Big Headed Fly: Essential Facts and Fascinating Insights

Big Headed Flies are a fascinating group of insects that display some intriguing behaviors and characteristics.

They belong to the family Pipunculidae, which are known for their unique egg-laying tactics and their parasitic nature.

These flies have a distinct appearance, often featuring a thick, broad head with large eyes and antennae. The overall size of these flies varies by species, but some can be quite sizable.

Big Headed Fly
Source: Katja Schulz from Washington, D. C., USACC BY 2.0, via Wikimedia Commons

One interesting aspect of Big Headed Flies is their method of reproduction.

The larvae of these flies are parasitoids, meaning they develop inside the bodies of other insects such as ants, bees, and wasps.

The adult female fly is known to intercept incoming bees in-flight, laying her eggs on them in a “heat-seeking missile” fashion, without the heat, of course.

This behavior allows the larvae to have a ready food supply once they hatch.

Some common features of Big Headed Flies include:

  • Robust body with a wide head
  • Large eyes and distinct antennae
  • Larvae that parasitize other insects
  • Unique egg-laying behavior

Big Headed Flies are fascinating insects rich in interesting behavior and adaptations, making them a noteworthy subject in the world of entomology.

Learning more about these intriguing creatures can deepen our understanding of the complex relationships that exist within ecosystems.

Big Headed Fly Basics

Taxonomy and Family Pipunculidae

Big-headed flies belong to the family Pipunculidae and are part of the arthropod phylum, as they have jointed legs.

They are also classified as hexapods because they possess six legs. There are two major subfamilies within Pipunculidae: Pipunculinae and Nephrocerinae.

Physical Characteristics

Big-headed flies are typically:

  • Small, ranging from 1/8 to 1 inch long
  • Possess a wide head, as wide or wider than their abdomen
  • Have large eyes and distinct antennae

Some species may have black bodies with bands or stripes of orange, yellow, or white, to resemble stinging bees or wasps, while others are mostly brown or metallic.

Geographical Distribution

Big-headed flies are commonly found throughout North America and other regions. Their distribution is influenced by factors such as:

  • Climate conditions
  • Availability of food sources
  • Presence of host insects

They are known to be parasitoids of ants, bees, wasps, grasshoppers, crickets, and other small insects.

Life Cycle and Reproduction

Egg Stage

The life cycle of the Big Headed Fly begins with the egg stage. Female flies intercept bees in-flight to deposit their eggs on them

Larval Stage

Once the eggs hatch, the larvae emerge.

Once the eggs hatch, the larvae burrow into their host, consuming it from the inside out.

This parasitic relationship often results in the death of the host insect.

The larvae undergo several stages of metamorphosis, transitioning through the pupal stage before emerging as adult flies

Pupal Stage

After completing the larval stage, the Big Headed Fly enters the pupal stage.

This stage lasts about 55-65 hours and serves as a transitional period between the larval and adult stages.

Source: Zeynel CebeciCC BY-SA 4.0, via Wikimedia Commons

Adult Stage

Finally, adult Big Headed Flies emerge from their pupae.

These adults have distinctive features such as large eyes, robust bodies, and banded patterns on their body, mimicking bees or wasps.

Adult flies play an essential role in the ecosystem as they often act as pollinators.

Feeding and Dietary Habits

Nectar Consumption

Big Headed Flies are known for their nectar consumption.

These insects primarily feed on the nectar from various types of flowers. Some examples of flowers they prefer include:

  • Aster
  • Goldenrod
  • Milkweed

The larvae, as mentioned earlier are carnivores that eat the insides of the host insect in which the eggs were laid.

Honeydew Consumption

Another dietary staple for Big Headed Flies is honeydew. Honeydew is a sugary substance produced by aphids and other sap-sucking insects.

Big Headed Flies are attracted to the sweetness of honeydew and can often be found feeding on it.

Insect Prey

In addition to nectar and honeydew, Big Headed Flies also consume insect prey. They are predators of various small insects, like aphids, mosquitoes, and other flies.

Parasitism and Host Relationships

Host Selection and Parasitization

Big Headed Flies, or Eudorylas, are known to parasitize various insect hosts. Some examples include:

  • Bees
  • Wasps
  • Ants
  • Grasshoppers
  • Planthoppers

These parasitoids choose their hosts based on specific cues, such as chemical, visual, and tactile signals.

For instance, certain ants give off pheromones that attract Big Headed Flies.

Parasitization occurs when a female fly lays its eggs in or on the host insect. Upon hatching, the larva feeds on the host, eventually causing its death.

Impact on Host Insects

The effects of Big Headed Flies on their host insects can be significant. For example:

  • Reduced host population
  • Stunted growth and development
  • Decreased ability to reproduce

While parasites sometimes help control pests, the impact on beneficial insects should be carefully considered.

Habitat and Environmental Factors

Preferred Habitats

Big headed flies, also known as Pipunculidae, thrive in various habitats. Their ideal environments include:

  • Grasslands: Tall grass provides ample shelter and resources for these insects.
  • Forest edges: The boundary of forests and open spaces offers an optimal mix of vegetation and sunlight.

