How Big Are Horse Flies? Uncovering the Buzzing Truth

Horse flies are a common nuisance to humans and animals alike, known for their painful bites and persistent behavior. These insects can be found in various parts of the world, often near bodies of water where they lay their eggs. Their size can vary, but typically, they are considered moderate to large flies.

Adult horse flies, in general, measure between 14 to 19 mm long, with clear wings and a grayish-brown thorax. On the other hand, smaller relatives called deer flies measure between 10 to 13 mm in length, and have wings that are tinted smokey gray-brown or with dark patterns, along with a greenish-yellow thorax that features dark stripes ¹. These size differences, as well as the color variations, can help in identifying and distinguishing between horse flies and deer flies.

By understanding the size and appearance of horse flies, people can take appropriate measures to protect themselves and their animals from these bothersome pests. Whether it’s through the use of repellents, traps, or other methods, managing their presence can lead to a better outdoor experience for all.

Horse Flies Identification

Size and Appearance

Horse flies are considered moderate to large flies, measuring between 14 to 19 mm in length. They have a stout body shape and are usually medium to large in size, making them easily distinguishable from other fly species.

Some key characteristics of horse flies include:

• Moderate to large size (14-19 mm)
• Stout body shape
• Clear wings
• Grayish-brown thorax

Color and Wings

Horse flies’ coloration is mostly drab browns, grays, and blacks. Their wings are generally clear, which contrasts with their dark-colored bodies. In some cases, they may have wings that are solidly colored or display dark patterns.

Eyes and Antennae

One of the most distinguishing features of horse flies is their large, brightly colored eyes, often displaying horizontal stripes. Additionally, as part of the Tabanidae family, their antennae are relatively short compared to other flies.

Male vs Female Horse Flies

It’s important to differentiate between male and female horse flies, as only female horse flies are known for biting. Female horse flies have a proboscis used for piercing skin and feeding off blood, whereas male horse flies have mouthparts suitable for feeding on nectar from flowers.

Here’s a comparison table of male and female horse flies:

When identifying horse flies, it’s helpful to consider these attributes alongside factors, such as their size, color, and environment, to correctly determine the fly’s specific identity, while also acknowledging regional variations of fly species.

Horse Flies Behavior

Biting Habits and Pain

Horse flies are notorious for their painful bites, which are caused by their unique mouthparts. The female horse fly’s mouthparts are designed to cut through skin and draw blood from animals and humans alike. Some common victims of horse fly bites include:

• Cattle
• Horses
• Humans

Not only are the bites painful, but they can also cause swelling and allergic reactions in some people. Males, on the other hand, do not bite as they feed on nectar.

Comparison of Horse Fly Bites

Blood, Nectar, and Pollen Diet

While female horse flies primarily feed on the blood of their victims, the male horse flies consume nectar from flowers. This difference in diet is due to the different roles each gender plays in the horse fly lifecycle:

• Male: Pollinator
• Female: Requires blood for egg development

Life Cycle and Breeding Grounds

The typical horse fly life cycle consists of four stages:

1. Egg
2. Larva
3. Pupa
4. Adult

Female horse flies lay their eggs on plants or other surfaces near suitable larval habitat, such as:

• Wetlands
• Ponds
• Marshy areas

Horse flies have relatively brief lifetimes, with the entire lifecycle typically completed in one summer season.

Habitats and Seasonal Activity

Horse flies are most active during the summer months and can be found in various outdoor habitats, including:

• Wetlands
• Forests
• Meadows

They are often attracted to livestock and other animals in barns or pastures, making them a significant pest for farmers and property owners. To reduce horse fly activity, it is recommended to:

• Provide shelters or canopies for animals (Penn State Extension)
• Avoid areas with high horse fly populations
• Turn livestock out at night rather than during the day, as horse flies are primarily active during daylight hours

Prevention and Control

Protecting Livestock and Pets

Protecting your livestock and pets from horse flies is important to prevent painful bites and allergic reactions. One way to protect animals is by providing them shelter in barns, which have screens on windows and doors to prevent the horse flies from entering. Examples of other measures include:

• Using fly-resistant mesh blankets for horses
• Applying pet-friendly repellents on dogs and rabbits

Ways to Repel Horse Flies

Several methods can be used to repel horse flies. Here are a few options:

• Essential oils: Some people use essential oils, such as eucalyptus and citronella, to repel horse flies.
• Plants: Growing plants around your home that have strong scents can help deter horse flies, for example, basil, lavender, and lemongrass.
• Carbon dioxide traps: Horse flies are attracted to carbon dioxide, so using traps that emit CO₂ can lure them away from humans and animals.

Fly Traps and Insecticides

Another way to control horse flies is with fly traps and insecticides. A popular type of trap is the sticky trap, which captures horse flies on a sticky surface. Insecticides like pyrethrin can be sprayed around the property to help reduce horse fly populations.

Fly Traps and Insecticides Comparison:

Keep Your Property Horse Fly-Free

Managing your property can play a significant role in preventing horse fly infestations. Here are some steps to follow:

• Eliminate damp areas around your yard, as horse flies breed in moist, organic debris.
• Keep swimming pools clean and properly chlorinated.
• Regularly maintain landscape, including trimming tall grasses and bushes.

In summary, controlling horse flies involves a combination of protecting animals, using repellents and traps, and maintaining your property. Proper management can help keep your home and surroundings free from horse flies and ensure the safety of your livestock, pets, and yourself.

Horse Flies in Literature and Science

In literature, horse flies have been mentioned by ancient Greek playwright Aeschylus in his works. They are often portrayed as persistent and annoying creatures.

In science, horse flies are known for their multicolored appearance and their roles as disease vectors. They transmit pathogens through their bites, causing potential health risks to humans and animals.

Horse flies can impact livestock, especially affecting milk output. This is due to the stress caused by their bites, which results in reduced milk production in cattle.

Their preferred breeding grounds are typically areas with damp soil and water. This helps ensure their reproductive success and population growth.

Features of Horse flies:

• Multicolored
• Disease vectors
• Impact on livestock
• Preference for damp breeding grounds

Some characteristics of horse flies that allow them to thrive as disease vectors include their resilience and strong flight capacity. This enables them to cover large distances, ensuring their continued existence and spread of pathogens.

Comparison table:

By examining the role of horse flies in literature and science, we’ve seen their enduring presence and impact throughout history. Their influences on both human and animal health, as well as their distinct traits, have made them a subject of interest across disciplines.

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Horse Fly: Male and Female Eye Comparison

Male and Female Horse Flies Feeding on Parsley
Location: Clarksburg, MA
August 18, 2011 10:04 am
Hello, saw your recent horse fly post and thought this might be a nice follow-up. I’m not sure of the species, but the images show a male and a female horse fly feeding on the flowers of parsley. The side view image is of a female. I can say both males and females have been coming around to my parsley the past few years, and pretty much have ignored me and my dog with the exception of one female, who I must have annoyed, after a particularly long photo session, because at the very end she bit my hand. (Picture of my resulting Stay Puft Marshmallow hand not included.)
Signature: Michael Marlow

Hi Michael,
Thank you so much for sending these awesome images for a wonderfully informative posting.  Most of the Horse Fly images we receive are the genus Tabanus, however, your flies appear to be Hybomitra cincta, which we identified on BugGuide.  Of the genus, BugGuide indicates:  “The 55 species are mostly northern, Canada and Alaska; being replaced in the remaining USA by Tabanus.”

Your close up photos beautifully illustrate how the eyes of the female, the blood sucker, have a space between them, while the eyes of the male, who feeds exclusively on nectar, have no spacing between them.

