R
Species Profile

Robber Flies

Asilidae

Hawks of the insect world
Gnilenkov Aleksey/Shutterstock.com

Robber Flies Distribution

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Found in 36 countries

robber fly

At a Glance

Family Overview This page covers the Robber Flies family as a group. Stats below are general traits shared across the family.
Also Known As Assassin flies, Assassin fly
Diet Insectivore
Activity Diurnal+
Lifespan 1.5 years
Weight 5.0E-5 lbs
Status Not Evaluated
Did You Know?

Asilidae span a wide size range-roughly ~3 to ~60 mm long, from tiny gnatsized hunters to "giant" robber flies.

Scientific Classification

Family Overview "Robber Flies" is not a single species but represents an entire family containing multiple species.

Robber flies (family Asilidae) are predatory true flies that capture other insects (and occasionally small arthropods) in flight or from perches, using strong legs and a piercing proboscis to subdue prey.

Kingdom
Animalia
Phylum
Arthropoda
Class
Insecta
Order
Diptera
Family
Asilidae

Distinguishing Features

  • Strong, spiny legs adapted for grabbing prey
  • Stout, forward-pointing proboscis used to inject saliva and feed
  • Dense facial bristles forming a “mystax” (beard) that helps protect the face during prey capture
  • Typically agile fliers; many hunt from a perch and sally out to intercept prey
  • Often mimic bees/wasps in coloration (especially genera like Laphria)

Physical Measurements

Males and females differ in size

Weight
♂ 0 lbs (0 lbs – 0 lbs)
♀ 0 lbs (0 lbs – 0 lbs)
Top Speed
25 mph
flying
Venomous

Appearance

Primary Colors
Black Brown Gray Tan Cream
Secondary Colors
Yellow Red Orange White Striped Spotted
Skin Type Hard chitinous exoskeleton with dense setae/bristles; often pruinose (dusty waxy bloom) and a robust, tapered abdomen; wing membranes clear to lightly tinted.
Distinctive Features
  • Size range across family: ~0.3-5+ cm body length (small to very large robber flies).
  • Lifespan range: adults typically weeks to a few months; total life cycle often ~1-3+ years depending on species, climate, and larval development.
  • Key facial trait: mystax (dense bristles on the face) protecting head during prey capture.
  • Predatory adaptations: strong spiny legs for seizing prey; stout piercing proboscis injecting saliva to immobilize and liquefy prey.
  • Hunting ecology: common perch-and-pounce strategy; many also intercept prey in sustained flight-strategy varies by habitat and species.
  • Global distribution and habitat breadth: found worldwide (especially diverse in warm regions) across deserts, grasslands, forests, savannas, dunes, wetlands margins, and montane zones.
  • Body form diversity: ranges from slender, long-legged aerial hunters to stocky, very bristly "beelike" mimics; some resemble wasps or bumblebees.
  • Larval ecology varies: larvae are predatory in soil, leaf litter, decaying wood, or under bark; microhabitats differ widely among genera.
  • Wing appearance varies: usually clear, sometimes smoky or lightly tinted; flight often fast, direct, and agile.

Sexual Dimorphism

Sexual dimorphism is usually subtle and variable across the family. Males often show more conspicuous terminal genitalia and sometimes different bristle patterns or abdominal shape, while females typically have a broader abdomen for egg production.

♂
  • More prominent terminal genitalia/claspers at abdomen tip
  • In some groups, slightly narrower abdomen or different abdominal tip shape
  • Occasionally denser facial/leg bristles or contrasting setae (varies by species)
♀
  • Broader abdomen, especially when gravid
  • Ovipositor/terminal segments shaped for egg laying in soil or substrates
  • Often slightly less conspicuous terminal structures than males

Did You Know?

Asilidae span a wide size range-roughly ~3 to ~60 mm long, from tiny gnatsized hunters to "giant" robber flies.

They often catch prey in midair, making them the fly world's analog of raptors.

Many species inject saliva that quickly immobilizes prey and begins digestion before they "drink" the liquefied contents.

Their trademark facial "moustache" (mystax) helps shield the head and eyes when prey struggles.

Larvae are also predators, commonly hunting other insect larvae in soil, leaf litter, or rotting wood.

Some robber flies mimic bees or wasps (and even fuzzy bumblebees), gaining protection through look-alike intimidation.

They occur on every continent except Antarctica, occupying habitats from deserts and dunes to forests, grasslands, and wetlands.

