Encountering a large fly often sparks curiosity about its identity. These insects, noticeably larger than common house flies, frequently draw attention due to their prominent size. This guide provides identification features for commonly observed large fly species.
Defining Large Flies
The term “large flies” is a descriptive classification rather than a scientific one, generally referring to flies noticeably bigger than the common house fly. While a house fly typically measures around 4 to 7.5 millimeters, flies considered “large” often start from about 6 millimeters and extend up to 25 millimeters or more. This perceived largeness can stem from various physical attributes, including extended body length, a robust build, or an impressive wingspan.
Beyond sheer size, characteristics like a hairy body or prominent eyes also contribute to a fly’s large appearance. These features help distinguish them from smaller species, making their overall presence more noticeable.
Common Types of Large Flies
Horse Flies and Deer Flies
Horse flies and deer flies belong to the family Tabanidae and are known for their robust bodies and often large, brightly colored eyes. Horse flies can range from about 12.7 to 31.75 millimeters in length, typically appearing black or gray with large, often brilliant green eyes. Their wings are usually clear or cloudy, distinguishing them from deer flies. Female horse flies have blade-like mouthparts, used to cut skin and feed on blood, which is necessary for egg production.
Deer flies are smaller, generally 7 to 10 millimeters long, with gray or light brown bodies that may have yellow and black stripes. Their clear wings are marked with dark bands or patches. Female deer flies are blood feeders and possess large, often brightly colored eyes. Both horse and deer flies are strong fliers, attracted to movement, carbon dioxide, and dark, large objects.
Crane Flies
Often mistaken for giant mosquitoes, crane flies (family Tipulidae) possess a slender body and notably long, delicate legs that are easily detached. Their size can vary considerably, from a few millimeters up to 60 millimeters in North America, and some tropical species can exceed 10 centimeters. Crane flies typically have a V-shaped groove on the back of their thorax, a distinguishing feature.
Adult crane flies do not bite; many species do not feed at all, or they consume nectar or water. Their flight pattern is often wobbly or erratic, and their wings can appear smoky or clear. Females have a pointed abdomen for depositing eggs, while males have blunt abdomens.
Blow Flies
Blow flies (family Calliphoridae) are identified by their metallic sheen, often blue, green, gold, or black. They are typically larger than common house flies, measuring 6 to 14 millimeters in length, and possess large compound eyes and short antennae.
Blow flies are frequently seen near decaying organic matter like carrion or garbage, where females lay eggs. Their larvae, known as maggots, develop within these materials. They are strong fliers, traveling considerable distances for food and egg-laying sites.
Robber Flies
Robber flies (family Asilidae) are predatory insects known for aggressive hunting. They vary widely in size, from 3 to over 50 millimeters, and their appearance can range from slender with a tapering abdomen to stout and hairy, sometimes mimicking bees. A distinguishing characteristic is a depression or “divot” on the top of their head, located between their prominent compound eyes.
They have hairy faces, often with a distinctive “mustache” of bristles around their mouthparts, and long, strong legs adapted for capturing prey mid-flight. Their coloration typically ranges from brown, gray, or black. Robber flies ambush prey, injecting enzymes to liquefy internal contents before consumption.