Letter 2 – Male Dark Giant Horse Fly from the UK

Subject: Large wasp like fly
Location: West Cumbria , UK
June 30, 2016 6:25 am
A large flying insect landed on our balcony and I would like to know what it is. We live in Cumbria, UK and it’s the first time I’ve ever seen one of these before. I attach a couple of photos and it measures about an inch long. Can anyone identify this bug please?
Signature: P & K

Dear P & K,
This is a Horse Fly in the family Tabanidae and the large eyes with no space between them indicates this is a male Horse Fly.  It looks very much like this female Horse Fly from our archives, and we suspect they are the same species.  We believe we have correctly identified your species as the Dark Giant Horse Fly, Tabanus sudeticus, thanks to images on Influential Points where it states:  “The dark giant horsefly is distributed widely in northern Europe into Russia. In Britain it mainly lives in boggy areas in the north and west, although it is also quite common in the New Forest.”  A species page on Influential Points states:  “The dark giant horsefly flies in July and August and commonly feeds on the blood of cattle and ponies. In Europe … Tabanus sudeticus flies from the end of June and through July and August. Krčmar (2005)  reports that it reaches its maximum abundance in third week of July. In Britain it mainly lives in boggy areas in the north and west, although it is also quite common in the New Forest. Tabanus sudeticus is distributed widely in northern Europe into Russia.”  Additionally, the site states:  “Tabanus sudeticus is anautogenous – it must first take a blood meal before it can lay eggs (Krčmar & Maríc, 2007 ). The dark giant horsefly undoubtedly prefers feeding on horses, cattle and deer, but it will bite man if available, as many have found to their cost. It makes a deep hum when flying around a host, but this stops abruptly just before it settles.”  If it is any consolation, only female Horse Flies are blood sucking biters.  Male Horse Flies take nectar.  BioPix has some excellent images of the Dark Giant Horse Fly.

Letter 3 – Horse Fly Larva

Giant Horsefly maggot?
April 2, 2010
Hi,
Several days ago I was taking pictures of the tadpoles and water striders at a local pond when I noticed this strange creature. I think it might be a horsefly maggot, but I don’t really know.  This  guy was about 2 inches long, with a tapered “head” and “tail.” It was translucent white with brown bands. In one of the pictures you can see the flicking motion it used to swim. Any help you can provide in identifying this strange creature would be appreciated.
Josh Kouri
Oklahoma

Hi Josh,
Thanks for sending your excellent photos of a Horse Fly Larva.  There is a report on BugGuide of a person being bitten by a Horse Fly Larva, so handle with caution.

Hi,
Thanks for confirming the I.D. I’ve found these maggots several times in the past. The first time I found one my brother picked it up and it bit him! I don’t think he had any itching or swelling like the person on BugGuide, though.
Josh

Letter 4 – Horse Fly Larva from Canada

pond larva
Location: Southern Ontario, Canada
March 31, 2011 7:30 pm
We have found a larva from our pond. It was in the outflow of the pond. The pond is just below freezing. The larca is a little more than 1 inch in length. It is segmented, white with black banding at the segments. It comes to the surface for air. Hopefully the attached pictures will help identify it. The penny is for size comparrison.
Signature: John

Hi John,
This is the larva of a Horse Fly.  These aquatic larvae are predatory and they feed upon small pond creatures.

Letter 5 – Horse Fly from Turkey

Subject:  green eyed wasp imposter
Geographic location of the bug:  Kiyikoy, Turkey
Date: 06/23/2019
Time: 04:41 AM EDT
Your letter to the bugman:  Can anyone tell me what this is and a bit more about it? I’ve been told it’s not a wasp and doesn’t sting but I don’t trust yellow and black flying things. It’s larger than a normal UK wasp.
How you want your letter signed:  Domino

Dear Domino,
This is a Horse Fly in the family Tabanidae, and females bite larger animals like horses, cattle and other livestock, sucking blood as they feed.  They are opportunistic and will bite humans if there is no other available prey.  Based on Diptera Info, it might be Tabanus promesogaeus and the person who submitted the images claims:  “Common and very ready to bite painfully as I know to my cost…”

Letter 6 – Korean Horse Fly

HUGE fly in Korea
I was hiking on Hyoja Island in the southern part of South Korea when I ran across this giganto-sized specimen (maybe an inch or so long) What is it? I’ve never seen anything quite like it. It’s got some great eyes though, eh? Thanks,
Kian

Hi Kian,
All we can tell you is that it is a Horse Fly in the family Tabanidae and that she is a female. The females are the biters.

