Understanding Horse Fly Differences: Male vs Female

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Horse flies and deer flies are two types of insects that are commonly found in various regions. Both male and female horse flies and deer flies can create problems for humans and animals due to their painful bites. However, it is important to understand the differences between the male and female of these species in order to identify them and implement proper control measures.

Male and female horse flies and deer flies are often differentiated by their sizes and eye patterns. While female horse flies and deer flies typically have a larger body size and separated eyes, males have smaller bodies and their eyes touch at the top. In addition to these physical differences, female horse flies and deer flies, unlike males, require blood meals to reproduce. Consequently, females are the ones causing painful bites to animals and humans.

When dealing with horse flies and deer flies, it is essential to consider effective control methods, particularly for the biting female population. Some methods include using traps, repellents, and physical barriers to protect animals and humans. Being aware of the distinctions between male and female horse flies and deer flies can help in developing targeted strategies to prevent and reduce their impact on daily life and recreational activities.

Horse Fly Basics

Male vs Female

Male and female horse flies have notable differences. Males primarily feed on nectar, while females require blood meals for egg production. Female horse flies are equipped with a painful biting mouthpart called the proboscis, which they use to pierce the skin of their host animal, such as a horse or human. Let’s compare the characteristics of male and female horse flies:

Characteristic Male Female
Feeding habits Nectar Blood meals for egg production
Mouthparts No proboscis Painful biting proboscis
Role in life cycle Mating, not involved in egg-laying Blood-feeding, egg-laying

Species and Family Tabanidae

Horse flies belong to the family Tabanidae, which also includes deer flies. There are over 100 species of horse and deer flies known to occur in Texas.

Common features of horse flies in the Family Tabanidae:

  • Large size compared to other flies.
  • Distinctive eyes with colorful, iridescent patterns.

Here is a list of general horse fly characteristics:

  • Size: Can be around 5-7 mm long.
  • Appearance: Resembles house flies, with circular black spots on a gray abdomen.
  • Habitat: Larvae can develop in aquatic habitats.

Horse flies can be a nuisance and harmful to both humans and animals. The female’s painful bite can disrupt recreational activities and even affect harvesting some agricultural crops. Taking preventative measures, such as using fly repellents, can be useful in reducing horse fly populations and minimizing their impact.

Biology and Life Cycle

Larval Stage

  • Horse fly eggs are laid on plants and grass near water sources, hatching into larvae within a week.
  • Deer fly larvae live in moist soil and feed on small insects and decaying organic matter for protein.

The larval stage is critical for growth, typically lasting from 1-3 months, depending on species and environmental conditions.

Pupal Stage

  • The pupal stage is where the larvae transform into adult flies in a protective cocoon.
  • The duration of the pupal stage varies, usually between 1-3 weeks.

During this time, the maggots complete metamorphosis and emerge as fully developed adult horse and deer flies.

Adult Stage

A comparison of adult male and female horse flies:

Characteristic Male Horse Fly Female Horse Fly
Mouthparts Modified for feeding on nectar Adapted for piercing and sucking blood
Diet Primarily nectar from flowers Blood meals from hosts
Size Slightly smaller Slightly larger
Life Span Shorter Longer

In the adult stage, both male and female horse and deer flies feed on energy-rich sugars found in nectar, plant sap, or honeydew produced by sap-sucking insects such as aphids and scale insects. Female horse flies also require blood meals from hosts for reproduction. Adult flies emerge from late spring into summer and have a short life span, typically ranging from a few days to a few weeks.

Feeding Habits and Diet

Female Horse Flies

Female horse flies are known for their biting behavior. They feed on the blood of livestock and other animals, including horses. This is done using their specialized mouthparts, which include:

  • Mandibles: These sharp, cutting structures help the flies slice open the skin.
  • Sponging mouthparts: These parts assist in lapping up the blood, similar to the way a sponge absorbs liquid.

The diet of female horse flies primarily consists of:

  • Blood from various animals
  • Occasional plant sap and nectar to supplement their nutrition

For example, a female horse fly might feed on a cow in a pasture, causing discomfort to the animal and possibly leading to irritated skin or infections.

