Thick Headed Fly: Quick Guide to These Fascinating Insects

folder_openDiptera, Insecta
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Thick-headed flies may not sound like the most appealing creatures, but they’re undoubtedly fascinating insects with unique behaviors. These flies belong to the family Conopidae and are known for their distinct, under-curved abdomen, making them easily identifiable. Found in various environments, these winged critters have a particularly intriguing method of reproduction.

In order to lay their eggs, thick-headed flies have an excellent precision technique. They target specific insects like bees, wasps, and other prey in mid-air, swiftly depositing an egg on the unsuspecting host’s neck before releasing their grip. This airborne interception is truly a remarkable sight, showcasing the thick-headed fly’s agility and speed in action.

As you continue to learn about thick-headed flies, you’ll discover their unique role in different ecosystems and the fascinating behaviors they exhibit. With further knowledge, you can better appreciate these often overlooked insects and the incredible adaptations they’ve developed to survive and thrive in their surroundings.

Overview of Thick-Headed Fly

Thick-headed flies belong to the family Conopidae within the order Diptera, which falls under the class Insecta in the phylum Arthropoda. These insects are fascinating little creatures with some unique traits that separate them from other flies.

As a member of the order Diptera, thick-headed flies share similarities with other two-winged insects. However, they have some distinctive features that make them stand out. They have an elongated abdomen with a pointed, often curved end, resembling a wasp’s stinger. Don’t be fooled though, as this structure is used for laying eggs rather than stinging.

One notable characteristic of thick-headed flies is their parasitic nature. Many species are known to parasitize bees and wasps. They do this by laying their eggs in the bodies of these hosts, where the larvae develop and eventually consume their host from within.

A few key features of thick-headed flies are:

  • Elongated, wasp-like abdomen
  • Pointed, non-stinging end
  • Parasitic nature (targeting bees and wasps)

Thick-headed flies can be encountered in various habitats, including meadows, gardens, and forests. They are often observed perching on flowers, where they feed on nectar and wait for potential hosts to parasitize. So, the next time you see a peculiar-looking fly on a flower, it could be one of these fascinating insects.

In conclusion, thick-headed flies are remarkable insects with distinct features and behaviors. Remember to appreciate their intricacies as you encounter them in nature, and maybe you’ll even find yourself enjoying these often underappreciated members of the insect world.

Species and Varieties

Identifying Common Species

There are several species of thick-headed flies (Conopidae) that you might encounter. Some common genera include Conops, Myopa, Physoconops, and Zodion. They all share a distinctive body shape, with an under-curved abdomen.

Here are a few examples:

  • Conops quadrifasciatus: This species has a yellow and black striped pattern, resembling a wasp.
  • Physocephala tibialis: This North American species has a unique wing pattern and red eyes.

These flies have some fascinating features:

  • They attack their target prey in mid-air, quickly depositing an egg.
  • Their larvae are parasites of various insects, such as ants, bees, wasps, and grasshoppers.


Thick-headed flies can be found in various habitats across the globe, including North America and the Pacific islands. They can even be found in urban gardens.

When comparing the distribution of different genera, consider the following:

Genus Distribution
Conops Worldwide
Myopa Worldwide
Physoconops North America, Europe
Zodion North America, Australia

So, you can find these interesting flies in various places, from your backyard to the other side of the world. Keep an eye out for their unique features, and remember to appreciate their role in the ecosystem.

Physical Characteristics

Appearance and Size

Thick Headed Flies are elongated insects with a body length that ranges from 3 to 15 mm. These flies exhibit various colors, such as brownish, black and yellow, or black and white patterns on their body. Their size and appearance depend on the species. For instance, some species have a long proboscis, while others have a smaller, retractable one.

Unique Body Features

A unique feature of Thick Headed Flies is their ptilinum, a sac-like structure that helps them emerge from the puparium during the adult stage. They also have a distinct wing venation that is specific to their family. Some key characteristics include:

  • A long, thin body
  • The presence of a ptilinum
  • Unique wing venation patterns

Wing and Flight Characteristics

Thick Headed Flies have well-developed wings that allow them to fly efficiently. Their wing and flight characteristics are determined by their wing venation, which can vary among species. These flies are agile fliers, capable of quickly changing direction to catch their prey or evade predators.

