What Are Robber Flies?
Robber flies are members of the family Asilidae, one of the most predatory groups within the order Diptera (true flies). With over 7,500 species worldwide and approximately 29 species in the UK, they are formidable aerial hunters that capture other insects in flight, including prey considerably larger than themselves. Often described as the “birds of prey” of the insect world, robber flies combine powerful flight, exceptional eyesight, and a potent bite to subdue their victims.
The Name “Robber Fly”
The common name “robber fly” reflects their bold, aggressive hunting style. These flies will attack almost any flying insect they can overpower, including bees, wasps, dragonflies, grasshoppers, and even other robber flies. The scientific family name Asilidae derives from the Latin asilus, meaning “gadfly” or “horsefly.”
Identification
Robber flies vary considerably in size and appearance, but they share several distinctive features:
- Mystax – A dense tuft of bristles (the “moustache”) on the face between the eyes. This is the most characteristic feature of the family and serves to protect the face during struggles with prey.
- Large compound eyes – Widely spaced and prominent, providing excellent binocular vision for tracking prey in flight.
- Concave vertex – A depression on the top of the head between the eyes, creating a “hollowed-out” appearance.
- Strong, piercing proboscis – Short and stout, used to stab prey and inject saliva.
- Powerful thorax – Well-developed flight muscles give robber flies the speed and manoeuvrability to catch prey on the wing.
- Long, spiny legs – Used to grasp and secure prey in flight.
Common UK Species
| Species | Size | Appearance | Habitat |
|---|---|---|---|
| Machimus atricapillus | 12–18 mm | Grey-brown with dark markings | Grassland, heathland |
| Dioctria rufipes | 10–12 mm | Black with orange-red legs | Woodland edges, hedgerows |
| Leptogaster cylindrica | 9–12 mm | Very slender, wasp-like | Tall grassland, meadows |
| Dysmachus trigonus | 14–20 mm | Stout, hairy, grey | Sandy heathland, dunes |
| Neoitamus cyanurus | 12–16 mm | Dark with bluish sheen on abdomen | Open woodland, gardens |
Hunting Behaviour
Robber flies are ambush predators that combine patience with explosive speed. Their hunting strategy follows a consistent pattern:
1. Perching – The robber fly selects a prominent perch—a twig, leaf tip, stone, or bare patch of ground—with a good view of the surrounding airspace.
2. Detection – Using their large, forward-facing compound eyes, they scan for passing insects. Research suggests they can detect prey at distances of several metres.
3. Interception – When suitable prey is spotted, the robber fly launches from its perch in a rapid, direct flight to intercept the victim. The attack is typically from below and behind.
4. Capture – Prey is seized in mid-air using the fly's long, spiny legs, which form a basket-like cage around the victim.
5. Injection – The robber fly immediately stabs its prey with its short, powerful proboscis and injects a cocktail of neurotoxic and proteolytic saliva. This paralyses the prey and begins to digest its internal tissues.
6. Feeding – The fly typically returns to a perch to feed, sucking out the liquefied contents of its prey. The empty exoskeleton is then discarded.
Did you know? Robber flies are so confident in their hunting abilities that some species regularly attack and consume bees, wasps, and even dragonflies—insects that are themselves formidable predators or armed with stings. Their neurotoxic saliva acts so quickly that stinging prey is usually incapacitated before it can deploy its defences.
Lifecycle
The lifecycle of robber flies is relatively poorly studied compared to their dramatic adult behaviour. Females lay eggs in soil, decaying wood, or vegetation. The larvae are elongated, pale, and live in soil or leaf litter, where they are predatory, feeding on beetle grubs, other fly larvae, and assorted soil invertebrates. The larval stage typically lasts one to three years. Pupation occurs in the soil, and adults emerge in late spring or summer.
Ecological Importance
As apex predators within the insect world, robber flies play an important role in regulating populations of other insects. They are generalist predators that take a wide range of prey, and their impact on local insect communities can be significant. In ecosystems where robber flies are abundant, they may help to control populations of pest species such as flies and beetles.
Robber flies are also part of the food web themselves, serving as prey for birds, spiders, and larger predatory insects.
Conservation
Several UK robber fly species are considered nationally scarce or rare. Species such as Asilus crabroniformis (the hornet robber fly), one of Britain's largest flies at up to 28 mm, has declined significantly due to habitat loss. It is now largely restricted to grazed pastures in southern England and Wales, where it breeds in cattle dung. Conservation grazing and dung beetle habitat management benefit this impressive species.
Key Takeaway
Robber flies are among the most impressive predators in the insect world, combining acute vision, explosive speed, powerful grasping legs, and neurotoxic saliva to catch and consume a wide range of prey in flight. With 29 UK species and over 7,500 worldwide, the family Asilidae represents a fascinating and ecologically important group that deserves far more attention from naturalists.