Male Horse Flies

On the other hand, male horse flies have a less aggressive feeding habit, as they do not bite or feed on blood. Instead, they rely on plant-based sources for their nutrition, including:

  • Nectar: This sweet, sugary substance is obtained from flowers and serves as the primary food source for male horse flies.

Here’s a comparison table highlighting the differences in feeding habits between female and male horse flies:

Feature Female Horse Flies Male Horse Flies
Primary Food Source Blood from animals Nectar from flowers
Mouthparts Mandibles, sponging parts Adapted for nectar feeding
Biting Behavior Yes No
Impact on Livestock Irritation, infections N/A

Overall, the feeding habits of female horse flies cause more concern due to their biting behavior and potential impact on livestock, while male horse flies pose less of a threat as they feed only on plant-derived nutrition sources such as nectar.

Habitat and Environments

Home and Wooded Areas

Horse flies and deer flies are commonly found in wooded areas, making their buzzing presence known to both humans and animals. Male horse flies tend to feed on plant nectar and usually avoid coming too close to human homes.

Female horse flies, in contrast, are known for their painful and persistent bites to obtain blood from humans and animals. This biting behavior is more prominent near wooded areas, where they have a better environment for breeding.

Streams and Farm

Horse flies and deer flies are also found around streams and farm environments. Streams provide an ideal habitat for the larvae of these flies. In farm settings, they can become a nuisance to both humans and livestock alike.

For example, horn flies typically adapt to cattle manure as a requisite habitat for their larval development, and adults feed mainly on cattle Penn State Extension. This type of fly can negatively impact farm operations and animal health, especially for horses.

Comparison Table

Flies Home and Wooded Areas Streams and Farm
Male Feed on plant nectar; avoid human homes Less nuisance; found around streams
Female Biting behavior; looking for blood meals Can negatively impact farms/ livestock; larvae development in cattle manure
  • Horse flies usually found around wooded areas and streams
  • Deer flies typically found in farm environments
  • Male flies feed on plant nectar
  • Female flies exhibit biting behavior for blood meals
  • Farming and livestock in areas with high fly populations may experience challenges

In conclusion, horse flies and deer flies are usually found in wooded areas, near streams, and on farms. Understanding their habitat and environments can help in implementing effective pest control measures.

Health Risks and Protection

Effects on Humans

Horsefly bites can cause swelling, pain, and rash in humans. Female flies require a blood meal for reproduction and may transmit bacteria and viruses during their bites. Some common diseases caused by horsefly bites include tularemia and anthrax. Symptoms from these bites may involve dizziness and weakness.

To protect against horsefly bites, people can use:

  • Repellents: Apply effective insect repellent containing DEET or essential oils like eucalyptus and citronella.
  • Fly traps: Use commercial fly traps or make DIY ones with reusable bottles.

Effects on Pets

Pets, such as horses and dogs, can also suffer from horsefly bites. Their bites can lead to infections and diseases transmitted through the blood meal taken by the female flies. Protecting your pets from horseflies includes:

  • Treatments: Apply approved repellents or ointments to prevent bites.
  • Shelter: Provide shaded areas for pets to minimize exposure to horseflies.
Humans Pets
Effects Swelling, pain, rash Infections
Risks Tularemia, anthrax Protozoa, bacteria

Remember, minimizing exposure to horseflies and using appropriate repellents and fly traps are essential for protecting both humans and pets. Monitoring the healing process after a horsefly bite can also help identify any possible complications.

Species and Identification

Genus Tabanus

Horse flies belong to the genus Tabanus, within the family Tabanidae. They are commonly found in the continental US, with over 160 species of horse flies. A key feature used in identification is their eyes:

  • Male horse flies: Eyes are large and often brightly colored; they touch each other in the center
  • Female horse flies: Eyes are medium-sized and spaced apart

Northern Horse Fly

The Northern Horse Fly is a species that can be found in the USA. They share general characteristics with other horse flies but can be distinguished by their appearance and size.

