Introduction

Robberflies are members of the Asilidae and this is one of my favourite groups of flies. They are generally rather elongate, with many of them being large and conspicuous while others are seemingly delicate and elusive. All members of the family share the same defining characteristics of a relatively small head with a deep notch on top separating the large eyes and a noticeable bristly ‘moustache’. They also have long legs that give them a gangly appearance, often with spines on them which help them grasp their prey, much like the ‘basket’ on the front legs of dragonflies. 

All are fearsome predators and have piercing mouthparts that deliver the most potent toxin known of any fly (although, reassuringly, harmless to humans!). Most species hunt their prey by taking up a suitable perch and then sitting on high alert, the highly mobile head tilting and turning on the look-out for anything that might stray within range, and then darting out to capture it. Prey items vary depending on species but consist largely of other flies (including robberflies in some instances) but also parasitic wasps and sawflies, bugs, beetles and even the odd spider. Although they generally give the impression that they are just insouciantly lounging around, they will quickly dart away as soon as you get anywhere near them.

There are 28 species of robberfly in Britain with a reasonable number of them in Norfolk so I have been able to find and observe my fair share of them. Many species have a southern distribution as they prefer warm climates, although a number of species are only found in Scotland, notably the Bee Robberfly (Laphria flava), one of the most spectacular species which is confined to areas of Caledonian pine forest. Another magnificent species is the Hornet Robberfly (Asilus crabroniformis) which used to be found in Norfolk. This species requires old cow or horse dung for its larvae to develop in where they feed on the larvae of dung beetles. Changes in agricultural practice and the routine dosing of livestock with avermectins has probably led to its decline in Norfolk and other places too. 

Several of the larger species like sandy soils so in Norfolk the Brecks and coast offer ideal habitat. In fact the Breckland Robberfly (Machimus arthriticus), as its name suggests, is found only in the East Anglian Brecks. Another specialist species that can be found in Norfolk is the Dune Robberfly (Philonicus albiceps) which should be looked for around the coast where there are large areas of sand dunes. 

The vicinity of nice fresh cow pats is a good place to look for some of the larger robberflies as they hang around waiting for other flies to appear so that they can prey upon them. On one memorable occasion at Weeting Castle, I carefully stepped around some dog poo causing a cloud of greenbottles to fly up. Instantly a robberfly zoomed out of nowhere, snatched a greenbottle out of the air and landed on my leg with it and waited for its toxin to take effect, affording me superb views.

Typical Robberflies

These are the medium to large robberflies with stout bristly legs and a prominent moustache. They can be observed habitually sitting about waiting for prey or conjoined in copulation. Although many are a rather drab grey-brown they often have tessellated abdomens that show subtle patterns depending on the angle of light. They have exceptional eyesight and participate in capture-dart tactics to catch their prey, sitting primed and ready to dart out and catch prey either as it flies past or picking it off the ground.

Dysmachus trigonus

Dysmachus trigonus or the Fan-bristled Robberfly is found in dry, sparsely vegetated habitats, typically grassland and heathland and also coastal areas including sandy cliff tops. They often settle on paths so are easily seen as you walk along, usually flitting off ahead a few feet before alighting again. It is a distinctive robberfly with a nicely patterned abdomen and prominent bristles on the thorax and sides of the abdomen. The female is one of two species that have a broad, flattened blade-like ovipositor, the other one being Eutolmus rufibarbis. 

Eutolmus rufibarbis 

Eutolmus rufibarbis is a scarce species nationally but is relatively common in East Anglia. In fact I have recorded it several times in my garden in Norwich and also in my local cemetery. It is a large robberfly with a tessellated abdomen and all black femurs. The female has a wide, flattened blade-like ovipositor distinguishing it from most species other than Dysmachus trigonus. The male has a tuft of hairs on the underside of the abdomen similar to Machimus atricapillus but these are golden apically rather than totally black (hence the name Golden-tabbed Robberfly). I generally observe them sitting on low vegetation, often grass stems, from where they launch their attacks. Prey items usually comprise other species of fly. In my garden this includes Lucilia sp greenbottles, which can be rather ubiquitous some years.

Machimus atricapillus

Machimus atricapillus is called the Kite-tailed Robberfly from the projection of black hair tufts on the 8^th sternite on the underside of the male’s abdomen. It is a relatively common and widespread species and one that I regularly encounter in my local cemetery where I often see them sitting on gravestones on warm sunny days. The female lays her eggs in the flower buds of various plants. Once when I was checking Canadian Goldenrod for solitary bees, I disturbed a robberfly which I presumed was using the plant as a perch to catch prey from. I retreated and waited and the robberfly returned and I realised that it was a female ovipositing. After she had gone, I carefully searched the flowerhead but was unable to locate any eggs. They must have been secreted within the unopened buds.

Machimus cingulatus

Machimus cingulatus is relatively scarce in Norfolk but I have recorded it in my garden. It is appreciably smaller than the previous two species. There are five species of Machimus on the British list, all of which are rather similar and care is needed to distinguish them, especially the females. 

