Horseflies are a widespread group of insects known for their persistent biting habits. Found globally, these flies can be a nuisance to both animals and humans. Identifying horseflies accurately involves recognizing their distinct physical characteristics, which helps differentiate them from other common insects.
General Appearance
Horseflies are large, robust insects with a stout body shape. Their size can vary, with many species ranging from 0.2 to 1.0 inch (5 to 25 millimeters) in length, and some can be as large as a bumblebee. Most horseflies exhibit dark coloration, appearing in shades of brown, black, or grayish hues. Some species may have a metallic sheen or striped patterns on their abdomen.
Specific Identifying Features
Their large, prominent compound eyes are a distinguishing characteristic. These eyes display iridescent colors, such as green, blue, or purple, and can have intricate patterns or bands. A key difference between sexes is that male horseflies have eyes that nearly meet in the middle of their head, while female horseflies have eyes that are noticeably separated. Horseflies possess a single pair of wings, which are clear or slightly tinted, and may feature prominent dark veins or a speckled pattern. When at rest, their wings are held in a distinct V-shape or slightly overlapping.
The mouthparts of female horseflies are short, stout, and blade-like, designed to slice skin to access blood. This contrasts with the more delicate mouthparts of male horseflies, which primarily feed on nectar and plant juices and are not capable of biting. Horseflies also have short, stout antennae, composed of three segments. Their bodies and heads are covered in short hairs, but they lack the dense bristles.
Distinguishing From Similar Insects
Horseflies are often confused with other flying insects, but several features help differentiate them. Deer flies are smaller than horseflies, measuring up to 0.4 inches (10 mm) long. Deer flies also have patterned or banded wings, whereas horsefly wings are clear or uniformly dark.
House flies are typically about 0.25 inches (6-7 mm) long, much smaller than horseflies. They are dull gray with four dark stripes on their thorax and have sponging mouthparts, unlike the stout, blade-like mouthparts of female horseflies.
Wasps and bees can be distinguished from horseflies by their distinct body segmentation, featuring a narrow ‘waist’ between the thorax and abdomen, which horseflies lack. Wasps and bees have two pairs of wings, while horseflies have only one pair. Wasps and bees possess stingers, whereas female horseflies inflict painful bites using their specialized cutting mouthparts.