Life Stage Differences

The lifecycle of a Thick Headed Fly consists of four stages: eggs, larvae (maggot), pupae, and adults. Here are some characteristics for each stage:

  • Eggs: Laid on or near host insects, they are oval and tiny.
  • Larvae: The maggot stage is legless, with a segmented body. It feeds on the host insect.
  • Pupae: Encased in a protective puparium, they transition from larva to adult fly.
  • Adults: Fully developed wings and a proboscis are present in this stage.

By understanding these physical characteristics of Thick Headed Flies, you can easily identify them in your surroundings and learn more about their unique features.

Life Cycle and Reproduction

Mating Habits

Thick-headed flies have unique mating habits. Males usually wait on flowers for females to arrive, then catch them mid-air. After mating, the female searches for suitable hosts on which to lay her eggs.

Egg Laying and Larval Development

During the egg-laying process, female thick-headed flies target a variety of insects, such as bumblebees, ants, and crickets. They’ll lay eggs on burrow and ground nester bumblebees, for example. Once hatched, the larvae penetrate their host, living and feeding on it until they reach their final larval stage. Hosts for thick-headed fly larvae can also include aquatic insects.

  • Host variety: bumblebees, ants, crickets, aquatic insects
  • Larval stage: living and feeding on the host

Pupal Stage and Metamorphosis

When the larva is ready, it leaves the infested host and finds a suitable location to pupate. It then forms a pupa and undergoes metamorphosis, transforming into the adult thick-headed fly. The newly emerged fly begins its life cycle anew, starting with mating and continuing the cycle of reproduction.

Here’s a comparison table of the life cycle stages:

Life Cycle Stage Features
Mating Males catch females mid-air; mating takes place
Egg Laying Female lays eggs on suitable insect hosts
Larval Development Larva penetrates host, living and feeding on it
Pupal Stage Larva leaves host, forms pupa, and undergoes metamorphosis
Adult Fly Emergence Newly emerged adult thick-headed fly begins mating and cycle repeats

By understanding the life cycle and reproduction of thick-headed flies, you can better appreciate their role in the ecosystem and their fascinating behaviors.

Behavior and Habits

Feeding Habits

Thick-headed flies typically feed on nectar from flowers. They use their proboscis to suck up the sugary liquid which provides them with energy. You can often find them visiting various types of flowers in gardens or wild habitats.

Mimicry and Survival Tactics

The remarkable quality of thick-headed flies is their ability to mimic other insects, particularly bees and wasps. This strategy helps them avoid predators, as they appear to be potentially dangerous or unpalatable. Given their striking resemblance to these insects, they can easily deceive both predators and prey. For instance, they may appear similar to honey bees, sweat bees, or Anthophora bees.

Host Manipulation

One unique aspect of thick-headed fly behavior is their host manipulation, especially during reproduction. Female flies lay their eggs on or near potential host insects, such as bees or wasps. The larvae then parasitize the host, eventually killing it. They are known to target a range of insects, including ants, bees, wasps, grasshoppers, and crickets, allowing their offspring to develop inside the host and eventually kill it.

Behavioral Resistance

In response to host defenses, thick-headed flies have evolved behavioral resistance strategies. They often wait for the right moment to approach and lay their eggs on the host, ensuring the success of their offspring. This careful, strategic approach helps them overcome the challenges presented by host insects and increase the likelihood of successful parasitism.

Ecological Impact and Significance

The Thick Headed Fly is an interesting insect with a unique ecological role. As a parasitoid, they help control populations of their hosts, which include a variety of other insects. They mainly target wasps and bees, but can also parasitize other insects like beetles and grasshoppers.

These flies are internal parasites, laying their eggs on or inside their host. Upon hatching, the larvae feed on the host’s internal tissues, ultimately killing it. This parasitic relationship benefits the ecosystem by controlling insect populations and promoting biodiversity.

For example, in the case of wasps or bees, Thick Headed Flies may help manage their numbers, preventing excessive competition among pollinators. This can benefit plant populations, enabling them to thrive and maintain a balanced ecosystem. Additionally, by preying on other insects, they indirectly protect plants that those insects might otherwise damage or consume.

Features of Thick Headed Flies:

  • Parasitize various insects like wasps, bees, beetles, and grasshoppers
  • Lay eggs on or inside their host
  • Larvae feed on host’s internal tissues

In conclusion, the ecological impact and significance of Thick Headed Flies is primarily due to their function as parasitoids. They contribute to controlling insect populations, promoting biodiversity, and maintaining the balance within an ecosystem. Understanding their role helps scientists and environmentalists better manage and protect ecosystems. While their methods may seem brutal, they are an essential part of the natural world.

