Deer flies are small, stout flies measuring 10 to 13 millimeters long, roughly the size of a housefly. They’re easiest to recognize by their wings, which have distinctive dark bands or smoky gray-brown patterns, and their striking metallic eyes that shimmer with green and gold.
Body Size and Shape
Adult deer flies fall in the 7 to 13 millimeter range depending on the species, putting them on the smaller side of biting flies. Their bodies are compact and slightly flattened, with a greenish-yellow thorax (the midsection behind the head) marked by dark stripes. Some species, like the common North American deer fly Chrysops excitans, have a mostly black body with yellow or orange triangles along the back. Overall, they look stockier and more triangular than a typical housefly.
Wings With Dark Bands
The fastest way to tell a deer fly apart from other flies is by looking at the wings. Deer fly wings have bold dark bands or patches across them, visible even while the fly is resting. These markings range from solid black bars in the middle of the wing to a smoky gray-brown tint that covers most of the wing surface. This is the single most reliable identification feature and the clearest difference between deer flies and their larger relatives, horse flies, which typically have clear or solidly colored wings.
Iridescent Eyes
Deer flies have large compound eyes that are hard to miss. The eyes display vivid metallic colors, often green and gold, created by layered structures in the outer lenses that reflect light. In some species like Chrysops relictus, these reflective layers actually filter certain wavelengths of light, giving the eyes a brilliant gold-green shimmer. You can also use the eyes to tell males from females: males have eyes that touch together at the top of the head, while females have a visible gap between them.
Prominent Mouthparts
If you get close enough to notice, deer flies have large mouthparts that project downward and forward from the head. Females have two pairs of blade-like cutting structures that work like tiny scissors to slice open skin and start blood flowing. A sponge-like pad then soaks up the blood. Males have similar but much weaker mouthparts and cannot bite. Only females feed on blood, which they need to produce eggs.
How They Differ From Horse Flies
Deer flies and horse flies belong to the same family, so confusion between them is common. The differences are straightforward once you know what to look for:
- Size: Deer flies measure 10 to 13 mm. Horse flies are noticeably larger at 14 to 19 mm, and some species reach over 25 mm.
- Wings: Deer flies have dark banded or patterned wings. Horse flies generally have clear wings.
- Body color: Deer flies tend toward greenish-yellow with dark stripes. Horse flies are typically grayish-brown.
- Where they bite: Deer flies target the head and neck. Horse flies go for the legs.
Both have colorful iridescent eyes, so eye color alone won’t help you distinguish them.
What Their Bites Look Like
Since many people searching for deer fly identification have just been bitten, here’s what to expect. A deer fly bite produces immediate, sharp pain that some compare to a bee sting. The bite site typically develops into a red, raised welt that itches and may swell. Because the fly cuts the skin open rather than piercing it with a needle-like probe, the wound can bleed more than you’d expect from such a small insect. Some people develop larger allergic reactions with significant swelling. In most cases, the stinging, swelling, and itching resolve within a few days.
Where and When You’ll See Them
Deer flies are most active from May through September, with peak activity in June and July. You’re most likely to encounter them near swamps, marshes, ponds, and stream banks, particularly at the edges of wooded areas. They patrol the borders of broken woodlots looking for hosts, and they rest on tall grass or leaves between feeding attempts.
They fly during the day and are attracted to dark moving objects and carbon dioxide from your breath, which is why they tend to swarm persistently around your head. They’re strong fliers capable of covering long distances from their breeding sites, so you don’t need to be standing in a swamp to encounter them.