Horsefly (Various)


Class: Insecta
Order: Diptera
Family:    Tabanidae
Size:    Length: ½ inch to 1 inch (1.27 to 2.54 cm)
Weight: Unknown
Diet: Plant sap, flower nectar
Distribution: Worldwide
Young:  100 to 1,000 eggs
Animal Predators:  Frogs, lizards and spiders
IUCN Status: No special status
Terms: Group: Swarm
Lifespan: 3 weeks to 2 years, depending on the species and location



·         There are more than 3,000 species of horseflies worldwide, 75 of which can be found in Canada.

·         They are sometimes called gadflies or greenheads.

·         Deer flies—which are typically smaller and have more boldly coloured wings—are related to horseflies and can transmit tularemia, anthrax, and other diseases to mammals, including humans.

·         Horseflies can fly at speeds of up to 25 miles (40 km) per hour.   



Horseflies have a stocky body and colourful eyes, usually green. The body of an adult consists of a head, thorax and abdomen. They have two wings and six legs which are attached to the thorax. Each leg ends in a pair of claws with sticky pads at the end so that they can walk upside down. The males eyes touch but the female’s eyes are spaced slightly apart. They use antennae to feel and smell. 



Horseflies are found worldwide, and have resting places in wooded areas or dense grass, as well as cattails. They are also found in moist soil and mud.


Feeding Habits

Both males and females dine on the nectar of flowers or plant sap, but when females need to reproduce, they drink up to three ounces (88.7 ml) of blood at a time from one person or animal. Horseflies’ tongues are tube shaped and are used as a pump. When the females bite, they suck the victim’s blood. Females drink blood so they can get protein to develop their eggs. They are painful biters and often leave an open wound on the victim’s skin. Horsefly larvae have venom that they inject into their prey, usually the larvae of other insects, but also other horsefly larvae. The venom immobilizes the prey almost immediately. The bite of the larva is as painful as that of the adult horsefly. 



In the summer, female horseflies lay egg masses on top of plants near water. After hatching, the larvae either make their way to moist soil or drop into the water. At this stage, they feed mostly on insects and worms. In cooler regions, they hibernate during the winter months, and develop into the pupal stage in spring, becoming adults by June. Females also lay eggs on the front legs of horses, mules or donkeys. When the eggs hatch they cling to the animal’s tongue when the animal licks its leg, and are then swallowed and transported to the animal’s stomach where they incubate for nine to 10 months. They are excreted alive. 



Horseflies are impressive flyers who can hover in one place or quickly zoom off on the spur of the moment. They often are found in swarms and as many as 500 of them have attacked one cow at one time. During the warm, humid season when these flies are in abundance, farmers leave their work until after dark, when it is cooler and the flies have gone to rest. 



Horseflies are not a conservation concern. 



Horsefly Wildlife Fact File, IM Pub, US