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Today on Animal Fact Files we’re discussing deer flies. Deer flies are classified in the same family as horse flies. Both types of insects have bright, colorful, complex eyes but deer flies are smaller and often sport banded wings, or sometimes even banded eyes. The name “deer fly” could be in reference to their host choice as these flies are seen around deer, but their hosts also include ungulates such as livestock animals, humans, reptiles such as turtles, and birds such as ducks, so don’t let the name confuse you! Similar to mosquitoes, female deer flies require blood to nourish their eggs. The male’s don’t bite. Deer fly females most frequently attack the head and shoulder area of their hosts, though they can bite anywhere. They’re often attracted to dark spots such as the patches on a cow. These insects find their hosts through color, movement, odor, carbon dioxide emission, and body heat, so simply standing still or holding your breath isn’t going to drive away a determined deer fly. Fascinatingly, deer flies are often attracted to large, colorful objects, such as cars, so it’s not uncommon to see them congregating around vehicles, especially those parked near water! What’s more, not even smoke can keep them away as some species are actually attracted to wood smoke!

When a deer fly bites, it makes an incision in the host’s skin and spits. Deer fly saliva acts as an anticoagulant which means the blood pools at the site of the incision and flows freely. This makes the blood easy for the fly to drink! In many cases, the bite itself isn’t what causes itchiness and inflammation but the fly’s saliva. Like mosquitoes, another type of biting fly, the saliva can cause an allergic reaction which brings about the itchiness! Of course, while mosquito bites tend to go more unnoticed at first, deer fly bites can be painful! Some people describe the bite as similar to a bee sting. What’s more, deer fly females can also transmit parasites with their feeding behavior. One example is the transmission of eye worms which is a parasitic infection that can occur in humans. It's treatable but still freaky!

Adult deer flies are much smaller than adult horse flies in much the same way most deer are smaller than most horses. Deer fly adults are shorter than an inch (2.5 cm) long, but their larvae can nearly triple this in length! These insects undergo complete metamorphosis meaning they begin their life as an egg that hatches into a larva that pupates to form the adult. Deer fly eggs are often laid in moist soil or leaf litter. There can be dozens to hundreds, sometimes even thousands of eggs all laid at the same time. The larvae hatch after a few days to nearly two weeks later depending on the species and environmental conditions. Deer fly larvae are the longest lived life stage for these animals. They spend their time hidden in soil or detritus, rotting plant roots or sometimes even aquatic areas for upwards of three years before they pupate. Deer fly larvae eat detritus and other animals including other deer fly larvae! They're cylindrical with tapering ends and often appear white, green, brown, or cream with obvious rings.

The deer fly pupal stage lasts about one to three weeks while adults only live about a month before perishing. They eat, mate, reproduce and die. Males wait in vegetation for females to pass by, or form mating swarms that attract females to them. The males don't need blood, so they only eat nectar, pollen, and tree sap. They're rarely encountered by humans for this reason. While a female deer fly will find you, you have to actively search for a male deer fly in order to see one! The females need blood to help their eggs grow and may return to the same host multiple times if they're able to successfully feed from it, but female deer flies also eat plant-based foods similar to the males.

Although these insects can fly for miles to disperse, once they've settled on a spot, they typically remain there for the rest of their adult life. Deer flies are most closely associated with water and wet habitats including swamps, streams, and even coastlines. There are more than 100 classified deer fly species living on most tropical and temperate continents and islands, though they're absent from some places such as Hawaii. Deer fly predators include birds, spiders, dragonflies, wasps, robber flies, mice, wading birds, frogs, toads, and many more. Their larvae are also eaten by larvae and nymphs of other invertebrate species. Although some deer fly larvae can live for years in the soil, some species undergo multiple generations per year in which the flies survive only a few months before perishing. 

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