Horsefly bites are painful, the bites of large specimens especially so. Most short tongued (short proboscid) species of horse flies use their knife-like mandibles to rip and/or slice flesh apart. Flies with longer proboscides bite more like a mosquito, their stylet-like mouthparts piercing the host's skin like needles.
The horse fly's bite is more immediately painful than that of its mosquito counterparts, although it still aims to escape before its victim responds. Moreover, the pain of a horse fly bite may mean that the victim is more concerned with assessing the wound, and not swatting the interloper. In any case defense is difficult, considering the agile nature of the fly. However, inhabitants of regions where the flies are a pest usually learn to swat immediately at the first hint of the bite. That usually gets the fly, especially if its escape is hampered by its having bitten through clothing. The bites may become itchy, sometimes causing a large swelling afterward if not treated quickly.As Rutgers University explains about the Greenhead fly in particular:
The Greenhead Problem: The salt marsh greenhead fly, Tabanus nigrovittatus, is an abundant and bothersome summertime pest along our coastal marshes. Because the females bite during daylight, and because they occur in large numbers, have a long flight range, and attack persistently, they interfere with the enjoyment of coastal areas throughout much of the summer.
To anyone who has not visited the New Jersey coastal areas during "fly season," the impact of these flies on daytime activities is hard to imagine. We have collected in traps over 1000 greenhead flies per hour all seeking a blood meal. Greenhead fly populations reach peak numbers during July, but extend from late June into September.
Conventional methods of biting fly control, such as those used for mosquitoes are either environmentally undesirable or economically impractical. Both adults and larvae of greenhead flies are large in comparison to other, non-target organisms. Generally, more insecticide is needed to kill larger insects. The higher concentrations or greater amounts of toxic materials needed to obtain - greenhead control have undesirable effects on other insects and animals. Marsh water management by ditching may actually enhance greenhead production. Although high-level impoundments reduce the numbers of developing greenhead larvae, this is a costly and impractical approach to fly control for much of our coastal wetlands. . . .
Where Greenheads Come From: Greenhead flies are produced from our coastal marshes. We have found as many as 70 larvae in a single square yard of marsh sod. Developing larvae concentrate along the upper vegetational zone reached by daily high tides. Foraging through wet thatch, surface muck, and vegetation, the predaceous larvae attack and devour a variety of invertebrates, including some of their own kind. Larvae overwinter and form a pupa after a brief period of spring foraging. The adult emerges from the pupa in late spring.
Adult flies mate on the open marsh. Within a few days and without seeking a blood meal, the female lays her first egg mass, consisting of 100 to 200 eggs. To produce additional egg masses, the female needs a blood meal. Among biting flies, blood serves as a rich protein source necessary for egg development. In the case of the salt marsh greenhead, protein for the first egg mass is obtained when the predaceous larva eats other insect larvae or small animals, but to lay additional egg masses she must obtain a blood meal.
Adult female greenheads move from the salt marsh to nearby -wooded or open areas along the marsh edge to seek suitable blood sources. There they await and attack wildlife, livestock, and people that venture close enough for them to detect.
Females live for three to four weeks in the uplands before they become too weak to bite. Because of this long life, larger numbers of' blood hungry flies build up in areas near salt marshes. The physical removal of large numbers of flies can reduce this buildup and thus decrease the greenhead fly problem locally.While these flies can carry infectious agents, no serious diseases are currently associated with them in the Northeastern U.S. and they are apparently not venomous.
Note that while the adult females feed on blood in order to reproduce, that adult files who are not in a reproduction phase, like bees, feed and nectar and pollen and are an important pollination species; not as important as bees, but an important supplement to bee pollination nonetheless.