Horse flies are among the vampires of the fly world. Here is our comprehensive rundown of these blood sucking pests that covers what they eat, their habitat, types, life cycle and larva, size, and the various elements that may potentially be attracting them to your house or neighborhood.
Horse Fly Facts
The term “horse fly” is used to refer to numerous species of insects in the Tabanidae family. Here are some horse fly facts that may help you with classwork or understand these little creatures better in your quest to control them:
Horse flies are biologically classified under family Tabanidae and order Diptera. The two most common horsefly genera (or genuses if you like) are Tabanus and Chrysops. The most common species is Tabanus lineola which is notable for its bright-green eyes. Its common name is green head.
Various common names are used to refer to horse flies: Green headed monsters, ear fly, gad fly, breeze fly, deer flies, klegs, cleggs, zimbs, bull dog flies (in some parts of Canada), stouts, and march flies (in Australia).
Horse flies are found all over the world except in some remote islands and in far north and south latitudes.
These biting flies are notable for their yellowish-brown or brown colored bodies with hairs all over. They have a characteristic huge head. A horse fly looks very similar to a honey bee except for the fact that it has only a single pair of wings.
The beautifully colored compound eyes with lustrous metallic hues make these insects very notable. The eyes are dichoptic (separated) in females but holloptic (continuous) in males.
Horseflies are commonly found around aquatic habitats (e.g. ponds, marshes, tanks, creeks, streams, riparian zones of freshwater habitats etc.) where they are known to burrow into the sand or gravel. They prefer such environments as they provide ideal conditions for development through the immature (larval) stages.
The adults feed on nectar, honeydew, and other plant juices, and sometimes pollen. The females of most species however anautogenous which means that they require a blood meal to assist in egg development. As such, females feed on mammalian blood even though certain species are known to bite birds and reptiles as well.
Horse fly bites are particularly painful since, unlike those inflicted by most biting flies and insects, they are more of a razor-like cut than a piercing so to speak.
In immature stages, their larvae feed on small organisms present in their moist breeding areas including insects, earthworms, crustaceans, snails, and other invertebrates. The larvae of most species are also cannibalistic meaning that they can as well feed on other larvae.
Types of Horse Flies
There are way too many types of horse flies – more than 4500 species are known worldwide – and discussing all of them would take an entire separate article if not a book. Anyway, here are just a few of the most common types that you may delight to know for now:
This is the most prevalent horsefly species, notable for bright-green eyes. Its common name is green head.
Small sized horsefly species with a body length of between 13 and 15 mm and. It has yellowish-brown and black markings on the abdomen and the base of each wing has a broad alula. Its eyes have a characteristic band which earns this species the name band-eyed brown horsefly. They are abundantly found in northern Europe and Russia.
Sometimes referred to as the hairy legged horsefly, this one is notable for hairy eyes, three stripes in each eye, and whitish triangles along the dorsal midline of its dark-grayish abdomen. It’s commonly found in Mongolia, China, Europe, Russia, and Japan.
Black Horse Fly
The term black horse fly is used to refer to Tabanus atratus, a species of horseflies most commonly found in eastern United States. Black horse fly is notable for its pitch black, dark brown, or blackish purple body (including the wings) which is covered with short stout hairs.
Adult black horse flies range in size from 20 to 25 mm and have compound eyes. Females are known for their dichoptic eyes (with a space in between them) while males have holloptic (continuous) eyes. They can survive in a variety of habitats with the exception of extreme climates such as far north and south, mountain tops, and deserts.
Female black horseflies require mammal blood to provide it with essential proteins before it can start reproducing. Males however feed on nectar and plant saps.
As is to be expected with horseflies, Tabanus atratus depend on aquatic environments for development through the immature stages (larval stage). The larval stage undergoes 6-9 instars before pupation and the pupal stage takes between 1 and 3 weeks after which ready to fly and mate adults emerge.
An adult female lays 3-4 masses in its lifetime, each with between one hundred and one thousand eggs. The eggs are usually laid in an aquatic environments as these are suited to supporting the development of the flies through the larval stage.
