the flies' rapid life cycle, makes them ideal subjects for studying inherited characteristics. Drosophila melanogaster is the common species in the lab and in the wild.

Humpbacked flies (family Phoridae) look like pomace flies, but run more than fly, scurrying with a jerky gait. They have varied life histories. Depending on the species, larvae develop in rotting organic matter, or as internal parasites of insects, other arthropods or mollusks.

Entering the bathroom, you may find a small, hairy insect clinging to the side of the basin. That would be a moth fly, family Psychodidae. Moth flies can reach staggering numbers in locations close to their typical larval habitat of sewage. The few found in the average home usually develop in the drain trap.

Your porch light at night attracts a different set of flies. Relax. Those giant mosquitoes, or daddy longlegs on wings, are harmless crane flies (family Tipulidae). Most do not even feed during their brief adult lives. The larvae, known as "leatherjackets," may be aquatic or terrestrial, depending on the species. They eat mostly decaying organic matter, though some kinds are vegetarians and a few are predators.

Also mistaken for mosquitoes are the midges (Chironomidae). Nearly identical to skeeters, midges are incredibly abundant, especially near water. Males often gather in great swarms over prominent objects. The whining of their wingbeats is annoying, but they do not bite. The larvae of most species are aquatic, feeding on microorganisms. Many species build cases of sand or debris.

Mosquitoes (Culicidae) rarely show up at lights, so bugzappers are useless. Ultrasonic devices don't repel them, either. Carbon dioxide in your breath, and other chemicals like lactic acid, draw the bloodsuckers.

Walking through the garden in daylight will introduce you to some flies that resemble wasps or bees. These flies have bold patterns of black and yellow that mimic the colors of species that predators find dangerous or distasteful.

Flies have only one pair of wings, and usually very short antennae, while wasps and bees have two sets of wings and longer antennae. Flies also have sponging or sucking mouthparts, in contrast to the chewing mandibles of stinging insects.

Taking this mimicry even further, some hover flies in the family Syrphidae even hold their front legs in a position to imitate antennae, and buzz their wings at the same frequency as their venomous counterparts. Syrphids are diverse, important pollinators of flowers, and the larvae of several species are voracious predators of aphids.

The larva of the drone fly, Eristalis tenax, is known as the rat-tailed maggot. Living in putrid waters, it breathes through a telescoping tail it stretches to the surface. The narcissus bulb fly, Merodon equestris, is another convincing bee impostor. Its larva is unusual in being an occasional pest of daffodils.

In addition to impersonating bees, some of the fuzzy bee flies (Bombyliidae) parasitize them. A female Bombylius major hovers over the burrow of a solitary bee, and lobs eggs down the tunnel. The larvae that hatch crawl into the cells of their host, and feast on the pollen and nectar stored for the bee's offspring. Other bee flies are parasites of solitary wasps, tiger beetles or other insects. Several species have long beaks through which they sip flower nectar.

The bumblebee disguise of Laphria robber flies (family Asilidae) can even fool scientists who know better. Look for Laphria at rest on foliage, logs, and tree trunks in sun-splotched woodlands. They cock their heads as other insects pass overhead. When suitable prey presents itself, the fly dashes out, grabs its victim and returns to its perch to feed. A sharp, stout beak administers the kiss of death.

Robber flies of other types are more streamlined. Large Promachus species buzz loudly as they fly short distances from perch to perch. These and similar varieties, like Efferia, Proctacanthus, Asilus and Diogmites, are most abundant in open fields, glades and prairies, and along riverbanks and forest edges. They can kill large beetles and grasshoppers, and even cicadas. The habits of the larvae remain a mystery for most species, but some are external parasites of beetle larvae.

Horse flies and deer flies

Many of Missouri's forty-plus species of horse and deer flies have dazzling striped or spotted eyes. Horse flies (genus Tabanus) can be huge, while deer flies (Chrysops) are smaller, with dark spots or bands on clear wings. The blood-feeding females slice, dice and slurp with blade-like mouthparts and a sponge-like pad similar to a house fly's. In livestock, blood continues to flow after the flies have departed. The open wounds invite infection and provide sites for screwworms and other parasitic flies to lay their eggs. Bites to humans can result, rarely, in severe allergic reactions, and our sensitivity increases with each bite.

Female tabanids are attracted to dark, moving objects and shiny surfaces. They often chase cars and frequently wind up as roadkill. Males, with wrap around eyes, seek only nectar and females. They sometimes swarm above treetops and man-made structures, the better to attract the fairer sex.

Another biter is the stable fly, Stomoxys calcitrans. It is related to the house fly and closely resembles one, except for the needle-like beak. Both sexes take blood from a variety of hosts. Harassment of cattle by stable flies, as well as horse flies, can cause a decrease in milk production and disrupt grazing patterns. Stable flies rarely breed in manure, preferring decaying grass clippings, wet hay and compost.