Letter 7 – Horse Fly Maggot

Mysterious large water creature
Please look at these 4 images and determine what this alien life form may be. It came from our pond. It’s about 4cm long. It’s translucent. It lived for 15-20 minutes completely submerged in 99% isopropyl alcohol. Found in Travis County in central Texas near the town of Manor. Thanks,
Jim

Hi Jim,
This amazing creature is a Horse Fly Maggot. There is an image on BugGuide of the Western Horse Fly and it looks very similar to your image, though your image might be of a different species in the same genus, Tabanus. There is another image on BugGuide from Texas that is just listed by the genus name. The larvae are carnivorous, and are reported to bite.

Letter 8 – Horse Fly Larva

pond larvae
Location:  Noblesville, Indiana, USA
September 13, 2010 11:30 pm
Found this in our backyard Koi pond. It’s an all-natural pond, with a bog pond for a filter. This was hanging around the water lettuce. It seems to swim around pretty well. and it’s about 2 inches long.
Signature:  Koi pond amatuer

Dear Koi pond amateur,
This is the larva of a Horse Fly.  It is our understanding that they might bite.  You can compare your individual to this image on BugGuide.

Letter 9 – Horse Fly Larva

Found this in the Water…….
Location: Great Bend Zoo, Great Bend Kansas
February 2, 2011 5:06 pm
While tending to a busted waterline at the Great Bend Zoo, we found 2 of these swimming around in the freezing water. Temperature was about -2 with a windchill factor of -10 at the time we found them.
Signature: Johnny Z.

Hi Johnny,
You found a Horse Fly Larva.  You can compare your photo to the one on the Horse Flies and Deer Flies of Kentucky website.  Female Horse Flies which are also known as Gadflies feed upon blood and lay their eggs on plants near the water’s edge.  Most Horse Flies have aquatic larvae, but some Horse Fly larvae develop in damp earth.

Daniel Marlos,
Thank you so very much for such a fast response.  We weren’t quite for sure what it was, but we had guessed a larva of some sort.
Again, thank you for the fast response 🙂
Johnny Z.
-Zoo Keeper/Safety Officer, Great Bend-Brit Spaugh Zoo

Letter 10 – Horse Fly Larva

Subject: Weird Bug From Pond
Location: Lake View, AL, 35111
September 9, 2014 11:00 am
Hey Bugman,
I have some strange bug crawling around my pond and sidewalks. They seem to move like worms.
They’re greyish brown and I haven’t been brave enough to pick one up and check it out.
I live in Lake View, AL
Thanks,
Jared
Signature: McCalla Bugster

Dear McCalla Bugster,
This looks to us like the immature stage of a fly, possibly a Horse Fly.

Letter 11 – Possibly Wasp Pupae

Subject: some sort of pupae? larvae?
Location: Missouri, United States
April 1, 2015 11:34 pm
found these while digging out at our pond. I suspect they were underground and I dug them up. I picked them out of the water and returned back to the house to investigate them.
P.S. if you’re wondering why I’m digging the pond, it dried up and now has a hard time staying full of water. if I don’t dig it, the tadpoles can’t mature! 🙁
Signature: Michael

Dear Michael,
We are guessing that these might be the Puparia of Horse Flies.  Many Horse Flies have aquatic larvae.  According to BugGuide:  “larvae found in streams, ponds, and marshes, especially along the edges and in the sea shore.”  Flies have an interesting metamorphosis process, and the pupa is encased in a puparium.  Though the information is about Blow Flies, the USA.Gov Visible Proofs Forensic Views of the Body page states:  “The larvae becomes shorter and stouter and the outer cuticle (skin layer) of the larvae hardens into the puparium and slowly darkens over a period of about 10 hours.”  We have several images of Horse Fly larvae on our site, and we cannot locate matching images to substantiate our guess at this time, but we are going to seek a second opinion.

