Tabanus sp. (Diptera: Brachycera: Tabanidae)
Today’s insect is probably familiar to most of us. I’m referring to Tabanus sp. (Horseflies). With their strong proboscis they have been known to ruin an afternoon picnic or two. They are most commonly found near bodies of water or in forested areas. Tabanid species have been associated with several animal pathogens (including humans) but they are not the primary vectors of any human diseases. Among the rest of the vertebrate world, tabanids are of importance in the mechanical and cyclical transmission of Trypanosoma evansi (Surra) and Trypanosoma vivax in horses, camels and other domestic animals in Latin America and Africa. In Africa, most of the important trypanosomal pathogens are transmitted by Tsetse flies (Glossina species) which are found only on that continent.
Interestingly, people usually confuse Tsetse flies with tabanids. However, while both male and female Tsetse take a bloodmeal, male tabanids do not bite. In that, they are like other disease vectors such as mosquitoes. Also, people will often say that they’ve been “stung” by a tabanid or a Tsetse fly. However, neither Tsetse flies nor horse flies have a sting apparatus as do the wasps and bees. But, while Tsetse flies and horse flies are not able to sting, both have a nasty bite.
In this week’s image of Tabanus sp. some of the important morphological characters are labeled and notes on them follow here. To the left are a calypter and beneath it sits the haltere (The small, brown, round body lying beneath and protected by the calypter). Flies have only two wings while the rest of the insects have four. In the Diptera (Di=two; ptera=wings) what remains of the 2nd pair of wings (after evolution had finished its work) are the halteres, gyroscopic-like devices that whirl around during flight keeping the fly on a controlled, predictable flight path. Not visible in the photograph is the stick-like bottom part of the haltere to which the round part is attached. On the right side of the image are the antennae, short and narrowing to its apex (a note; tabanids are in the suborder Brachycera [Brachy=short; Cera = horn] referring to the short length of the antennae, a character shared with the many groups in that suborder (contrast mosquitoes [suborder Nematocera] with their long thread-like antenna). Beneath the antennae is the tabanid’s proboscis. The outer (visible) part is sponge-like. Hidden away are the blade-like mouthparts that slice through the host’s tissues leaving the pooled blood to be sponged up. Finally – the tabanid’s eyes are clearly made up of two groups of different-sized ommatidia (each a tiny light sensor). The larger, upper ommatidia are better able to see in the ultraviolet while the lower and significantly smaller ommatidia allow for finer visual acuity. One last observation. The eyes of many insects give away the sex of the specimen. This is so for Tabanidae, in which the eyes of males are holoptic (touching each other), whereas the females’ are separated (dioptic). We have a male. It was captured in Mundere forest, next to Lake Victoria in western Kenya.
Credits: Dr Robert Copeland