Members of the Conopidae are some of my favourite flies, so much so that two out of the fifteen species depicted on F is for Flies in ‘Arthropedia: an illustrated alphabet of invertebrates’ are members of this family, one of which is further favoured with a line drawing in the same chapter.

Introduction

So what’s so great about these flies? They are an interesting bunch both in looks and lifestyle. Several superficially resemble wasps, a bit of Batesian mimicry going on here where a harmless creature resembles one armed with a sting. Others look decidedly odd, some might even say deformed, with a hunched back and oddly curled abdomen. They are all parasitoids that target various aculeate Hymenoptera, ie bees and wasps.

In Britain there are 23 species of Conopid flies from seven genera. They are split into two subfamilies based on morphological characteristics, the Conopinae and Myopinae. Collectively they are also known as Thick-headed Flies and have been given the attribute of bee or wasp grabber from the way they pounce upon their prospective hosts.

The larva develops inside the living host, feeding on the haemalymph (the insect equivalent of blood) and eventually killing it. When the host dies the fly larva pupates inside the dead husk, passing the winter in a ready-made shelter and the adult fly emerges the following year. As well as flower-rich habitats like meadows and chalk grasslands, Conopid flies can be encountered in woodlands and wetlands, anywhere that their hosts are to be found.

The Myopinae

The strangest looking ones are the Myopa and Sicus species. When I first encountered one I thought it looked quite sinister which does rather go with their lifestyle. Sicus ferrugineus in particular has a distinctive hunched appearance (think Shakespeare’s Richard the Third). Fortunately, this is one of the commonest species and as such even has a commonly used English name – the Brown Bee-grabber (or Bee-jumper). They are widely distributed around the country and can readily be seen visiting flowers for nectar. They also hang around flowers in order to intercept their preferred prey – bumblebees and some of the larger solitary bees, notably the Andrena mining bees.

Sicus ferrugineus can be encountered singly or ‘doubled-up’ either as a mating pair or, more likely, a male mate-guarding by continuing to piggy-back on the female after copulation to prevent another male attempting to mate. A female will often shadow a bee, following it around from flower to flower but you will be extremely lucky to see one actually attacking a bee. This generally happens in flight with the fly jumping on the back of the bee, levering apart the abdominal segments with the specially developed tip of her abdomen and inserting an egg. All done in the blink of an eye.

I have yet to witness this event despite hours of observation including watching one following a female Andrena denticulata around on Ragwort. The fly attempted to land on the bee which avoided it by dropping down underneath the flowers then quickly flying off to another plant.

The nine species of Myopa are quite similar to Sicus but the antennae are set higher on the head and they have comparatively smaller eyes. They are generally darker looking too, often with dark markings on the wings. Most are on the wing in spring and I generally see them when I am searching early flowering shrubs and trees for Andrena mining bees which are the primary hosts. Cherry Laurel has proved productive with flies often sitting on the leaves and I have also seen them visiting Sallow catkins.

In my experience they are more secretive and less often encountered than Sicus. Several of them are indeed rare and elusive and their hosts have yet to be discerned. They can be a difficult group to identify, even with an actual specimen to hand under the microscope.

Myopa buccata has several features that readily help distinguish it from other species which often allows it to be identified from photographs. These include the whitish cross-vein on the wing and the presence of dark dots behind the eyes. It is the commonest species in Scotland which is where I happily encountered my first one.

Thecophora atra is the diminutive little cousin and is easily overlooked because of its small size. As such it is infrequently recorded and a second species, Thecophora fulvipes, is even scarcer. It is thought that various Halictine bees (Lasioglossum and Halictus species) are used as hosts. Like many of the other species it is most often encountered on flowers in good habitats for bees. Even so, I was pretty amazed when one turned up in my small suburban garden.

The Conopinae

Compared to the Myopinae, these are very striking flies with a long slim abdomen, very long antennae and, in most, a long proboscis. The largest genus with five species is Conops, four of which are similarly coloured black and yellow to mimic wasps. Subtle differences can often enable them to be told apart from good photos. Every black and yellow Conops that I have encountered thus far has turned out to be C. quadrifasciatus but I live in hope of eventually finding one or more of the others. None appear to be particularly common, even those that are relatively widespread. The larvae of four of them mostly develop inside bumblebees.

The fifth species, Conops vesicularis has a browner colouration with areas of yellow and black which make it a Hornet mimic. And it does indeed have the temerity to target Hornets (Vespa crabro) as its primary host. It is by far the rarest of the five and I was fortunate indeed to see one very briefly in a wood in Surrey while looking for Wood White butterflies.

Leopoldius signatus is a black and yellow wasp mimic that is a parasitoid of social wasps, particularly Vespula vulgaris and V. germanica. It is best searched for around Ivy flowers where its hosts readily congregate around this bountiful late supply of pollen and nectar, which is a vital food source for so many insects from bees to butterflies. It is similar to Conops spp but lacks the elongate proboscis and is on the wing much later in the year.

They were once considered quite scare but it turns out that, as is often the case, no-one was looking for them in the right way or place. With the arrival of the Ivy Bee (Colletes hederae) on our shores more naturalists started taking an interest in what was visiting Ivy flowers and records began mounting up. So not so scare after all then.

Two other species also occur; Leopoldius brevirostris which is on the wing in July and August and L. calceatus which is a recent addition to the British list first recorded in 2018. L. calceatus also haunts Ivy flowers and is presumed to use social wasps as a host. It is much darker than the other two Leopoldius sp and also differs in having a long proboscis like Conops. One I shall be on the look-out for come Ivy blossom time.

Physocephala rufipes is the elegant member of the tribe and resembles the Conops species with its slender abdomen and long antennae. It is an overall darker fly though, being reddish brown with only narrow yellow bands on its abdomen. It is one of the commoner Conopids and can be seen throughout summer and into autumn. I have seen them on dry heathland as well as wet flushes where one was visiting the flowers of Common Fleabane and Water Mint. They have occasionally turned up in our garden and on the allotment too. Look for them where bumblebees are plentiful.

A second species, Physocephala nigra, is much scarcer and is found mainly on Dorset heathland but also Heather moorland in Scotland with a few other scattered localities. Sadly it doesn’t occur in Norfolk and is another Conopid I have yet to see.

Vanna Bartlett.

Further Information

Discover more fascinating invertebrates in my book ‘Arthropedia: an illustrated alphabet of invertebrates’ published by Mascot Media.

For more information on Conopid flies and superb photographs of the other species see Steven Falk’s excellent Flickr pages.