Eric Eaton Responds
Hi, Daniel:
Don’t think they are horse fly puparia.  Almost remind me more of wasp larvae inside….How long has the pond been dry?
Eric

Thanks Eric,
Good question. I will write back and ask.  You think a wasp that digs a hole as a nest?
Daniel

There are hundreds of species of solitary wasps that dig burrows for nests.  If the puparia were large, I’m thinking cicada killer or horse guard.
Eric
author, Kaufman Field Guide to Insects of North America
http://bugeric.blogspot.com/

Michael provides additional information
it’s certainly possible they could be wasps, the pond has been dry since late last year and only recently started filling up from the rains this year. I wonder what sort of wasp though, very interesting.

unfortunately I cannot, I already threw them away. but I’d say the biggest was about the length of a nickel if that helps at all.

Letter 12 – Horse Fly Larva, we believe

Subject:  Beauty and a beast
Geographic location of the bug:  Nova Scotia, Canada
Date: 07/19/2018
Time: 05:41 AM EDT
Your letter to the bugman:  Hi Bugman!
I was recently working on stream habitat assessments …  On another day, we were looking at rocks for freshwater benthic macroinvertaebrates and I found this worm-like creature that was not on our super-simplified ID guide. It was translucent and you could see everything shifting around when it moved. As I was trying to take photos and video of it moving/wriggling, it bit me (or stung/poked me), drawing blood and I dropped it. Luckily (or unluckily depending on how you look at it, I suppose), we came across another one later. As I watched it move this time, I believe what I might have gotten stuck with its back end grippers which it seems to use to grip onto the rock face. I was looking at some other aquatic larval stages for different insects and cam across an image of crane fly larvae that looks similar, but again, I’m not really sure and was hoping you might have a better idea.
Here’s hoping!
How you want your letter signed:  Many thanks, Van

Hi Van,
We believe your guess that this is a Crane Fly larva is incorrect, but we do believe you have the insect order correct.  We believe this is an aquatic Horse Fly larva and according to the Missouri Department of Conservation:  “The larvae of horse and deer flies are fairly straight, segmented, wormlike maggots that are tan, whitish, or brownish. Several fleshy rings circle the body. They are robust, circular in cross-section, and taper at both ends. There are no true legs, although fleshy, nobby pseudopods or prolegs are present. In relaxed specimens, a thin, pointed breathing tube extends from the hind end to protrude above the water surface.”  BugGuide has an account of a person being bitten by a Horse Fly larva.

Letter 13 – Horse Fly: Tabanus marginalus

Subject:  Large fly like bug
Geographic location of the bug:  Ontario, Canada
Date: 08/08/2018
Time: 10:08 PM EDT
Your letter to the bugman:  Hello! What’s this bug? It was on the wall outside my store …It’s horrifying and I must know if it is a horsefly or cicada or another insect I haven’t thought of..
How you want your letter signed:  Sparkledemon

Dear Sparkledemon,
This is definitely a Horse Fly and it looks to us like Tabanus marginalus pictured on BugGuide, and it is very curious that though BugGuide states: “The most common biting horse fly throughout the world” there are only three images posted there and all are from a limited area in the Northeastern portion of North America, including Ontario.

Letter 14 – Horse Fly Larva found frozen in pond

Subject:  Frozen Like Han Solo
Geographic location of the bug:  a pond in northern IL
Date: 12/10/2018
Time: 11:07 PM EDT
Your letter to the bugman:  Hi.  We were hiking in the woods and saw a few of these trapped in the ice of two different shallow ponds near our home.  Each specimen was about 2 to 2.5 inches long.  I thought it must be a larvae of a pond insect, but I haven’t been able to find any that are supposed to be that big.  Any ideas?
How you want your letter signed:  Mary

Dear Mary,
This looks to us like the larva of a Horse Fly.  There is a matching image on Quora where it states:  “Most horse flies are associated with water, and the carnivorous larvae can be found therein.”