Temperature and Climate Impact

Big headed flies are sensitive to temperature and climate changes. Some important factors include:

  • Temperature range: They typically prefer temperatures between 20°C to 30°C (68°F to 86°F).
  • Humidity: High humidity levels can promote increased fly activity, while drier conditions may hinder their development.

A visual comparison of preferred habitats and temperature conditions:

HabitatTemperature RangeHumidity
Grasslands20°C to 30°CHigh
Forest edges20°C to 30°CVaries

By understanding the habitats and environmental factors that impact big-headed flies, you can better appreciate their ecological role and behavior.

Big Headed Fly Subgroups


Tomosvaryellini is a tribe of big headed flies known for their interesting appearance and behavior. Here are some key characteristics:

  • Broad head
  • Unique wing venation patterns
  • Predator of small insects

These flies are commonly found in various habitats, from forests to urban areas.


The genus Tomosvaryella includes a variety of big headed fly species. Some distinguishing features of these flies are:

  • Antenna-like palps
  • Parasitic nature: they use other insects as hosts for their larvae
  • Predominantly found in the Northern Hemisphere

Examples of Tomosvaryella species include T. borealis and T. pulchra.


Nephrocerus is another genus of big headed flies, characterized by:

  • Smaller body size
  • Distinct wing shapes
  • Larvae develop inside the bodies of bees

These flies are typically found around flowers, as they mimic bees to help them approach potential hosts.

HabitatForests & urbanNorthern HemiFlowers
HostsSmall insectsOther insectsBees

Significance and Interactions with Other Species

Big Headed Flies and Human Observations

Big headed flies, known for their large, compound eyes, provide fascinating subjects for scientific study. They are often found in:

  • Forests
  • Grasslands
  • Near water sources

Researchers have observed them exhibiting unique behaviors, such as their distinct flying patterns and mating rituals.

Relationship with Predators

There are three main predators of big headed flies: spiders, wasps, and ants. In areas that are near water bodies, frogs can also kill and eat these flies.

Another common predator of big headed flies are robber flies, who inject a fluid venom into their victim to turn their insides into liquid.

Defense Mechanisms

Big headed flies have developed defense mechanisms to protect themselves against predators, such as:

  • Camouflage
  • Evasive flying techniques


In summary, Big Headed Flies are remarkable insects with distinct physical features, including a broad head, large eyes, and antennae.

Their parasitic nature is evident in their reproductive method, where larvae develop inside other insects like ants, bees, and wasps.

The adult female’s unique egg-laying behavior involves intercepting bees mid-flight.

These flies play a significant role in ecosystems, often found in forests and near water sources.

Reader Emails

Over the years, our website, has received hundreds of letters and some interesting images asking us about big headed flies. Scroll down to have a look at some of them.

Letter 1 – Snipe Fly: Chrysopilus basilaris

Strange fly from Texas
Location: Fort Worth, Texas
May 13, 2011 8:19 pm
I have recently noticed a few of these flies on my apartment patio that I have never seen before. They started appearing the last week in April. I haven’t seen more than 3 at any one time, usually only one is hanging around.

They are not scared easily and I was able to get VERY close to take pictures without any problem. Seems like they appeared after some recent heavy rain, but I’m not certain there is a connection.

Please help me identify this insect, I have more pictures if needed. Thank you for your time.
Signature: Robert E.

Snipe Fly

Dear Robert,
Though we were unable to locate an exact visual match, your fly has the necessary characteristics for us to deduce that it must be a Big Headed Fly in the family Pipunculidae which is profiled on BugGuide

The identification description is:  “Hemispheric head almost completely made up of the huge compound eyes. Body usually black. Wings tend to be elongated and to be narrowed at the base.

Antennae are aristate and the (usually long) arista arises on the dorsal side of the antenna. Dissection of genitalia normally required for identification of species.” 

We eagerly welcome our readership to either confirm or make a correction to our identification.

Update:  June 28, 2014
Thanks to a comment, we now know that this is a Snipe Fly,
Chrysopilus basilaris, which can be viewed on BugGuide.

Letter 2 – Big Headed Fly, we believe

Subject: Is this a wasp?
Location: Coastal San Diego County
July 22, 2012 12:36 pm
Found this in my backyard in San Diego in mid July. It is 3/8” long. Can you identify it?
I’m guessing it’s a beneficial parasitic wasp of some kind?
Signature: Clint

Big Headed Fly, possibly

Hi Clint,
This is most definitely not a wasp.  It is a fly.  We believe it is either a Big Headed Fly in the family Pipunculidae which is represented on BugGuide by a few species that look generally like your individual, though none are an exact match. 

If it is not a Big Headed Fly, our second choice would be a Flower Fly or Hover Fly in the family Syrphidae, but there are too many possibilities for us to want to undertake that task after a long busy day at the day job and an evening of local activism regarding transportation issues. 

Big Headed Flies and Syrphid Flies are grouped in the not taxonomically recognized category that is a between the Order and Family, though it is not considered a Suborder. 

The insect diversity in Southern California is great, so we try to post unusual submissions from there whenever possible. 


  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. 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.

  • 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.

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