Fan-bristled Robberfly

Fast Facts

Latin name: Dysmachus trigonus

Notable feature: A distinctive medium sized species of robberfly with a faint pattern of dark broad-based triangles down its back and grey dusted wedge-shaped spots at the side, along with prominent bristles on the thorax and sides of the abdomen.

Conservation Status: Not Evaluated

Where in the UK: Appears common and widespread in England and Wales. In Scotland only found in a handful of locations, most notably Tentsmuir Forest in Fife and in Ardeer and Shewalton Sand pit in North Ayrshire. Recorded also along the Solway coast.

Fan-bristled Robberfly (Dysmachus trigonus) © Steven Falk


The Fan-bristled Robberfly (Dysmachus trigonus) is a native, medium-sized robberfly.

Robberflies are members of the Family Asilidae.

Like others in the family, the Fan-bristled Robberfly is rather long in appearance. All species in this family share the same defining characteristics of a relatively small head with a deep notch on the top that separates a pair of large eyes and a noticeable bristly ‘moustache’. Legs are almost entirely black, the tibiae being only narrowly orange at the base.

Robberflies are often seen settling on paths and are easily disturbed when walking, usually flitting off ahead a few feet before alighting again.

    • Size: Body length 12-17 mm; wing length 8-11 mm
    • Life span:  Bi-annual life cycle or longer; adults can be found May to mid-September. Larvae can take 2-3 years to fully develop (depending on environmental conditions as well as the species), and pupal stage can take 2-6 weeks.
    • Diet: The Fan-bristled Robberfly is a predator, feeding on other insects.  It has been recorded feeding on moderately large flies including Stiletto Flies (genus Thereva), Cluster Flies (genus Pollenia) and Blowflies (genus Calliphora), as well as other species of Robberfly, and even beetles!  They lie in wait for prey and launch a “capture-dart” approach.  An important adaptation is the presence of stout spines and long hairs which form a catching basket suspended below the fly.
    • Reproduction:  The female has a broad, flattened blade-like ovipositor (potentially designed to lay eggs between plant tissue). Robberflies have limited or minimal courtship behaviour, with males pouncing on the female simply as if to catch prey. There is not much information regarding reproduction specifically in relation to the Fan-bristled Robberfly.
    • When to see: Adults can be seen during the day from May to mid-September with a peak in population in June and July.  Most likely to see it resting and less active in morning and evening when it is cooler.
    • Population Trend:  One of the most widespread species of robberfly the Fan-bristled Robberfly is mainly a southern species; there are fewer records from Scotland. Due to habitat requirements, it can be confined in area – often to quite discrete parts. It is a difficult species to find because it is well camouflaged, wary and will elude sweeping nets except in cooler weather.
    • Threats: Loss of habitat – in Wales, Northern England and Scotland it is found almost exclusively on the coast; associated with fixed dunes and found along sparsely vegetated paths and grasslands.  At sites inland, it is restricted to sandy soils such as those on heaths.
    • Fun Fact:  This is a powerful species of robberfly. It has been recorded feeding on the Dune Robberfly (Philonicus albiceps) as well as on dung beetles and even spider-hunting wasps (Pompilidae species)!

How you can help: 

Buglife is working to increase awareness of invertebrates and the Fan-bristled Robberfly through specific projects and campaigns such as “Ardeer Peninsula – Scotland’s bee haven” but we need your help!

Join a recording scheme and log your finds – get involved with the Soldierflies and Allies Recording Scheme or download the iRecord app and get recording!

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