The Difference Between a Horse Fly & a Deer Fly

Appearance Differences

• The horse fly is the larger of the two flies, ranging from ¾ to 1 inch in length. Its body is brown or black, with a pattern of stripes or triangles on its abdomen, and its wings are either clear or completely dark. Its antennae are shorter than its head, with a thick base. In contrast, the deer fly ranges from ¼ to ½ inch in length. Body color may be yellow or black with stripes or shapes on the abdomen. The deer fly's wings are usually marked with dark patterns or bands, and the antennae are longer than the head, slender and uniform in size.

Feeding Behaviors

• Humans are most likely to be concerned with the females of both flies, as they are the ones that bite people and animals, seeking a meal of blood to produce their eggs. However, the two types of flies exhibit marked differences in their feeding behavior. Most horse fly species avoid humans, preferring to bite standing animals on the legs and body, while deer flies include humans among their prey. In contrast to horse flies, deer flies generally attack moving animals and bite on the shoulders and head.

Larval Environments

• Research shows that deer fly larvae seem to require a high amount of moisture in their environment, so they naturally prefer swamps, streams and ponds. During the larval stages, deer flies feed on organic matter. On the other hand, horse fly larvae survive in a larger variety of habitats, including those with a drier environment. These larvae feed primarily on insect larvae, crustaceans and earthworms; some species are known to be active predators, hunting and killing for their food.

Effects on Humans

• One area in which there is little difference between these two flies is their effect on humans, aside from the fact that deer flies are somewhat more likely to bite people than are horse flies. For most people, either fly is nothing more than a painful nuisance, although people rarely become ill or develop an allergic reaction after a bite. However, these flies do feed on the blood of their prey and therefore have the potential to transmit blood-borne diseases from one host to another.