Study and Research

You may wonder how researchers study thick-headed flies. To begin with, they collect specimens from various locations and habitats. This helps them understand the diversity and distribution of species. For example, thick-headed flies can be found parasitizing ants, bees, and wasps.

As you study these flies, it is essential to observe and document their behavior. This may include recording details about their flight patterns, host selection, and reproductive strategies. One interesting aspect of their reproduction is their unique egg delivery system, which works like a heat-seeking missile.

In the lab, you can examine the specimens under a microscope, which can aid in identification and classification. Additionally, you can compare their morphological traits, such as body size, wing structure, and color patterns. This information is vital in understanding the evolutionary relationships among thick-headed fly species.

Research on thick-headed flies also holds practical implications, such as in biological control. Certain species of these flies can potentially help manage pests that harm crops or spread diseases. However, more studies are needed to evaluate their effectiveness and potential risks.

Some key features of thick-headed flies that you should consider during research include:

  • Their distinct parasitic lifestyle
  • A wide range of hosts that they parasitize
  • Unique reproductive strategies
  • Potential applications in biological control

Remember to keep a detailed record of your observations and findings. Sharing your research with other scientists and the public can contribute to the growing body of knowledge about thick-headed flies and their fascinating lives.

Role in Horticulture and Pest Control

Thick-headed flies play a significant role in horticulture and pest control. They are considered good bugs due to their predation on various pests. Some thick-headed flies target caterpillars, which can cause foliage damage to ornamental and vegetable plants.

These flies lay eggs on the bodies of their host pests. Once the eggs hatch, the larvae penetrate the host’s body and feed on it. This process helps in controlling unwanted pests in your garden or farm.

In some cases, thick-headed flies can be more effective at controlling pests than chemical methods. This is because they target specific pests without harming non-target species. It is essential to encourage the presence of these good bugs by providing plants that attract them.

Some ways to attract thick-headed flies to your garden include:

  • Planting a variety of flowers, particularly those with umbel-shaped blooms such as dill, Queen Anne’s lace, and yarrow
  • Avoiding the use of broad-spectrum pesticides, which can harm beneficial insects
  • Maintaining a diverse garden ecosystem with natural habitats like hedgerows and brush piles

While thick-headed flies can be beneficial for horticulture and pest control, they may not eliminate all the pests in your garden. It’s essential to combine their usage with other integrated pest management strategies, such as cultural, biological, and chemical methods, for more effective results.

Frequently Asked Questions

You might be curious about the Thick-headed Fly and its behavior. Let’s address some frequently asked questions to help you better understand this unique insect.

What are Thick-headed Flies?

Thick-headed Flies belong to the family Conopidae. They are parasitoids of ants, bees, wasps, grasshoppers, crickets, and a few other insect groups. Their larvae develop inside the body of their host, eventually killing it.

How do Thick-headed Flies reproduce?

Female Thick-headed Flies have a unique egg-laying process. When she spots a potential host, she intercepts it in mid-flight and deposits her eggs on it. The larvae then penetrate the host’s body and develop inside.

What do adult Thick-headed Flies eat?

Adult Thick-headed Flies consume nectar and pollen. They are particularly attracted to umbelliferous plants, such as carrot, dill, and other herbs, as well as composite flowers like asters.

How can I identify Thick-headed Flies?

To identify a Thick-headed Fly, look for its distinct features:

  • A conical or thickened head shape
  • Elongated body
  • Wings held flat over the abdomen when at rest
  • Mimic the appearance of wasps or bees

Are Thick-headed Flies harmful to the environment?

Thick-headed Flies play a vital role in controlling the population of their host insects. In some cases, they may even contribute to pest control in agricultural settings. However, they can also affect beneficial pollinators like bees.

Now you have a better understanding of Thick-headed Flies, their behavior, and their impact on the environment. Remember that these insects are an essential part of the ecosystem and play a crucial role in keeping their host populations in check.