Unique Adaptations

  • Mystax (dense facial bristles) that helps protect the face and eyes from struggling, spiny, or stinging prey.
  • A rigid, piercing proboscis designed to penetrate prey and deliver saliva that both subdues and pre-digests tissues.
  • Powerful, spiny legs that function like a raptor's talons for grabbing and restraining prey in flight.
  • Exceptional visual performance: large compound eyes and rapid flight control support precise midair interception.
  • Camouflage and mimicry across the family: many are cryptically colored for bare ground or bark; others mimic bees/wasps to deter predators.
  • Flight agility suited to ambush and pursuit: quick takeoffs from perches, abrupt turns, and controlled landings to feed safely.

Interesting Behaviors

  • Perch-and-pounce hunting: many sit on exposed twigs, rocks, or bare ground, then launch to intercept passing insects; others patrol continuously in flight-strategy varies by species and habitat.
  • Broad prey menu: typically other insects (flies, beetles, bees/wasps, true bugs, butterflies/moths), and sometimes spiders or other small arthropods; larger species can take relatively large prey.
  • Aerial grappling: prey is seized with spiny legs, often carried to a perch, then pierced with the proboscis for immobilization and feeding.
  • Territoriality is common in some groups: individuals may defend sunny perches or patrol routes, especially males during breeding periods.
  • Thermoregulation behavior: many species bask in sunlit spots to warm up for fast flight; activity can peak in the hottest parts of the day in open habitats, but shifts to cooler windows in very hot regions.
  • Reproduction and development vary widely: adults often live weeks to a few months, while the full life cycle commonly spans months to multiple years (often ~1-3+ years) depending on species, climate, and larval growth rate.
  • Larval ecology is diverse but consistently predatory: larvae hunt in substrates like soil, sand, decaying wood, or under bark, targeting other invertebrates.

Cultural Significance

Robber flies (Asilidae) are called helpful predators because they cut many insect numbers, including farm pests. Some eat pollinators like bees, so beekeepers worry. Their dramatic hunts make them popular in macro photos and nature education.

Myths & Legends

In Greco-Roman writings, "asilus" was a feared biting fly blamed for attacking cattle and causing panic. Later scientists used the name Asilidae for robber flies, though the ancient asilus wasn't the same.

Across English-speaking natural history traditions, robber flies picked up folk-like epithets such as "assassin fly" and "hawk fly," reflecting a long-standing habit of comparing their ambush attacks to birds of prey.

In some beekeeping groups and rural stories, robber flies (Asilidae) are called "bee killers" for grabbing bees near hives, a reputation that shaped local views though real impact changes by place and season.

Victorian-era and early 20th-century insect writers often used robber flies as vivid examples of "nature's bandits," a storytelling framing that helped popularize them in educational nature essays and field guides.

Conservation Status

NE Not Evaluated (family-level). IUCN assessments are typically at the species level; within Asilidae, assessed species span from Least Concern to threatened categories (including some localized endemics), with many species remaining Data Deficient or Not Evaluated.

Has not yet been evaluated against the criteria.

Population Unknown

Looking for a specific species?

Hornet robberfly

Asilus crabroniformis

Frequently depicted and referenced in field guides and popular media as an archetypal 'robber fly' due to its large size, hornet-like appearance, and conspicuous predatory behavior; it is one of the best-known named species within Asilidae in Europe.

  • Family context (not species-limited): Asilidae span very small to very large flies (~0.3-5+ cm body length across the family), with strong legs and a piercing proboscis used to immobilize prey.
  • Predation is typically diurnal and visual; many species hunt from sunny perches (sit-and-sally), while others actively patrol, and some show prey preferences (e.g., bees/wasps, grasshoppers, other flies) that vary by genus and habitat.
  • They inject saliva that both subdues prey and begins external digestion; prey fluids are then consumed-this feeding mode is broadly characteristic across the family.
  • Life cycles vary: larvae are predators in soil, leaf litter, rotting wood, or other places. Development usually takes months to several years (often about 1–3+ years). Adults live weeks to a few months.
  • Asilidae are highly diverse in form (often bee/wasp mimics, slender 'stiletto' forms, shaggy 'bee-like' forms) and occupy habitats ranging from dunes and grasslands to forests and wetlands margins.

You might be looking for:

Asilus crabroniformis (Hornet robberfly)

28%

Asilus crabroniformis

Large European robber fly; hornet-like appearance; a well-known Asilidae species.

Laphria flava (Bumblebee robberfly)

22%

Laphria flava

Bee-mimicking robber fly (genus Laphria); common exemplar of Asilidae mimicry.

Promachus rufipes (Red-footed cannibalfly)

18%

Promachus rufipes

Large North American robber fly noted for preying on other insects; often cited in field guides.