Appearance:

  • Gray or yellowish body
  • Solid wings

Comparison Table

Aspect Male Horse Fly Female Horse Fly
Eyes Large, touch in center Medium, spaced apart
Feeding Nectar Blood
Body color Gray / yellowish Gray / yellowish
Wings Solid Solid

Keep in mind that Northern Horse Flies share characteristics with other horse flies. Always use caution when attempting to identify or interact with these insects, as their bites can be painful, especially from female horse flies.

Horse Fly Control

Prevention on Livestock

To protect your livestock from horse flies, it’s important to take preventive measures. One effective method is using fly sheets, masks, and boots on animals, which prevents flies from accessing their face, legs, or body1.

  • Pros:
    • Provides physical barrier against flies
    • Can minimize fly-induced stress in animals
  • Cons:
    • May require frequent checks and adjustments
    • May not be suitable for all types of livestock

Prevention in Home

For effective horse fly control in and around your home, consider the following approaches:

Manure Management

Regularly clean and dispose of manure piles to eliminate breeding sites. This will help in controlling the horse fly population.

Pest Control

Use approved repellents and insecticides, and follow the manufacturer’s instructions for proper application. This can help to reduce the horse fly population and protect both humans and animals from bites.

Physical Barriers

Install screens on windows and doors, and use fans to create air movement, which can interrupt fly flight patterns and prevent them from landing1.

Feature Comparison: Livestock Prevention vs Home Prevention

Livestock Prevention Home Prevention
Main Focus Physical barriers Combined approaches
Examples Fly sheets Manure Management
Fly masks Pest Control
Fly boots Physical Barriers
Effectiveness for Mammals12 High Moderate to High
Suitability for Livestock and Home12 Livestock-specific Home-specific

Footnotes

  1. Penn State Extension – Filth Fly Control on Horse Farms 2 3 4
  2. Horse and Deer Flies – Purdue University 2

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 – Female Black Horse Fly

 

Subject:  Huge Fly?
Geographic location of the bug:  Dubuque, Iowa
Date: 08/02/2018
Time: 10:26 AM EDT
Your letter to the bugman:  What is this thing – I’ve never encountered one of these before…  An inch or more in length!
How you want your letter signed:  Rich

Female Black Horse Fly

Dear Rich,
Thanks so much for providing such excellent images of a female Black Horse Fly,
Tabanus atratusMale Horse Flies have much bigger eyes and only females bite.

Female Black Horse Fly
Female Black Horse Fly

 

Letter 2 – Female Black Horse Fly

 

Subject:  Is this a deer fly?
Geographic location of the bug:  Elicott City, MD
Date: 07/31/2018
Time: 10:34 AM EDT
Your letter to the bugman:  A friend of mine spotted this little B@#$%^d on his car.   I sort of remember this as a deer fly from my childhood in the northern midwest, but never saw one on the east coast.  If I remember correctly, a bite from these guys will raise a respectable welt.
He said it was HUGE for a fly, maybe 3/4 inch?
Thanks for the help.
How you want your letter signed:  Ray Oberg

Female Black Horse Fly

Dear Ray,
This is a Black Horse Fly, not a Deer Fly, but Horse Flies and Deer Flies are both in the family Tabanidae, so they do resemble one another.  Deer Flies and Horse Flies are both biting Flies, but only the females bite and suck blood, and the bite is reported to be quite painful.  This is a spectacular image of a female Black Horse Fly,
Tabanus atratus.

Letter 3 – Female Black Horse Fly laying Eggs

 

Hello from Florida!
Female Black Horsefly preparing the "cradle" for her offspring on Sago Noted several pictures of this type of fly but w/o the egg sac.
Evi

Hi Evi,
The female Black Horse Fly, Tabanus atratus, deposits her eggs on a plant near the water’s edge.

Update: (07/14/2008)
Hello from Florida!
If not Tabanus atratus – what is it?? In any case, we collected egg sac including substrate directly below (damp and marshy) Hopefully, the maggots will hatch for us, helping us with ID. Thanks for your immediate response!
EVI

hi Evi,
We are confused by your new email. We agree that this is a Black Horse Fly, Tabanus atratus. We posted your photo and replied directly to you. Perhaps there was some garbling with the communication.