Neoitamus cyanurus

Neoitamus cyanurus, the Common Awl Robberfly, is named after the elongate tapering ovipositor of the female which she uses to lay her eggs into the buds of various trees.  This is a woodland species, often recorded from ancient woodland. Unlike other species that are seen low down, these are usually perched higher up on the leaves of trees and shrubs, often in dappled light on the edge of glades or pathways. It is an instantly recognisable species, having smartly bicoloured legs with black femora contrasting with orange tibia and dark tarsi.

Philonicus albiceps

Philonicus albiceps is the aptly named Dune Robberfly. This is a specialist species which is found almost exclusively on sand dunes around the coast. Although this restricts their distribution, they can be pretty common where they do occur. The female lays her eggs into the sand on the lee side of sand dunes between blades of marram grass, plunging her ovipositor deep into the sand and then using the bristles on the end of her abdomen to sweep sand back over the hole. As a large species with very stout bristly legs, Philonicus is capable of tackling larger prey items, including other species of robberfly.

Elegant Hunters

Robberflies in the genus Dioctria are rather elegant slender species. Some are associated with woodland whilst others are found in open grassland. Many of them take sawflies and parasitic wasps as a large proportion of their prey. 

Dioctria baumhaueri

Dioctria baumhaueri, the Stripe-legged Robberfly, hunts form perches on shrubs or trees around 1 to 2 metres up. It preys largely on parasitic wasps so ivy and oak are good hunting grounds for it. It is by far the commonest species I have found in Norfolk, accounting for most of my records for this genus, although that could be because I have found it in my garden and local cemetery where I regularly hunt for invertebrates!

Dioctria atricapilla

While Dioctria baumhaueri hunts from a high perch, D. atricapilla is more of a grassland species so should be looked for low down amongst plant stems. I have often found it along wide woodland rides and in the older, more overgrown area of my local cemetery where there are thick stands of Cow Parsley. It can also be found around rough field margins and hedgerows.

Dioctria rufipes

Dioctria rufipes is known as the Common Red-legged Robberfly and is primarily associated with grassland but can also be found around scrub and along woodland edge. It has distinctive red fore and mid legs contrasting with black hind legs. Considering that it is a widespread species it is odd that I have only ever encountered it once.

Dioctria linearis

Dioctria linearis is a rather small species that hunts low down and could easily be overlooked. It is usually found in woodland where it hunts from low perches along the edge of rides or glades, usually in dappled shade where it can be difficult to spot. It is easily recognised from the yellow fore and mid legs and by the presence of yellow patches on the sides of the dark abdomen. They are reported to take mainly small flies.

Choereades marginatus

This species looks very like one of the larger Dioctria and could be mistaken for D. atricapilla except for the golden hairs on the abdomen which give it the common name of Golden-haired Robberfly. It is quite exquisite and rivals D. atricapilla in the beauty stakes. It is another woodland species, hunting from a high perch from sun-dappled leaves around rides and clearings.

Leptogaster cylindrica and Leptogaster guttiventris

Leptogaster species are very slender delicate robberflies that look more like a small damselfly or even cranefly, with rather weak flight. They hunt by flying low amongst grass stems, almost drifting along aimlessly until they find some prey to snatch. For all this apparent weakness, they are still competent predators and have the same capture-dart skills and toxic saliva as their more robust relatives. 

There are two species of Leptogaster in Britain which are told apart by the colour pattern of the abdomen. Leptogaster cylindrica, the Slender-striped Robberfly, has a continuous dark stripe down the length of the abdomen whereas Leptogaster guttiventris has a broken line which is why it is called the Dashed Slender Robberfly. It isn’t always that easy to see whether the line is dashed or not, especially in photos. L. cylindrica is by far the commonest of the two although both are probably under-recorded due to their inconspicuous nature.

Lots more to learn

There are still a few Norfolk species that I haven’t seen and if I ever get up to Scotland again then there are Caledonian Pine Forest specialities to look for but I am just as interested in trying to observe more about the species that I regularly see, especially those in my garden. There is still a lot of detail to be learnt about different aspects of the life of these lovely charismatic flies including what they prey upon, if only they let you near enough to see!

References

‘British Soldierflies and their Allies’ by Stubbs and Drake is the work for the British fauna and includes a wealth of information as well as keys to all species with photos of museum specimens of most of them. Baldock and Early’s ‘Soldierflies, their allies and Conopidae of Surrey’ is a great introduction, especially if you are in southern half of England, and has the advantage of photos of live specimens. I started off with it and it spurred me on to buy Stubbs and Drake. A very useful addition is ‘Field Guide to the Robberflies of the Netherlands and Belgium’ which covers all the British species and has excellent photos taken in the field with the added bonus of being written in English! I find the combination of all three books works well for me. They are all available from NHBS.

Steven Falk’s Flickr pages are very useful and have excellent photos of live and pinned specimens to aid identification.