Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance/Horseflies and similar

Horse-flies and similar flies (Diptera)
(to Table of Contents)

Characters of brachyceran Horse-flies and Clegs (Tabanidae) and Snipe-flies (Rhagionidae)
These are typical flies of the sub-Order Brachycera. Their antenna are highly characteristic. These are short, projecting in front of head, and consist of three dissimilar and asymmetric segments, with sparse or no setae, and no arista. The outermost segment has a cylindrical extension with rings (or annulations) that appear like miniature segments.
 * Photograph shows a typical tabanid fly, with large eyes and wings for strong hunting flight during daytime. The short antennae are typical of brachyceran flies.

Females feed on blood of their hosts, males are not blood-suckers. Larvae inhabit wet soil. Mouthparts are complex with a set of two pairs of piercing and slashing mouthparts, two parts making a blood-sucking tube, and a large labellum adapted for sponging up blood released from the wound. These flies are painful to their hosts and are messy feeders; they often go from host to host in rapid succession, attempting to feed persistently and aggressively. This makes them potential mechanical vectors of many pathogens. Trypanosoma protozoa are the most important of these pathogens, but also Anaplasma bacteria and filarial nematode worms are transmitted.
 * Diagram of feeding at skin represents a tabanid fly cutting into the layer of dermal capillaries of its host to release blood for feeding (proportions are not accurate).

Eyes are exceptionally large relative to head, and on living flies they reflect colored patterns. These flies hunt their hosts during daytime. Legs are medium length and end in two claws, two pulvilli, and a central empodium in the form of a pad.

Thorax and abdomen bear many fine setae, giving a slightly furry appearance, in contrast to the large bristle-like setae of the muscid and calliphorid flies. Wings have complex patterns of venation and in many species there are patterns of light and dark colors; when the fly is at rest the wings are only partly folded over the abdomen.

There are many genera of tabanids of veterinary and medical importance; five examples only are described here. The greatest diversity of species and density of populations of tabanids occurs in the Americas, but these flies are widely distributed worldwide. The Snipe-flies belong to a varied Family (Rhagionidae) related to the tabanid group. They are either predatory on other invertebrate animals or feed on the blood of vertebrates, including livestock species.

Glossary

 * Annulation = Rings on the outer segment of antenna of some flies; appear similar to small segments (2 on Chrysops).
 * Antenna = Paired sensory organs on head of insects, sensitive to odors; of characteristic form in brachyceran flies (2 on Tabanus).
 * Arista = A long thin extension of the outermost segment of the antenna, usually seen on  Blow flies  and similar (1 on Symphoromyia).
 * Labellum = A large single component of the mouthparts of brachyceran flies, usually functions to sponge up liquids (6 on Tabanus).
 * Spur = A stiff and sharp extension of the surface of a leg (8 on Tabanus).
 * Tibia = The second to outermost segment of legs of arthropods (8 on Tabanus).

Tabanus (Tabanidae)
Characters: female, lateral and parts. 1- Large and robust tabanids. 2- Antennae have 3 segments of asymmetric shape; the outermost segment has an extension with 4 additional rings or annulations. 3- Eyes have a horizontal pattern, with green and brown colors; gap between the eyes is small; some species have simple eyes (ocelli) between the compound eyes. 4- Legs end with 2 claws, 2 pulvilli and 1 central empodium in form of a pad. 5- Wings are usually clear but may be black overall. 6- Labellum is large; piercing mouthparts are long and finely pointed. 7- Palps consist of 2 segments. 8- Mid-tibiae have spurs; fore- and hind-tibiae have no spurs.

Hosts: Tabanus species are commonly known as Horse-flies, Greenheads and similar. They feed on cattle, sheep, goats, horses, camels, and many other mammals including humans. Signs: Severe irritation and avoidance behavior are caused. Cattle and sheep may pack in a tight circle, all attempting to protect their heads in the center This is a herding defense called Fly-syndrome. Disease: Numerous Tabanus species in North America cause serious biting-stress and blood loss to livestock leading to costly lost production. Important pest species are: Tabanus atratus (Black-horsefly) and T. lineola (Lined-horsefly), and T. sulcifrons (Autumn-horsefly). Species in this genus are mechanical vectors of the protozoans Trypanosoma evansi protozoa causing surra, and of T. vivax causing nagana.

Chrysops (Tabanidae)
Characters: adult, lateral. 1- Small to medium-sized tabanids. 2- Antennae are long and the outermost segment has an extension with 4 additional rings. 3- Eyes are relatively small compared to the head, with a wide gap between them; they have patterns of green or red in curved shapes. 4- Simple eyes (ocelli) are present as a group of 3 between the compound eyes. 5- Hind-tibiae have spurs; mid- and fore-tibiae have no spurs. 6- Wings have a black band along the leading edge and across the center.

Hosts: Chrysops species are commonly known as Deer-flies, but they readily feed on cattle and other livestock animals.

Signs and disease: They cause irritation and biting-stress. Species in this genus are mechanical vectors of the protozoans Trypanosoma evansi protozoa causing surra, and of T. vivax causing nagana.

Haematopota (Tabanidae)
Characters: female, lateral. 1- Medium-sized tabanids. 2- Antennae are medium length, the 3 segments have symmetrical shapes and the extension of the outermost segment has 3 rings. 3- Eyes are medium-sized, with a large gap between them; they have a conspicuous angular pattern of red and green. 4- Spurs are present on the tibiae of the mid-legs only. 5- Wings have a complex pattern of pale spots on a grey background.

Hosts: These flies are commonly known as Clegs. They feed on cattle and other livestock animals.

Signs and disease: They cause irritation, biting-stress, and they transmit the protozoans Trypanosoma evansi causing surra, and T. vivax causing nagana.

Hybromitra (Tabanidae)
Characters: female, lateral. 1- Large tabanids. 2- Antenna consists of 3 dissimilar segments, the outermost is distinctly asymmetric and its outer extension has 3 rings. 3- Eyes are large, with reflective colors but not distinctly patterned. 4- Wings are clear, without patterns of dark areas. 5- Tibiae of all legs are without spurs.

Hosts: Cattle and other livestock species are hosts.

Signs and disease: These are responsible for causing irritation, biting-stress, loss of production, and transmission of the protozoans Trypanosoma evansi causing surra, and T. vivax causing nagana.

Lepiselaga (Tabanidae)
Characters: female, lateral. 1- Small tabanids. 2- Antennae consist of 3 symmetrical segments; the extension of the outermost segment has 4 rings. 3- Eyes do not usually have distinct patterns of color. 4- Tibiae of the mid-legs have spurs. 5- Wings have a large basal black band. Hosts: Cattle and other livestock animals are hosts.

Signs and disease: These flies cause irritation and biting-stress. They transmit the protozoans Trypanosoma evansi causing surra, and T. vivax causing nagana.

Distribution: They are restricted to forests of South America.-

Symphoromyia (Rhagionidae)
Characters: adult, lateral. 1- Antenna has three segments; the terminal segment is wide and bears a plain arista. 2- Eyes are of medium size and there are three ocelli between the eyes. 3- Body is black, with an elongated abdomen. 4- Hind-legs bear pairs of spurs. 5- Legs each have two pulvilli and a central empodium in the form of a pad. 6- Wings have venation pattern typical of brachyceran flies.

Hosts and disease: The genus Symphoromyia represents here a varied group of flies, often known as Snipe-flies, some of which are adapted for feeding on the blood of vertebrates, included livestock animals. They are not known as transmitters of pathogens to livestock.

(to Table of Contents)