Letter 4 – Female Black Horse Fly

 

Big Black Winged Insect
Location: Chincoteague, VA
August 8, 2011 7:47 pm
Hello,
This bug was on my car when I visited Chincoteague Island off the coast of Virginia. It was near the coast in the woods. Between 1 and 2 inches long. Any ideas?
Signature: Megan

Female Black Horse Fly

Dear Megan,
You have photographed a female Black Horse Fly,
Tabanus atratus, the same species we posted a photo of yesterday after being snared in the web of a Golden Orbweaver.  The eyes of female Horse Flies are spaced farther apart than those of the males which have no separation.

Letter 5 – Female Horse Fly

 

horse fly eyes
Location: Possum Creek metropark, ohio
August 9, 2011 12:58 pm
saw this at one of the local parks, i knew it was a horse fly, but the eyes amazed me, i want a pair of sunglasses that look like them!
Signature: BIBEF

Female Horse Fly

Dear BIBEF,
When we return back to the office after running errands, we will try to determine which species this lovely female Horse Fly is.  

Update
This photo of Tabanus abdominalis on BugGuide is a close match.  The eye pattern, leg markings and general coloration seem close, though the camera angle of the photo we are posting is not ideal for identification purposes.  The sighting data in the nearby states of Kentucky and Illinois also supports the possibility that this fly is Tabanus abdominalis.

Letter 6 – Female Horse Fly

 

Buzzing in the Brown eye Susan
Location: northeast Missouri (backyard)
August 18, 2011 1:30 pm
Hi! This creature was found hovering near Brown eye Susan’s on 8/17, at 7:53pm , heavy twilight. Actually there were 3 of them. They made a loud buzzing sound. They made no attempt to sting us, or get near us, they would hover in mid air and ”look” at us. My brain could only think a moth creature. In my moth guide the only one I could find resembling the not very clear pictures is a ”bagworm” moth. What do you think?
Signature: Jan

Female Horse Fly Hovering

Hi Jan,
Your photos of a hovering female Horse Fly are a wonderful addition to our site because they reveal the female of the family as more than a blood sucking fiend.  She also feeds from flowers.  You can tell she is a female by the space between her eyes.

Hovering Horse Fly

Letter 7 – Female Horse Fly

 

Subject: Fly ID
Location: Spring Hill, Florida
May 11, 2014 12:22 pm
Took this shot of a fly on my screen porch. Interesting color on the eyes. My guess would be some type of Hoverfly but I’m not sure.
Signature: S. Hunter Spenceley

Horse Fly
Horse Fly

Dear S. Hunter,
Your fly is a Horse Fly, and the spacing between the eyes indicates it is a female.  Only female Horse Flies bite.  The closest match we could find on BugGuide is
Tabanus lineola.

Letter 8 – Female Black Horse Fly

 

Subject: Huge fly out cicada?
Location: NW Iowa
August 30, 2015 8:26 pm
Found this sitting outside camper in Iowa and got a picture after the wife freaked out. Is this a horse fly?
Signature: Curious in Iowa

Female Black Horse Fly
Female Black Horse Fly

Dear Curious in Iowa,
Because of the spacing between the eyes, one can tell that this Black Horse Fly,
Tabanus atratus, is a female.  Only female Horse Flies bite and suck blood from mammals, and if they cannot find a four footed host, they will bite humans.  According to BugGuide:  “Although Tabanus atratus do not often bite humans, when it does happen it leaves painful memories. This fly can also transmit bacterial, viral, and other diseases such as surra and anthrax, to both humans and other animals through its bite.  The effect of T. atratus on livestock can be a serious problem. Blood loss and irritation from the flies can severely affect beef and milk production, as well as grazing. Livestock usually have no way of avoiding the painful bites, and millions of dollars have been spent trying to control these pests. (Long 2001)”

Letter 9 – Female Dark Giant Horse Fly from Scotland

 

Subject: Hover fly in Scotland
Location: Scottish West Highlands
August 2, 2016 10:58 am
This creature was about 1 inch long, seemed really massive! I’m in the West Highlands of Scotland. It also seemed on its last legs but disappeared after a while.
Signature: Alison

Female Dark Giant Horse Fly
Female Dark Giant Horse Fly

Dear Alison,
This is a female Dark Giant Horse Fly,
Tabanus sudeticus, and you can tell she is a female by the spacing between her eyes.  Your three different views are a wonderful way to provide an excellent identification guide for our readership.  According to Influential Points, it is also known as the Dark Behemothic Horsefly.

