Tabanid flies, also known as horseflies, deerflies, mango flies, and mangrove flies, are medium sized, 5 - 25 mm long, and have a very painful bite. In addition to loiasis, they also transmit tularemia and come arboviral diseases. They are the vector and the intermediate hosts of this parasite.
In addition to humans, tabanids feed on cattle, horses, deer, and other small mammals and reptiles. They lay their eggs on objects hanging above water, such as leaves or branches, and the larvae " live in mud, rotting vegetation, humus, damp soil and shallow, muddy water at the edges of pools, swamps and streams", with development taking anywhere from one to three years from egg to adult.
They feed during the day, particularly during the sunniest time of the day, which perfectly coordinates with the diurnal periodicity of Loa loa microfilariae. They are common in "forests and swampy areas with woody vegetation", don't enter houses to bite, and can fly for long distances. Additionally, they are often interrupted while feeding due to their painful bites, and so they bite and potentially infect many individuals trying to obtain a single blood meal.
Protection from Tabanid bites and control of Tabanid populations is difficult due to their biting habits and widespread distribution. (Information and picture from h ttp://www.who.int/docstore/water_sanitation_health/vectcontrol/ch06.htm#b6-Horseflies%20and%20deerflies%20%28tabanids%29)