Reader Emails

Over the years, our website, 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 – Thick Headed Fly from Canada


Subject:  Fly ID
Geographic location of the bug:  Aurora, Ontario. Canada
Date: 05/04/2019
Time: 11:14 AM EDT
Your letter to the bugman:  Hi!
Could you please help me with ID of this fly? I guess it’s some kind of hoverfly
How you want your letter signed:  Vladimir Morozov


Dear Vladimir,
Your images are beautiful.  We are relatively confident this is not a Hover Fly, but we are uncertain of its family.  We are leaning towards Tachinidae, but we are currently battling very slow internet in Ohio which makes research ponderous.  Perhaps one of our readers will recognize this Fly.

Thick Headed Fly

Comment from Cesar Crash
Mouthparts, calyptra and halteres don’t look like Tachinidae. I think it’s Zodion sp.  See BugGuide.

Daniel, Cesar  thank you very much. Really looks like some Zodion

Letter 2 – Mating Thick Headed Flies


Subject:  thick headed fly
Location:  Lancaster, PA
October 30, 2013  10:12 AM
Hi again
Don’t mean to be a pest, but thought you might be able to use some shots of thick headed-flies making maggots. I took these in my flower garden on Joe Pye weed, Lancaster PA. There are abundant bumble bees which I imagine is why they’re hanging around.
Melody McFarland

Mating Thick Headed Flies
Mating Thick Headed Flies

Hi Melody,
Pest?  You have provided our archives with two marvelous additions of underrepresented species on our site.  We only had two Thick Headed Fly images in the category, but because of posting your submission, we realized that we had several additional older postings that were never categorized.  According to BugGuide, Thick Headed Flies in the family Conopidae are “usually found on flowers” and “Adults take nectar. Larvae are endoparasites of wasps, bees, ants, crickets, cockroaches, and some Diptera (mostly calyptrate); host group varies by subfamily.”

Mating Thick Headed Flies
Mating Thick Headed Flies

We are trying to post a few last minute submissions as we will be away from the office for the next week without internet connectivity.

Thick Headed Fly
Thick Headed Fly

Letter 3 – Mating Thick-Headed Flies from Ontario


Subject: Chalcid wasp?
Location: Peterborough ON
May 26, 2016 9:02 pm
I found these small (1mm+) wasp-like insects mating in my backyard on the weekend – May 24. It was sunny and warm: around 26C.
Signature: Rob Tonus

Mating Tiny Flies
Mating Thick-Headed Flies

Dear Rob,
Our initial impression that the faces on your mating insects looked more like Flies than Chalcid Wasps proved correct when we zoomed in on your very high resolution image, which revealed the presence of halteres which are defined on Entmologists’ Glossary as “modified wings. In the Diptera (true flies) it is the hind wings that have become halteres. …  Halteres are shaped like ‘drum sticks’ with a slender shaft connected to the thorax at one end and ending in a thicker structure at the other. Halteres are highly sophisticated balance organs and they oscillate during flight.”  So these are mating Flies.  We are going to post your submission as unidentified while we continue to research the identity of your mating pair of Flies.  We will also contact Eric Eaton to get his input.

Eric Eaton Responds
Sure:  Thick-headed flies, family Conopidae, maybe Myopa for genus?

Ed. Note:  This image on BugGuide looks very close, but it is listed as 12mm, not 1mm.  According to BugGuide:  “Myopa species are parasitic on Honey Bees Apis mellifera, Andrena and Mustache Bees Anthophora.”

Thanks so much for the quick feedback, Daniel.  I appreciate you investigating these mystery insects for me.

Thanks again for this additional information.
The flies were larger than 1 mm, but much smaller than 12 mm – perhaps 5 or 6 mm at most . . . that’s only an estimate, though, since I saw them mating, and they could have had their abdomens twisted, which made them look shorter.
Are these presumptions on genus the closest we’ll get to identifying them?  Is it difficult to determine species without having them in hand?

Perhaps a Dipterist may be able to do a conclusive species ID, but alas, we have not the necessary skills.

Letter 4 – Thick Headed Fly, not Syrphid Fly


Identify a Wasp with a white face?
Location:  Fairfield, Maine
August 3, 2010 7:45 pm
I found this on some goldenrod along with dozens of paper wasps. I seemed very camera shy or very busy, so I was only able to get this one picture. I looked through a lot of the potter wasps but did not find anything with the same markings and colors. Is this even a wasp at all?
James R

Unknown Syrphid Fly

Hi James,
This is not a wasp, but a fly that mimics a wasp.  We suspect it is in the family Syrphidae, the members of which are called Flower Flies or Hover Flies.  We are posting your photo as unidentified until we get an actual species or at least genus name.  Perhaps our readers can assist us.