Their larvae feed on any small organisms they come across in the gravel or sandy substrate of the moist environment in which they burrow after being hatched. Earthworms, snails, small insects, and other invertebrates are all common preys.
What Attracts Horse Flies
Their bites are painful and wow unto you if their scissor-like cuts make you end up with a bacterial infection. The question though is, “what attracts horse flies?”
To start with, horse flies are attracted to sunny, warm climate and t is thus not surprising that they peak during warm seasons, being most active in summer and early fall.
Horseflies are also attracted to moist environments e.g. marshes, swamps, bogs, etc. as these provide them with an ideal environment for their larvae to grow and develop into adults once they have hatched from the eggs.
Adult horse flies are also attracted to large, dark objects, more so moving ones e.g. horses, humans, etc. This is how females they locate their prey while looking for blood. They also find their prey by detecting certain odors and gases especially the carbon dioxide released by humans and other animals during respiration.
Are Horse Flies Dangerous
Female horse flies are notorious for their particularly painful bites to human beings in their search for blood protein. They also attack cattle, avian species, and even reptiles.
This makes them dangerous as they can transmit parasites and diseases among their victims. They are known to transmit diseases such as leucocytozoan, anaplasmosis, hog cholera, anthrax, and tularemia. They also cause blood and weight loss to livestock. They as well reduce milk output in dairy cattle.
In spite of their harmful roles as pests and disease transmitting vectors, these insects serve important roles in pollination. Some S. African species in particular have long proboscides which are suited to extraction of nectar from flowers that have long, narrow corolla tubes e.g. Lapeirousia and certain Pelargonium flowers.
Horse Fly Size – How Big Are Horse Flies
“I have an infestation of large flies which are without doubt not house flies. My son thinks they are horse flies but I am not sure. What is the actual size of a horse fly?”
The actual size of a horsefly varies from one species to another ranging from those that are as small as the common housefly to those that are as huge as the bumble bee.
If you are not sure, your best bet would be to look for other characteristic features. To start with, they are most notable for their huge head and having hairs all over their body which makes them look so much like bees. They however only have one pair of wings.
Their eyes also have a characteristic metallic or iridescent luster. The eyes meet dorsally in males, but in females they are separate.
What Do Horse Flies Eat
As we have already mentioned, male horseflies feed exclusively on nectar, honeydew, and other plant liquids. They may also feed on pollen which makes them good pollinators.
Females must however ingest a blood meal before they can start laying eggs; this provides them with protein. Mammalian blood is a preferable choice for them, but some species also feed on avian, reptile, and even amphibians blood.
The males lack the mandibles that their female counterparts use to bite and suck blood from their victims. This in context means that only female horseflies are ectoparasitic, just as is the case with mosquitoes.
Horse Fly Larvae and Horse Fly Life Cycle
The horsefly life cycle has four distinct stages which means that they undergo complete metamorphosis. These stages are egg, larval, pupal, and adult.
The life cycle begins with mating which happens in swarms. The female then lays fertilized eggs on vegetation that is in close proximity to (typically hanging over) an aquatic environment. The eggs are usually dark, shiny, and spindle-like in shape and each female can lay anything between a few to several hundreds of them. The eggs hatch in 5-12 days. On hatching, the larvae drop into water or moist earth.
The larvae burrow into the ravel or sand substrate of the aquatic environment they are in or along the banks of the river or stream where they feed voraciously on earthworms, snails, and other invertebrates going through 6 to 13 instars.
The larvae then move to dryer soils ready for the next stage of development; pupal stage. Pupal stage takes 6-12 days depending on the species as well as the prevailing conditions, especially temperature. During this stage, the larvae develops adult features such as wings while encapsulated in a hard pupal casing.
Finally, an adult horse fly emerges from the pupal casing. The emerged adults are ready to fly and mate and will start looking for mates immediately. The female also start feeding on blood immediately to prepare for reproduction.
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