A pair of Kite-Tailed Robber flies at Gait Barrows
Pairs of butterflies or dragonflies are easy prey for the (digital) photographer; they seem wholly unconcerned with what may be going on around them. So it was a bit of a change when Betty spotted what turned out to be a pair of (true) flies in September, whilst on the Limestone Trail at Gait Barrows. It was not immediately apparent what they were until they were 'potted up' temporarily for id purposes. At that point, a shot was taken which fortunately turned out to be an acceptable one: we were too trusting that they might be static and amenable but, unlike butterfly pairs, they were off in tandem in a flash when the top of the tube was removed for a better view.
The general view of true flies as a group is highly coloured by their sometimes less desirable habits and, in some cases, their preferred habitats. They are frequently observed but not in enough detail for identification. The robber fly group had been only an illustration in a book and not 'real' to us; so it was a bit of a surprise to learn that what we had found at last made one of their species tangible. Without a specialist 'fly book', their identity remained unresolved, so we resorted to sending the picture to a couple of entomologists on email: Steve Hewitt, of the Tullie House Museum in Carlisle, who is always helpful, and Bill Hardwick, who is listed as the fly recorder for Cheshire (for the Lancashire one we could not locate an address). Their responses were in agreement and both are almost certain the species is Machismus atricapillus, but what was particularly fascinating was Bill's intricate and detailed analysis of both the advantages, and otherwise, of identification from photographs. His excellent response is reproduced here with his kind permission.
"You are correct - they are certainly robber flies, and are indeed unusual for your neck of the woods. Identification from the photograph, although a good one, is slightly tricky as the most clearly conclusive features are obscured for both sexes. The first bit is easy: from the general jizz and the extensively red legs, they can only be one of two species - Machimus atricapillus or M. Cingulatus. All other British robber flies are either of a completely different size and/or shape, and/or have totally black legs (or almost so).
In case you have retained the specimens, the male of atricapillus is easily distinguished by the presence of a distinct tab with a forked or rounded end on the underside of the 8th sternite (last major abdominal segment before the genital capsule). The female of atricapillus also has a characteristic feature on the same sternite: a pair of long bristles. Both of these features are absent in the corresponding sexes of cingulatus.
Assuming you have no voucher, the other characteristics that can be used to separate the two species include the colour of the hairs on the frons: these are entirely black in atricapillus (rarely one or two white), whereas they are about 50% white in cingulatus. Now, if you've not done much work on identifying flies, the next statement is going to sound odd. Unfortunately it is often easy to confuse black and white hair colouration in flies: much depends on the angle of view, the background, the angle of light and degree of reflection. What can seem white from one angle appears black from another! So this isn't the best thing to judge by in a photo where you can't rotate the specimen, move the light or change from a light to dark background.
The general colour female of atricapillus is described as being "darker" (a term that might be more helpful if one had reference material of both species to hand); that of cingulatus as "browner". Again similar lighting caveats apply.
The best remaining discriminatory character in the photo is in the female antennae. In cingulatus, the arista (the fine hair-like segment at the tip) is about half the length of the preceding thicker elongate segment. In atricapillus it is at least two-thirds the length of that segment. Using a magnifying glass and ruler, I reckon the arista on the left antenna (which is luckily almost in profile) is about 70% the length of the preceding segment. That points to atricapillus.
Body length doesn't really provide much corroboration: atricapillus is normally in the range 12-15mm, whereas cingulatus is slightly smaller at 10-13mm. Either could fit with your description.
Both species are on the wing at this time. Habitat preferences again favour atricapillus which frequents a wider range of habitats including limestone areas, chalk, dry clay grassland and heathland, usually on open grassland with shelter from either bushes or rather uneven ground. Cingulatus is a decidedly local species of dunes and sandy heaths. Both are mainly southern to midlands species, atricapillus being the more widespread and generally extending further north including the Peak District, but both have few known localities in northern England or Scotland. Cingulatus is much scarcer by the time you get as far north as the south midlands and is often absent from apparently suitable dunes even in its relative strongholds of Dorset, Hampshire and Surrey along the coast to East Anglia and the Brecks.
There are no previous records of cingulatus from your county on the NBN database and only two records for atricapillus, both in 1999, one from West Lancashire, the other from Cumberland.
All in all, then, I think it is a pretty safe bet that your flies are Machimus atricapillus - a good record for the Barrows. Cheers, Bill"
Betty & John Holding
Photograph by John.