Correction thanks to Karl
August 4, 2010
This is a Thick-headed Fly [see BugGuide] (Conopidae), so named because of their relatively large heads. According to the Kaufman Field Guide to Insects of North America “They are mostly parasites of adult solitary bees, and sometimes wasps. The female fly assaults the host in midair, often forcing it to the ground and ramming an egg between the victim’s abdominal segments before releasing it.”  I believe the genus is either Physocephala or Phyoconops, the difference apparently being that in Physocephala the hind femur is somewhat swollen at the base, whereas in Physoconops it is not. This feature is not always easy to distinguish but the femurs do appear slightly swollen in this individual. There are many very similar looking species but based on color patterns of the face, legs, wings and abdomen, I think this may be Physocephala marginata. Regards. Karl

Letter 5 – Possibly Thick Headed Fly from China


Subject:  Bug ID
Geographic location of the bug:  in the hills around Fuzhou, Fujian, [SE] China,
August 28, 2017 6:44 AM
found at about 800m alt. in the hills around Fuzhou, Fujian, [SE] China, 28/08/2017.
I guess that the orange one is an immature form of a solitary wasp but the other one is beyond my guessability.
How you want your letter signed:  Eric Smithson

Thick Headed Fly, we are guessing

Dear Eric,
The insect beyond your “guessability” is a Weevil, but your orange insect is the one that really interests us.  It is not an immature solitary wasp.  We wish the mouthparts were more evident in your image.  The antennae resemble those of a Fly in the order Diptera, and that is our best guess.  We haven’t time to research this right now, so we are posting it as unidentified and we are putting it out as a challenge to our readership to help us determine its identity.

Hi Daniel, thanks for your unexpectedly prompt reply. These things may be commonplace to you and your team but we find something new to us every time we go for a hike. The diversity of life on this planet is mind blowing and beautiful. Unfortunately I don`t have a better picture to show you but it looks to me like this might be an ? exuvia with the next stage emerging from underneath… but I really know nothing; just happy to keep discovering.
Thanks again.

Hi Again Eric,
Cesar Crash of Insetologia suspects this is a Thick Headed Fly in the family Conopidae.  We will do more research.


Letter 6 – Thick Headed Flies Mating


Thick-headed Bug Love
July 14, 2010
Found some thick-headed flies courting and thought you might like them for your bug love page. They were mating until I rudely interrupted with my camera, poor things.
central NJ

Mating Thick Headed Flies

Hi Sara,
We keep on saying this is the last letter we are posting today, and we keep finding awesome letters and photos, but we really need to stop with your awesome images of Thick Headed Flies in the family Conopidae.

Letter 7 – Thick Headed Fly


Thick-headed fly
July 25, 2009
Here is a photo I took today. Went to Buguide. It was identified as a conopidae, species unknown. Posably physoconops, or physocephala.
Terry Sincheff
Mound, MN

Thick Headed Fly
Thick Headed Fly

Dear Terry,
Thanks so much for sending us your photo of a Thick Headed Fly.  WE are linking to the BugGuide information page that states:  “The adults are usually found on flowers.  Food  Larvae are endoparasites, chiefly of adult bumblebees and wasps. Adults take nectar.

Letter 8 – Thick Headed Fly


Thick-headed fly
July 11, 2010
Thick-headed fly
Sure thought this was a smallish wasp, but finally found out it’s a thick-headed fly. Thought you might like a picture.

Thick Headed Fly

Hi Sara,
As we were working on your post, we realized we needed to create a new category for Thick Headed Flies.  We will need to search through our archives to see if we have any older postings of Thick Headed Flies in the family Conopidae, but this might be a first for our website.  We believe your specimen may be in the genus
Physocephala, which BugGuide describes as “adults feed on flower nectar  Females usually oviposit on hosts (mostly bumble bees and wasps) during flight. Larvae become internal parasitoids (usually kill the host).”  There is additional clarification of the life cycle on BugGuide:  “P. tibialis has been reported to parasitize workers of the bumble bee Bombus bimaculatus. Adults apparently alight and inject an egg into the abdomen of their host while in flight.  A study in Alberta showed that bumble bees parasitized by P. texana had the same lifespan as unparasitized individuals (Otterstatter et al, 2002; …).

Ed. Note: A previous posting of a Thick Headed Fly from 2009 was added to the new category.


  • 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.

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  • 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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