Nature Notes
Warm Weather Causes Early Hatch of Horseflies on Island
By Patricia Martin

Ogden Nash wrote, "God in his wisdom made the fly, and then forgot to tell us why." Those who have been around horses can understand Mr. Nash's feelings on the subject of flies. This year the stable flies came out early, and in great numbers, and a good number of horseflies have been buzzing around our equine friends.

Horseflies and stable flies are true flies and are found in the order Dioptera. As the name implies, Dioptera, from the Greek "di" (meaning two) and "optera" (meaning wing), the insects in this order have two wings, that is one pair, setting them apart from many other insect orders. In addition, they have sponging, sucking, or biting/ piercing mouthparts.

Many of the other insects, such as dragonflies, beetles, or butterflies, have two pair of wings as adults, or four wings, and yet it is the flies that are the most acrobatic of the flying insects. This is because sometime back in evolutionary development, the second set of wings on the flies disappeared, leaving behind two stumpy appendages called halteres. These rotating halteres, similar to gyroscopes, help give flies balance, hovering ability, and the capability of flying backwards.

A horsefly of the genus Tabanus.

There are estimated to be more than 25,000 species of true flies (while there are a number of other insects referred to as flies that are not in the true fly order, Dioptera) in North America, and of these, about 350 are horseflies (Family Tabanidae). Deer flies are also in this family. These insects are among the world's largest, and are known to most people because of their biting nature and their habit of circling the head, almost always out of reach and then unexpectedly biting on the leg or back, or biting your horse in sensitive places.

It's sometimes interesting to remember how insects have affected history. It's been written that horseflies may be partly responsible for Independence Day falling on the Fourth of July. It is said that the Declaration of Independence was signed on July 4, 1776, instead of a later date that would have allowed further debate and discussion, because the horseflies in Philadelphia were biting so aggressively that the delegates decided to adjourn to get away from them. Songs have even been written about horseflies. You might remember the old folk song about "Jimmy Crack Corn" and the horse that was bitten by a "blue tail fly." The fly that bit the master's horse was probably a horsefly of the genus Tabanus, native to the southeastern United States.

Horseflies of our region are characterized by their larger size (more than 1.25 inches in length), brownish or blackish bodies, and unbanded wings. There are several common species of horseflies in our area, including the green-headed horsefly (Hybomitra lasiophthalma), which has a black thorax with gray stripes and a brown abdominal tip. The eyes are green with purple bands and the wings are lightly spotted with brown. These flies are a problem at beaches, as they love to bite wet skin. The American horsefly (Tabanus americanus) is reddish brown with clear wings. The black horsefly (T. atratus) is uniformly black with wings dark brown or blackish. The striped horsefly (T. lineola) is brown with a reddish tint and its thorax and abdomen are striped with yellow or yellowish red. The abdomen has a distinct white strip down the center and the compound eyes are purple with three green stripes.

Though these flies are well known for biting mammals, a good portion of their diet, as adults, is nectar, and sometimes pollen. In their larval or immature stage, horseflies feed on other invertebrates in moist environments. It is the adult females that are responsible for the biting. The females usually feed on blood to aid in egg development. Males don't have the necessary mouthparts for blood feeding. Most female horseflies feed on mammal blood, but some species are known to feed on birds, amphibians, or reptiles. The females primarily use their large compound eyes to locate their prey. They wait in shady areas for a tasty morsel to pass by. They're attracted to large, dark objects, to certain animal odors and carbon dioxide, and to motion.

If you've ever been bitten by a horsefly, you know how painful the bite can be. Unlike insects that surreptitiously puncture the skin with a needlelike organ, female horseflies have mandibles like tiny, serrated scimitars, which rip or slice into flesh. This causes blood to seep out as the horsefly licks it up. They also can carve a chunk of flesh completely out of the victim to be digested at leisure. If you've ever been around a horse that has been bitten by a horsefly, you know it. One day I was riding my horse along Pottawatomie Road, when he got bitten by a horsefly. He gave a huge buck, and the next thing I knew, I was sitting on the road, looking up at my horse. He looked most apologetic.

These flies have come out earlier than usual this year owing to the warm weather we've been having. They're most active during the daytime in hot summer weather. Many species of horseflies prefer a wet environment because of their breeding cycle. Females lay eggs on vegetation, overhanging moist soil, or on stones close to water. Upon hatching, the larvae fall to the moist earth or into water to feed voraciously on invertebrates such as snails and earthworms, but they may also feed on small vertebrates. The development from the first larval stage to the adult often takes several months, and allows for wintering over in temperate climates. Temperature can greatly affect the length of time needed for growth into adulthood. Generally, the colder it is, the longer it takes.

In addition to the painful bites horseflies can give to mammals, they've also been known to carry a number of diseases to cattle, horses, dogs, and camels, as well as humans. A number of trypanosomes are transmitted to various organisms, causing "surra" and other diseases. In addition, equine infectious anemia virus and Western equine encephalitis can be carried by Tabanids (the horseflies in that genus, Tabaninae).

Not all flies are bad. Many of them play an important role in pollination and decomposition, but I have to say that, all in all, especially relating to horseflies, I agree with Ogden Nash.

P.S. Because of the warm weather and the lack of rain, many of our wildflowers are passing very quickly. Fireflies and grasshoppers are already out, and the beech trees are beginning to drop leaves owing to the lack of water.

Trish Martin is a yeararound resident of Mackinac Island, has earned a master's degree in botany from Central Michigan University, and owns Bogan Lane Inn.