Horse Fly vs Blow Fly
Horse Flies and blow flies are easy to mix up at a glance, but a few details separate them quickly. Here is how to tell which one you found and what, if anything, you should do about it.
Quick answer
Check for this first. A horse fly has large, heavy body with very large, often iridescent eyes, while a blow fly has metallic green, blue or bronze body.
Side-by-side comparison
| Horse Fly | Blow Fly | |
|---|---|---|
| Group | Flies & Mosquitoes | Flies & Mosquitoes |
| Scientific name | Tabanus spp. | Lucilia sericata |
| Size | 10–30 mm | 8–12 mm |
| Colors | Black, Brown and Gray | Green, Blue and Metallic |
| Legs | 6 legs | 6 legs |
| Wings | Yes | Yes |
| Bites or stings | Bites | No |
| Risk | Can bite or sting | Nuisance |
| Key feature | Large, heavy body with very large, often iridescent eyes | Metallic green, blue or bronze body |
Key differences
- Size: Horse Flies are larger, reaching 30 mm, while blow flies top out around 12 mm.
- Color: Horse Flies are black, brown and gray; blow flies are green, blue and metallic.
- Risk: the horse fly can bite or sting, usually with mild, local effects, while the blow fly is a nuisance pest but not a health threat to most people.
- Where found: look for horse flies in yards and on patios, in woods and along trails and near water; blow flies turn up in the kitchen and indoors.
- Signature feature: Horse Flies: large, heavy body with very large, often iridescent eyes. Blow Flies: metallic green, blue or bronze body.
How to identify horse flies
A large, stout fly with huge eyes that pursues people, horses and cattle for blood. Females slice skin with blade-like mouthparts, causing a sharp, painful bite.
- Large, heavy body with very large, often iridescent eyes
- Clear or dark-tinted wings held apart at rest
- Persistent circling around people near water
How to identify blow flies
A shiny, metallic green or blue fly that lays eggs on dead animals, meat and garbage. A sudden swarm indoors often signals a dead mouse or bird in a wall or attic.
- Metallic green, blue or bronze body
- Larger and louder than a house fly
- Often appears suddenly in large numbers indoors
Which one is more dangerous?
The horse fly carries more risk. Horse fly bites are painful and can swell; they rarely transmit disease to people in the US.
Horse fly bites are painful and can swell; they rarely transmit disease to people in the US. Blow flies don't bite but can carry bacteria from rotting material.
Where and when you'll see each
Horse Flies turn up in yards and on patios, in woods and along trails and near water, mostly in summer. Blow Flies turn up in the kitchen and indoors, mostly in spring, summer and fall.
Where you found the bug is itself a strong clue, since these two rarely share the same spots.
What to do if you find one
Horse Flies: Wear light-colored long sleeves near ponds and pastures. Read the full removal guide for more.
Blow Flies: Search for and remove any dead animal, then clean the area. Read the full removal guide for more.
Biology: flies & mosquitoes
Flies have only one pair of working wings; the second pair is reduced to small balancing knobs called halteres. They go through complete metamorphosis, and the larvae, called maggots, need moist organic material such as garbage, soil, drain film or standing water.
Frequently asked questions
What is the main difference between horse flies and blow flies?
Size: Horse Flies are larger, reaching 30 mm, while blow flies top out around 12 mm.
Which is bigger, a horse fly or a blow fly?
Horse Fly: 10–30 mm. Blow Fly: 8–12 mm.
Do horse flies or blow flies bite?
Horse Flies can bite. Blow Flies do not bite.
Where are they usually found?
Horse Flies: in yards and on patios, in woods and along trails and near water. Blow Flies: in the kitchen and indoors.