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Horse Botfly (Gasterophilus intestinalis)
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Suborder: Brachycera
Infraorder: Muscomorpha
Section: Schizophora
Subsection: Calyptratae
Superfamily: Oestroidea
Family: Oestridae



Oestridae (also called botfly or bot fly) is a family of Oestroidea. It is one of several families of hairy flies whose larvae live as parasites within the bodies of mammals, such as the Desert Woodrat. There are approximately 150 known species worldwide.

Only one bot fly species attacks humans, the Dermatobia hominis. For many years the deer botfly was cited as the fastest flying insect in the Guinness Book of World Records, but the claim has been refuted and is no longer considered probable.

Life cycle

  • Botflies deposit eggs in a host body, or sometimes use an intermediate vector: common houseflies for example.
  • Eggs are deposited in animal skin directly, or the larvae drop from the egg: the body heat of the animal induces hatching upon contact. Some forms of botfly also reside in the digestive tract when consumed by a licking action.
  • Myiasis can be caused by larvae burrowing into the skin (or tissue lining) of the host animal.
  • Mature larvae drop from the host and complete the pupal stage in soil.
  • They do not kill the host animal, and thus are true parasites (though some species of rodent-infesting botflies do consume the host's testes).
  • The bot fly presents annual difficulties to equestrian caretakers, as it lays eggs on the insides of horse’s front legs, on the cannon bone and knees, and sometimes on the throat or nose, depending on what type of bot fly does the laying. These eggs, which look like small, yellow drops of paint, must be carefully removed during the laying season (late summer and early fall) to prevent infestation in the horse. When a horse rubs its nose on its legs, the eggs are transferred to the mouth, and from there to the intestines, where the larva grows and migrates to the skin. When ready to emerge, a thumbnail-sized lump will appear on the horse, which is not painful, but if the lump happens to be where the saddle or bridle go, the horse will be out of commission until the wound made by the young bot fly’s emergence heals. Additionally, migrating larva may cause mouth sores, ulcers in the stomach, and blockage of the pyloric valve which could lead to colic. Removal of the eggs (which adhere to the host’s hair) is tricky, since the bone and tendons are directly under the skin on the cannon bones: eggs must be removed with a sharp knife (often a razor blade) or rough sand paper, and caught before they reach the ground. During this process the human can also become infected. Bots can be controlled with several types of dewormers, including dichlorvos, ivermectin, and trichlorfon.


  • Pape, T. (2001). Phylogeny of Oestridae (Insecta: Diptera). Systematic Entomology 26, 133–171. doi: 10.1046/
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Botfly". A list of authors is available in Wikipedia.
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