Gasterophilus intestinalis

Geographic Range

Gasterophilus intestinalis ranges throughout the Old and New World, but is most common in Western Europe and in the Southern and South Western United States. (Agneessens, et al., 1998; Roberts and Janovy, Jr, 2000)


Adult flies are found in open areas near equines. First larval instars are found in the tongue of the host. Second and third larval instars are found in the non-glandular region of the stomach. (Cogley and Cogley, 1999)

Physical Description

Adults range in size from 8 mm to 18 mm. Females are larger than males, and have a long pseudoovipositor. Both males and females have a constriction between the 1st and 2nd segments of their abdomens, separating it into a propodeum and a gaster. This is commonly referred to a "wasp-waist".

Larval instars are pill shaped. The majority of larval body segments have rings of barbs pointed toward the posterior. The last few segments do not have these spines. (Cogley and Cogley, 1999; Roberts and Janovy, Jr, 2000)

  • Sexual Dimorphism
  • female larger
  • Range length
    8 to 18 mm
    0.31 to 0.71 in


Females attach eggs to the hairs of horses, mainly on the knees and lower parts of the front legs. The horse cleans itself by licking these areas. Enzymes in the horse's saliva and heat from its tongue will cause the eggs to hatch and the first larval instar burrow into the tongue's surface. Larvae then burrow their way to the stomach of the horse. Using mouthhooks, the larvae feed from the blood of the horse. Both the second and third larval instars remain in the horse's stomach. In the Northern hemisphere, third instar larvae can be found year round while second instars are not present for the months of April-July. Come spring, the larvae will pass out with the feces of the horse and pupate in the manure or loose soil. Pupation takes between three and five weeks. (Cogley and Cogley, 2000; Lyons, et al., 2000; Roberts and Janovy, Jr, 2000)


Mating occurs shortly after the female emerges from her pupa. Mating commonly occurs in the same region as pupation becuase of the high density of emerging adults in the same area. Once mating has occurred, females immediately seek out a suitable host. (Cogley and Cogley, 2000)

After copulation, females seek out a suitable host for her eggs. Females hover near the horses front legs with their body curved forming a "C" shape. This places the ovipositor in a perfect position to attach eggs. Each female will lay eggs at multiple sites on multiple horses. This will increase the reproductive potential of the species. (Cogley and Cogley, 2000)

Once egg laying is complete there is no further parental involvement.

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female


Gasterophilus intestinalis larvae are obligate internal parasites. Adults are free-living and non-parasitic. The preferred hosts of this species are horses, giving it the common name of "horse bot fly". (Cogley and Cogley, 2000; Lyons, et al., 2000; Roberts and Janovy, Jr, 2000)

Food Habits

Larvae feed on the blood of their host. Using their mouth hooks and spines, larvae attach themselves to the gut wall of the host's stomach. Once attached, an ulcer forms around the larvae. Adult males are non-feeding while adult females have been observed feeding from waste material. (Cogley and Cogley, 1999)

  • Animal Foods
  • blood
  • body fluids
  • Other Foods
  • dung


Many species of insectivorous birds feed on the adults of this species.

Ecosystem Roles

Bot flies can cause damaging parasitic infestations. if populations become large enough in a single host, it can cause significant health problems and even death for the host animal.

Species Used as Host
  • Equines (horses, donkeys, and mules)

Economic Importance for Humans: Positive

There are no obvious economic benefits to humans from horse bot flies.

Economic Importance for Humans: Negative

Gasterophilus intestinalis is extremely common. The majority of the time, its presence is insignificant enough not to harm the host. However, when found in large number G. intestinalis can cause harm to the host and eventual death.

In extremely rare cases G. intestinalis attaches eggs to a different type of host, including humans. The effects of horse bot flies in humans have not been recorded. (Cogley and Cogley, 1999; Cogley and Cogley, 2000; Cogley and Cogley, 1999; Cogley and Cogley, 2000)

Conservation Status

This species is in no danger of extinction.

Other Comments

Moxidectin equine gel can be used to reduce and potentially eliminate G. intestinalis from a host. It has been shown to have 88-100% efficiency in eliminating third instar larvae from the host body. (Reinemeyer, et al., 2000)


Sara Diamond (editor), Animal Diversity Web.

Harlen Hays (author), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor.



Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map


living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map


living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


an animal that mainly eats the dung of other animals

  1. active during the day, 2. lasting for one day.

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season


offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.


Agneessens, J., S. Engelen, P. Debever, J. Vercruysse. 1998. Gasterophilus intestinalis infections in horses in Belgium.. Veterinary Parasitology, 77: 199-204.

Cogley, T., M. Cogley. 2000. Field observations of the host-parasite relationship associated with the common horse bot fly, Gasterophilus intestinalis.. Veterinary Parasitology, 88: 93-105.

Cogley, T., M. Cogley. 1999. Inter-relationship between Gasterophilus larvae and the horse’s gastric and duodenal wall with special reference to penetration.. Veterinary Parasitology, 86: 127-142.

Lyons, E., T. Swerczek, S. Tolliver, H. Bair, J. Drudge. 2000. Prevalence of selected species of internal parasites in equids at necropsy in central Kentucky.. Veterinary Parasitology, 92: 51-62.

Reinemeyer, C., P. Scholl, F. Andrews, D. Rock. 2000. Efficiency of moxidectin equine oral gel against endoscopically-confirmed Gasterophilus nasalis and Gasterophilus intestinalis (Diptera: Oestridae) infections in horses.. Veterinary Parasitology, 88: 287-291.

Roberts, L., J. Janovy, Jr. 2000. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology, Sixth Edition. Boston: McGraw-Hill Companies.