Efferia spp. (Efferia robber flies)

17%

Efferia

A very species-rich robber-fly genus, common in the Americas; representative of typical Asilidae form.

Diogmites spp. (Hanging thief robber flies)

15%

Diogmites

Slender robber flies often seen hanging while feeding; recognizable subgroup within Asilidae.

Life Cycle

Birth 200 larvas
Lifespan 2 years

Lifespan

In the Wild
0.3–4 years
In Captivity
0.03–3 years

Reproduction

Mating System Promiscuity
Social Structure Solitary
Breeding Pattern Transient
Fertilization Internal Fertilization
Birth Type Internal_fertilization

Across robber flies, adults are largely solitary; males often patrol or defend perches/territories and mate opportunistically with multiple females. Females can mate multiple times, then lay eggs on or in substrates; there is no pair bond or parental care.

Behavior & Ecology

Social No formal group Group: 1
Activity Diurnal, Matutinal, Vespertine, Crepuscular
Diet Insectivore Varies by species and habitat; commonly medium-sized flying insects-especially other flies and hymenopterans (bees/wasps).
Seasonal Hibernates

Temperament

Predatory, highly opportunistic hunters; generally non-social and intolerant of close conspecifics.
Often territorial around prime perches; chases and brief fights occur, intensity varies by species.
Bold ambush-and-pursuit behavior; rapid attack flights and strong prey restraint are typical.
Size range across family: adults roughly ~3-40+ mm body length (smallest to largest members).
Lifespan range across family: adults typically weeks to a few months; full life cycle often ~1-3+ years depending on species and climate.

Communication

None known; only incidental wing buzz during flight and handling.
Visual cues dominate: posture on perches, orientation to intruders, and aerial pursuit displays.
Tactile contact during mating (grasping with legs) and during prey capture/handling.
Chemical cues likely used for mate recognition and/or oviposition-site selection; evidence varies among taxa.
Flight-path signaling (chases, intercepts) functions in territoriality and courtship without coordinated group behavior.

Habitat

Forest Rainforest Deciduous Forest Coniferous Forest Woodland Grassland Savanna Prairie Steppe Shrubland Desert Tundra Alpine Meadow Mountain Wetland Swamp Marsh Bog Coastal Beach Rocky Shore Urban Suburban Agricultural/Farmland Plantation +19
Biomes:
Tropical Rainforest Tropical Dry Forest Savanna Desert Hot Desert Cold Mediterranean Temperate Grassland Temperate Forest Temperate Rainforest Boreal Forest (Taiga) Tundra Alpine Wetland +7
Terrain:
Mountainous Hilly Plateau Plains Valley Coastal Island Riverine Rocky Sandy Muddy +5
Elevation: Up to 14763 ft 9 in

Ecological Role

Widespread predatory insects and important mid-to-upper-level arthropod predators across many terrestrial habitats (grasslands, deserts, forests, wetlands edges), with substantial interspecific variation in prey specialization and hunting microhabitat.

regulation of insect populations (natural biological control) stabilization of local food webs as generalist-to-semi-specialist predators indirect influence on pollination dynamics by preying on pollinators and, in some species, occasional nectar feeding support of biodiversity via trophic interactions (as both predators and prey for birds, reptiles, and larger arthropods)

Diet Details

Main Prey:
Other flies Bees and wasps Beetles True bugs Butterflies and moths Grasshoppers and crickets Dragonflies and damselflies Spiders and other small arachnids Small arthropods +3
Other Foods:
Nectar Honeydew and other sugar sources

Human Interaction

Domestication Status

Wild

Robber flies (Asilidae) are wild predatory flies that have not been domesticated. People mainly meet them by chance—hunting along sunny trails, fields, gardens, or forest edges—or study them for science. They help control pest insects but sometimes eat pollinators (including honey bees) and can be a nuisance near apiaries or yards.

Danger Level

Low
  • Can bite/puncture skin if mishandled or trapped against the body; pain can be sharp and immediate
  • Localized redness, swelling, or itching; rare secondary infection if the wound is contaminated
  • Potential allergic reaction is possible but uncommon
  • Not aggressive toward humans; bites are typically defensive/accidental rather than predatory

As a Pet

Not Suitable as Pet

Legality: Generally legal to keep in many places, but collecting wild insects may be restricted in protected areas, parks, and some jurisdictions; permits may be required for collecting/transporting native wildlife, and international movement can be regulated. Captive-bred individuals are uncommon.