Letter 15 – Horse Fly Larva

Subject:  Weird worm like creature found in water
Geographic location of the bug:  Petersburg, Tennessee
Date: 04/29/2019
Time: 09:20 PM EDT
Your letter to the bugman:  I was going outside catching tadpoles to grow and I can across this worm like thing. I scooped him up and put him in with the tadpoles. Maybe he wasn’t originally in the water and he fell in? But I didn’t want to take the chance. I’ve looked up tons of worm like creatures and even asked my parents to no avail. It would be appreciated greatly if you could help figure this mystery out. Thanks in advance.
How you want your letter signed:  Sierra

Dear Sierra,
We believe this is a Horse Fly larva.  According to Quora:  “Most horse flies are associated with water, and the carnivorous larvae can be found therein. I have collected black horse fly larvae while searching through the muck and mud at pond edges. [T]Here’s a Colorado State University photo by Jennifer Bonnell of what is probably a black horse fly larva eating a small frog; they’ll also eat other insects, and, while I’ve never seen it, I’m sure they’ll eat any weakened or trapped minnows they might be able to.  Through the summer, the larvae grow in the water through 6–9 instars, and ultimately spend the winter in the the mud in their last instar. In spring, still in the muck and mire, they pupate and a few weeks later, the adults emerge.”  You might not want to keep this predatory Horse Fly larva with your tadpoles.

Letter 16 – Horse Fly Larva from UK

Subject:  What is this?
Geographic location of the bug:  Oxfordshire, UK
Date: 09/04/2021
Time: 06:11 PM EDT
Your letter to the bugman:  My friend’s son found this wee fella today (4th September 2021) near a boggy marsh in rural Oxfordshire, UK. We have no idea what it might be and wondered if you could help?
How you want your letter signed:  Sonja

Dear Sonja,
This sure looks like a Horse Fly larva to us, and we have received numerous reports this summer of Dark Giant Horse Flies from the UK here, and here, so it makes sense that this is the larva of a Dark Giant Horse Fly.

Letter 17 – Horse Fly Larva

Subject:  Interesting little guy
Geographic location of the bug:  Ashland, Virginia, USA
Date: 09/10/2021
Time: 05:02 PM EDT
Your letter to the bugman:  Found this guy in a shallow puddle. It was interesting how it flung it’s back end to “swim”.
How you want your letter signed:  Sgt_M

Dear Sgt_M,
This is a Horse Fly Larva.  Many species of Horse Flies have aquatic larvae.

Letter 18 – Large Female Horse Fly

Subject: Large Female Horse Fly
Geographic location of the bug:  Campbell, Ohio
Date: 07/20/2022
Time: 9:15 PM EDT
Gentle Readers,
Daniel was walking out the front door when he heard a loud buzzing (sounded like a smoke detector) coming from the inside of the house.  He was surprised to see the largest Horse Fly he has ever seen, about an inch long.  He trapped it in a stemmed glass and took some photos before releasing her outside. 

He believes based on the Buckeye Yard & Garden Online website and BugGuide  that it is Tabanus abdominalis, a species with no common name.  According to Joe Boggs on the former site hosted by Ohio State University extension:  “All horse flies are aggressive and vicious biters, but the bigger ones are particularly menacing.  Only the females bite; they require blood meals to be able to produce eggs.  When she finds a host, the female uses her sharp, knife-like mouthparts to slash upon a wound in the skin; the mandibles of large horse flies are powerful enough to cut through tanned leather!  After opening a wound, the female injects saliva that has anticoagulation properties and she then laps up the free flowing blood.  The bite is extremely painful, and blood continues to flow from the wound even after the female finishes feeding.”

There are several similar looking species and we would not rule out that this might be a wider ranging Tabanus sulcifrons also pictured on BugGuide.