Horseflies bite people because the female horsefly, like the female mosquito, needs a blood meal to lay her eggs. Like male mosquitoes, male horseflies eat pollen and nectar and rarely bite. Horseflies are called horseflies because they also bite horses, cattle and other livestock. Sometimes their bites can be so severe that it reduces milk production from cows and causes them and other animals to run mad and possibly injure themselves. Horseflies can be a scourge of humans as well.
The horsefly belongs to the family tabanidae and the order diptera and grow to .4 inches long, more or less. It has segmented antennae that look like curved blades and large, rounded, iridescent, separated eyes; the male's eyes are connected. The horsefly is pale gray and her back has a grayish fur. The abdomen and wings are black. The horsefly looks very much like a housefly but is larger and has the unusual antennae.
Horseflies are found most abundantly near wetlands or marshes where they lay their eggs. They like to attack their hosts in the shade, where the host has come to rest. After her blood meal the female horsefly will be able to lay 25 to 1,000 eggs on vegetation near wet sites. The eggs hatch in about one to two weeks.
Female horseflies are diurnal and are attracted to movement, shiny surfaces and the carbon dioxide and warmth from the respiration of living mammals. Unlike female mosquitoes the horsefly does not use subtle, syringe like mouthparts to get to her meal but mouthparts like little saws to tear up the skin and feed on the blood that results. This results in the bite being excruciatingly painful. As with mosquitoes there can be an allergic reaction. The bites should not be scratched and skin creams might help alleviate the pain and itching. The female horsefly, moving from host to host, might be a vector of disease.
After the eggs hatch the larvae fall to the ground. They're predatory and eat worms, other insect'a larvae and crustaceans found in the soil or the water. An unusually big horsefly larvae will even catch and eat fish and amphibians. The larvae take one to three years to mature. The horsefly undergoes a complete metamorphosis like all other flies as an egg, larva, pupa and adult. When it's time to pupate the larvae crawl to a dry area, pupate and then emerge as adults in the late spring or early summer. After all this, the adults only live a few days.