Among our largest flies are the mydas flies (Mydidae), which frequent forest margins. Mydas clavatus is a mimic of a large spider wasp, with a red band on its black abdomen, and shimmering violet wings. The larvae may be predatory on scarab beetle grubs.

A wealth of flies are parasites of other insects. Chief among these are the Tachinidae. Tachinids usually have very bristly or spiny bodies. Females can use devious tactics to access their hosts. Some species lay hundreds of minute eggs on foliage near their caterpillar targets. The caterpillars consume a few eggs in the course of feeding, and the maggots that hatch slowly consume their host internally.

Adult Trichopoda have feather-like scales on their hind legs that resemble the "pollen baskets" of bees. Trichopoda glues an egg atop a squash bug or stink bug, where the victim cannot reach to wipe it off.

Bot flies of the family Oestridae are parasites of mammals, living as larvae in such unthinkable places as the stomach lining of horses (Gasterophilus), sinus cavities of deer (Cephenemya), or under the skin of rodents (Cuterebra). The adult flies approach the size of bumblebees, but are rarely seen. Most species have no mouths, living off stored fat accumulated in the larval stage. Once thought to fly faster than the speed of sound, it is now known they seldom exceed 25 miles per hour. Still, livestock are known to panic at the approach of these insects.

Most blow flies in the family Calliphoridae dine on deceased bodies, and are indispensable agents of decomposition. The familiar "bluebottles" like Cynomyopsis cadaverina and Calliphora vicina, are named for their iridescent blue abdomens. Phaenicia sericata and Lucilia illustris are common, bright metallic "greenbottles."

Although they grow up in rotting flesh, they are competent pollinators of wildflowers. The black blow fly, Phormia regina, can be found as an adult every month of the year.

These species, and the secondary screwworm, Cochliomyia macellaria, are important to forensic scientists in Missouri. Some blow flies appear on the scene of death within minutes, while other species follow later, in a predictable sequence. Time of death can be accurately estimated by establishing which species are present on a corpse, and at what stage of development.

Phormia and Phaenicia have helped living people, too. During World War I, and shortly thereafter (early 1930s), sterile maggots were employed to clean wounds. The larvae eat only dead tissue, while secreting an antibacterial chemical that retards infection. To this day, maggots are still used in difficult cases of deep bone infection (osteomyelitis).

Flesh flies in the family Sarcophagidae also exploit carrion. Sarcophaga haemorrhoidalis are black and gray with scarlet eyes and "tails." The females arrive at a carcass later than blow flies, but catch up by laying live maggots instead of eggs. They can also find bodies indoors, where few other flies will search.

Tick-like, with flattened bodies and grappling hook claws, the louse flies (family Hippoboscidae) are truly bizarre. The wingless "sheep ked," Melophagus ovinus, is a familiar bane of livestock owners. Otherwise, hunters are the ones most likely to see these secretive parasites.

Larval hippoboscids develop one at a time, inside the female fly, feeding on milk-like glandular secretions. "Born" as fully grown larvae, they pupate immediately, on the host or on the ground. Most species emerge as flying adults, but some break off their wings after dispersing and settling on another host.

Louse flies feed on blood. Many, like the squab fly, Pseudolynchia canariensis, are common on pigeons or other birds, and sometimes serve as living ferries for feather lice. Lipoptena depressa occurs on deer.The world of flies is full of many more amazing stories, some yet to be deciphered. So, before you swat that noisy nuisance, you might want to contemplate its beneficial qualities. Or, maybe not.

Controlling Common Flies

Some folks swear by an unusual method of repelling house flies. They fill a clear plastic ziplock bag half full of water, and attach it to the outside surface of a door or window. No one is clear on why this works, but it does.

Flypaper still works well indoors, trapping flies that alight on the sticky yellow ribbon. Fly swatters are even more entertaining and rewarding. Pesticides are not recommended, to avoid contamination of foods and dishware.

A new trap, dubbed "Flybrella" by its creators at the USDA-ARS, is now approved for use in restaurants and other indoor settings.

Sanitation and exclusion are still the best preventive measures. Make sure trash cans are securely covered and emptied regularly. Dispose of manure and other organic waste quickly. Patch holes in screens and seal openings to the outdoors around pipes and electrical conduits.

Controlling horse and deer flies is virtually impossible. Avoidance is the best strategy. Donning a hat keeps deer flies off your scalp, and wearing light colored long sleeve shirts deters bites on extremities. Using repellents containing DEET is recommended, but never apply such chemicals to your skin. What residues are not absorbed will be quickly sweated off anyway. Using repellents on children is especially risky.

This Issue's Staff

Editor - Tom Cwynar
Managing Editor - Bryan Hendricks
Art Editor - Dickson Stauffer
Designer - Les Fortenberry
Artist - Dave Besenger
Artist - Mark Raithel
Photographer - Jim Rathert
Photographer - Cliff White
Staff Writer - Jim Low
Staff Writer - Joan McKee
Circulation - Laura Scheuler