Care Level: Expert Only

Purchase Cost: Up to $50
Lifetime Cost: $20 - $300

Economic Value

Uses:
Ecosystem services (biological control) Agriculture/gardens (pest suppression; occasional conflict near apiaries) Scientific research and education Biodiversity monitoring and conservation
Products:
  • Natural predation on insect populations (non-commercial ecosystem service)
  • Pinned/museum specimens for taxonomy and education
  • Field-guide, photographic, and educational value

Relationships

Predators 8

European bee-eater
European bee-eater Merops apiaster
Eastern kingbird
Eastern kingbird Tyrannus tyrannus
Barn swallow
Barn swallow Hirundo rustica
Common wall lizard Podarcis muralis
European mantis Mantis religiosa
Black-and-yellow garden spider
Black-and-yellow garden spider Argiope aurantia
Common green darner Anax junius
American bullfrog
American bullfrog Lithobates catesbeianus

Related Species 5

Mydas flies Mydidae Shared Order
Stiletto flies Therevidae Shared Order
Window flies Scenopinidae Shared Order
Bee flies Bombyliidae Shared Order
Dance flies Empididae Shared Order

Ecological Equivalents 5

Animals that fill a similar ecological role in their ecosystem

Dragonflies
Dragonflies Anisoptera Aerial, visually guided predators that often capture other flying insects on the wing. They use a similar sit-and-sally hunting strategy around sunny openings, though they are not true flies and have different capture anatomy.
Praying mantis
Praying mantis Mantodea Ambush predators that hunt from perches and seize passing insects. They occupy an analogous predatory role in terrestrial insect communities but are primarily ground- and vegetation-based rather than engaging in sustained aerial pursuit.
Assassin bug
Assassin bug Reduviidae Piercing-sucking insect predators that subdue prey with injected saliva. They occupy a similar functional role as invertebrate predators and sometimes share prey; they are true bugs and often hunt on surfaces.
Sand wasps Bembicini Fast, diurnal predators/parasitoids that capture insects (often flies) in open, sunny habitats; they overlap ecologically in prey-capture behavior and habitat use despite being Hymenoptera.
Tiger beetle
Tiger beetle Cicindelinae High-performance insect predators in open habitats; similar trophic niche (insect hunters) and reliance on speed and vision, though they are beetles and primarily hunt on the ground.

Types of Robber Flies

12

Explore 12 recognized types of robber flies

Hornet robberfly Asilus crabroniformis
Red-footed cannibalfly Promachus rufipes
Yellow bee-mimic robber fly Laphria flava
Giant hanging thief Diogmites neoternatus
Efferia aestuans robber fly Efferia aestuans
Giant robber fly Promachus hinei
Bumblebee robber fly Laphria bomboides
Slim robber fly Leptogaster cylindrica
Black robber fly Dioctria atricapilla
Spiny-legged robber fly Machimus atricapillus
Dasypogon robber fly Dasypogon diadema
Narrow-headed robber fly Holcocephala fusca

Summary

Robber Flies are flies that are known for their aggressive predator habits and can attack their prey mid-flight. They are found in abundance across the world and belong to a diverse insect family. Robber flies get their name because of their hostile behavior while attacking prey. They hide on their perches and suddenly attack prey by pouncing on them as soon as they get the chance. They also imitate bees or wasps to prey on other insects. 

Robber Flies Species, Types, and Scientific name

The Robber Flies belong to the family Asilidae and are of the order Diptera. They belong to the broad group of species that pounce on their prey mid-air. People often refer to these flies as assassin flies because of their predatory behavior. 

They are sometimes mistaken for biting flies or horseflies, but this is a misconception. Robber Flies may be aggressive to their prey, but they do not drink blood and they are not predators of humans. However, if you provoke them, they might bite in self-defense.

Robber flies are spread around the whole world and include more than 7000 species. Of this number, 1000 are only found in North America. This species is still being observed and new facts are being discovered even today. The currently accepted sub-families are as follows:

  • Asilinae
  • Bathypogoninae
  • Brachyrhopalinae
  • Dasypogoninae
  • Dioctriinae
  • Laphriinae
  • Leptogastrinae
  • Ommatiinae
  • Phellinae
  • Stenopogoninae
  • Stichopogoninae
  • Tillobromatinae
  • Trigonomiminae
  • Willistonininae

Appearance: How To Identify Robber Flies

Robber fly

Robber flies are aggressive and predatory. Those qualities are why they’ll also often be referred to as Assassin Flies.

©Seema Swami/Shutterstock.com

Eggs

The female Robber Flies lay their whitish eggs on low-lying grasses or in the crevices of bark or soil. The nest depends on the type of abdomen that the female possesses. Depending on its morphology, the female may lay the eggs away from the nest, within the soil, or in cavities of plant cells. Amounts of eggs laid are unknown.

