The insect family Tabanidae (Order: Diptera) comprises approximately 3000 species of flies worldwide, including the commonly known horseflies and deerflies. Females of most species require a blood meal to complete egg development and many species are important in human and animal medicine. This article briefly discusses the life cycle, biology, and medical significance of tabanids.
Tabanids are often encountered as wasp- or bee-like insects that bombard and often follow people outdoors. They tend to favor the head and upper body, where they usually alight completely unnoticed by the victim until the painful bite gives them away.
This fly is a member of the genus, Chrysops, commonly known as deerflies. These flies belong to the family Tabanidae that also includes the more widely recognized horseflies, genus Tabanus (fig. 2). Tabanids are mostly stout-bodied, fast-flying flies ranging in size from approximately 6-30mm. They occur worldwide, except in extreme northern and southern latitudes. Approximately 3,000 species are known worldwide and around 350 species occur in North America.[1] Many of these flies are well known to ranchers and those who enjoy the outdoors, for their viciously painful, and often relentless biting. In those species that bite, it is only the females that search out prey for blood meals to nourish their developing eggs. Male tabanids are nectar feeders and are incapable of biting, as they lack the modifications of the mouthparts found in females. Most tabanids feed on mammals, but some species feed on birds, reptiles or amphibians.[2] In addition to "deerfly" and "horsefly", some species of tabanids are commonly called gadflies, clegs or, in Australia, Marchflies (not to be confused with the Marchflies of North America, which belong to the non-biting fly family, Bibionidae).
Figure 2: Tabanus sp., courtesy James Castner and University of Florida
As with all other true flies in the order Diptera, tabanids are holometabolous insects. This means they go through the following life stages: egg, larva (a.k.a. maggot when referring to most fly larvae), pupa and adult. Though the life histories of the members of this fairly large family of flies differ, they can be generalized as follows. Eggs are usually laid in large, layered clusters of 100-1000 on vegetation or other objects overlying water or moist soil. The larvae, which are aquatic, semi-aquatic, or terrestrial, hatch from the eggs and drop to the water or soil below where they become voracious predators of other invertebrates or small vertebrates. The larvae are white to tan, with a slender, cylindrical body that is slightly tapered at the head (fig. 3). The head contains two sharp, slender mandibles that possess a hollow canal for transmitting venom into their prey. They can inflict a very painful bite to humans (personal experience) but their venom is not dangerous to humans. The larva undergoes several molts as it grows and depending on the species, the larval stage may last a several months or as long as two to three years. Once the larva is fully developed it moves into drier soil to pupate. Depending on the species, the pupal stage lasts approximately 5-21 days, and then the adult flies emerge from the soil. Mating occurs shortly after the adults emerge. Females then lay in wait in vegetation until a host for a blood meal wanders into range. Females are attracted to large, dark, moving objects and to CO2.
Figure 3: Horsefly larva, courtesy Jason M. Squitier and University of Florida
Many species of tabanids are known to play important roles in spreading diseases of livestock and other animals.[3] In addition, several species in three or four genera are medically important to humans. The mouthparts of female tabanids are modified into the equivalent of miniature scalpels or steak knives ideal for macerating the skin to the depth of the superficial dermal vessels. A pool of blood collects in the tissues and is lapped up by the tongue-like component of the mouthparts. The bite of tabanids is quite painful but often leads to little more than a transient wheal and flare reaction with minimal bleeding from the wound. Occasionally secondary bacterial infection is a problem. Some individuals have significant urticarial reactions to tabanid bites and cases of anaphylaxis have been reported in the literature.[4, 5, 6] Hemmer et al. recently isolated a 69kDa IgE-binding salivary gland protein from species of Chrysops.[7] A case of coexistent tabanid and hymenopteran (wasps and bees) anaphylaxis has also been reported.[8]
In addition to the affects directly related to their bites, tabanids vector at least one human disease and perhaps more. The best studied human disease vectored by tabanids is loaiasis (see link below) caused by the helminth, Loa loa, and vectored by species in the genus Chrysops. Tabanids have been suspected as vectors of human cases of tularemia (Francisella tularensis), and anthrax (Bacillus anthracis), though hard evidence is equivocal.[3]