Female Dark Giant Horse Fly
Female Dark Giant Horse Fly

Thank you so much. Lots of people guessing at my photo on Facebook but none of us got it right. I’m posted your link. Thank you!

Female Dark Giant Horse Fly
Female Dark Giant Horse Fly

Letter 10 – Female Dark Giant Horsefly from South West Wales

 

Subject: Female Dark Giant Horsefly?
Location: South West Wales
July 4, 2017 12:24 pm
Hi, found this insect on my window this morning, I’ve asked several friends to help identify it but the most likely contender seems to be the Dark Giant Horsefly and for it to be a female due to the band between the eyes. Would you agree?
Signature: Justin

Female Dark Giant Horsefly

Dear Justin,
We agree that this is a female Dark Giant Horsefly, though we do have one comment regarding the common name.  In North America, we reserve compound names using fly to refer to creatures that are not Flies, like Butterflies, Fireflies, Dobsonflies and Dragonflies, and the modifying naming strategy like Crane Flies, Fruit Flies and Flesh Flies, as well as Horse Flies, is used to identify true Flies in the order Diptera.

Letter 11 – Female Dark Giant Horsefly from England

 

Subject: Silly for not knowing…
Location: Carlisle, Cumbria
July 17, 2017 11:21 am
Hi please could you help me identify this beastie? He was about 2 inches and came home with me on my boot. I’d be most grateful. Thank you., Becky.
Signature: The bug

Female Dark Giant Horse Fly

This is a female Dark Giant Horse Fly.

Authors

  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. whatsthatbug.com is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

    View all posts
  • Piyushi Dhir

    Piyushi is a nature lover, blogger and traveler at heart. She lives in beautiful Canada with her family. Piyushi is an animal lover and loves to write about all creatures.

    View all posts
Tags: Horse Fly

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4 Comments. Leave new

  • Oh my gosh I just saw one of these monsters on my deck in Rhode Island. It seriously looks like a flying Darth Vader! Ugh…I tried to snap a pic, but it came after me so fast I rolled through my door like I was avoiding a sniper…not an easy feat for a 51 year old…Haha!
    Fierce looking bug for sure.

    Reply
  • Was driving home here in Georgia with window half way down, and nailed one of these females on the window jam. Must have been feeding cause I tore open the thorax which had red blood in it. And even though incapacitated, this horse fly was still able to crawl around for 5 mins in my car til I could pull over and remove it.. Terrified of stinging insects like rose flies and wasps etc… And about 6 miles prior I had a baby wasp get in my car and felt something like a splinter poking my back turned out to be that little guy and he stung me through my shirt but not
    Near as painful as an adult wasp.

    Reply
  • Was driving home here in Georgia with window half way down, and nailed one of these females on the window jam. Must have been feeding cause I tore open the thorax which had red blood in it. And even though incapacitated, this horse fly was still able to crawl around for 5 mins in my car til I could pull over and remove it.. Terrified of stinging insects like rose flies and wasps etc… And about 6 miles prior I had a baby wasp get in my car and felt something like a splinter poking my back turned out to be that little guy and he stung me through my shirt but not
    Near as painful as an adult wasp.

    Reply
  • Cynthia E VanLoan
    August 18, 2021 5:47 am

    we have these every year in Hudson Valley/Catskill Mtn area in NY. usually start becoming prevalent August – Sept. nasty bites. smart too. they tend to land on top of rump where horse cant reach with mouth or tail. mine will buck & head to nearest shed. good fly spray seems to help a lot.

    Reply

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