The eggs carry micro sculptures that are only visible under observation through a microscope. They are typically either oval or spherical but are always almost 0.07 inches long. 

Larva

The eggs hatch into a larva that is white or yellowish. The larva is predatory and may feed on eggs, other larvae, and small insects to survive. They migrate in the winter to pupate under the soil and emerge as grown adults. The total time for a larva to become an adult takes one to three years but their growth as larvae is accelerated in the warmer regions.

Adult

The Robber Flies measure up to 0.39 to 0.59 inches in length, making them the largest of all flies. Many Robber Flies are long and slender, with tapered abdomens similar to damsel flies. Some of the species are short and stout, resembling bumblebees.

The Robber Flies have beak-like heads that are pointed and stiff. This pointy head serves as a weapon. It is first used as a dagger to kill, then as a needle to inject the poison for killing the prey, and finally as a straw to gobble up the broken-down food. They have hairy bodies and their mouth parts are also covered in a tuft of hair. 

Most of the Robber Flies are black or brown. Sexual dimorphism is also quite visible but not to an extreme extent. The females have a broader abdomen to carry eggs after mating. 

Another interesting characteristic is their eyes. One compound eye is present on either side of its head and can see two different fields of view. This allows Robber Flies to observe a different stimuli at the same time. 

The Robber Flies have short and hairy antennae and long and slender legs. Their six legs have fleshy pads on their ends and are great for holding down prey. You can typically spot 6 to 8 hairy segments on their bodies but sometimes the last segments are obscured. Robber Flies have only one pair of wings.

Behavior

The behavior of the Robber Flies is both aggressive and predatory. They can eat any flying insect they see, which may be the reason why their population can be large in a very small area. 

Their mating behavior is quite effective. A female judges their partner and observes whether the male fellow is worthy. If they do not find the male to be a good mate, they pretend they are dead. Finding the female unresponsive, the male drops the female and flies away in search of another. 

Habitat: Where to find Robber Flies

The Robber Flies have a cosmopolitan distribution. This means that they are either found all over the world or in most parts of the globe. There are over 7000 species of this particular insect currently discovered world-wide, however, they are not found in Antarctica. 

Their population rate is quite high. They can spread quickly and are often found in huge numbers in small areas. This is likely due to their eating habits which suggest they can eat up to 1 to 2% of the flying insects in the region they have inhabited. 

More specifically, you can spot them in open areas so they can fly to hunt and then grasp their prey. Any open areas like grasslands, fields, ponds and streams, marshes, and crevices in woody areas are some examples. They usually have a perched area that allows them to give space to eat their prey safely after hunting. 

Diet: What do Robber Flies Eat?

The Robber Flies are aggressive eaters and predators. They can sight their prey from afar and grasp them in mid-air for a fulfilling meal. They have a needle-like head that injects venom to subdue the prey. This venom has an enzyme that can break down the body of the victim, allowing Robber Flies to digest their food easily.  

What Does Robber Fly Eat?

Their larvae eat other’s eggs, larvae, and even small insects. As for the adults, they can eat any flying insect in sight. These include beetles, grasshoppers, butterflies, bees, wasps, and even hummingbirds. 

How Much Do Robber Flies Eat?

They can easily consume one to two prey a day. 

What Eats Robber Flies?

Their predators include lizards, birds, bats, large spiders, frogs, etc.

Prevention: How to get rid of Robber Flies

The Robber Flies are not dangerous to people. They do not attack humans and are beneficial to the environment as they can help reduce pest populations. However, it is better to be careful around them because if provoked or mishandled, they might bite.

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Sources

  1. MDC / Accessed August 1, 2022
  2. Colorado Arts and Science Magazine / Accessed August 1, 2022
  3. UFL / Accessed August 1, 2022
  4. Wikipedia / Accessed August 1, 2022
  5. Wisconsin Horticulture / Accessed August 1, 2022
  6. Britannica / Accessed August 1, 2022
  7. Australian Museum / Accessed August 1, 2022
  8. NCSU / Accessed August 1, 2022
  9. Aggie Horticulture / Accessed August 1, 2022
  10. Your Conroe News / Accessed August 1, 2022
Alan Lemus

About the Author

Alan Lemus

Alan is a freelance writer and an avid traveler. He specializes in travel content. When he visits home he enjoys spending time with his family Rottie, Opie.
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Robber Flies FAQs (Frequently Asked Questions)

They are dangerous predators but are not harmful to a human. They may, however, attack if not treated right.

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