I first came across Jackal or Free-loader Flies when I was photographing a Misumena vatia crab spider that had caught a Honeybee (Apis mellifera). As I was taking pictures, I noticed several tiny flies alighting on the Honeybee. Intrigued, I consulted a knowledgeable friend who told me that they were almost certainly a species of Desmometopa colloquially known as ‘Jackal Flies’, come to feast on the juices of the poor bee. It is thought that, when captured, the bee gives off an ‘aargh, I’ve been caught’ pheromone to warn other Honeybees to stay well clear or suffer similar consequences. The Jackal Flies home in on this chemical signal to get a free meal, hence the common name and the alternative moniker of ‘Free-loader Fly’.

Several species of fly are known to exhibit this behaviour with most being in the family Milichiidae (which includes the Desmometopa spp). Banqueting alongside a top-notch predator like a spider seems an unlikely approach for a fly to adopt but the ones I watched looked to be in no danger whatsoever. The spider was busy hanging on to its prey as the venom took affect and the flies were so tiny in comparison that they were probably not worth the spider contemplating as a source of food although they may well merit nuisance value. I’m not sure I’d necessarily appreciate a cloud of flies around my dinner, unless it was a new or unusual species that is.

The second time I encountered them was while photographing a German Wasp (Vespula germanica) as it feasted upon the remains of an unfortunate Honeybee caught in the web of a Garden Spider (Araneus diadematus). This time I failed to actually spot the flies until I was looking at the photographs later. Social wasps regularly steal prey from spiders’ webs and in most instances that I have observed the target has been a Honeybee.

Usually the wasp cuts the bee in half with its sharp mandibles and flies off with a particular part, either the head or the abdomen. I have seen Hornets (Vespa crabro) doing this as well as German Wasps and Common Wasps (Vespula vulgaris). On one memorable occasion a wasp actually stole half a bee from a Misumena vatia, with the spider hanging on to one end of the bee while the wasp scissored through the ‘waist’ then flew off with the other end!

The next time I spotted a Honeybee caught in a Garden Spider web I was better prepared and on the lookout. I noticed some tiny flies hanging around in the vegetation supporting the web with a couple even sitting on the web itself (I assumed that they had got caught but they flew out and on to the bee once the spider had retreated). The spider quickly finished immobilizing the bee with some silken strands and then retreated to the edge of the web whereupon the Jackal Flies swarmed in to feast.

As more and more of the tiny flies converged upon the hapless bee, they began to make the bee spin and the web vibrate, alerting the spider to their presence. The spider ‘pulled’ on the strands of the web and started moving towards the bee, testing to see if something else had flown in.

It then moved rapidly onto the bee making most of the flies scatter in every direction but only to a short, safe distance. The few that remained quickly froze, sitting stock still on the bee. The spider sat guarding its prize briefly then went back to its retreat. It was a few moments before the flies slowly resumed feeding and for the ones that had flown off to return, seemingly aware that the spider was present and likely to come out and potentially catch them if they created too much movement.

More audaciously, I have even seen a Jackal Fly alight on the web of a large mature female Wasp Spider (Argiope bruennichi) to get a meal. These spiders are lightning fast at attacking and silk-wrapping any prey that flies into their web. So much silk is spun around the victim that it is impossible to tell what has been caught and it is pretty much impossible for anything to penetrate the all-enveloping silk cover. When the spider feeds, it has to tear a hole in the silk to expose the prey inside. At this point, any Jackal Fly wanting to dine alongside has to get perilously close to the spider’s jaws.

So a potentially risky but free meal, reliant on the interlopers being too small to be of significance, safety in numbers perhaps and applying a bit of stealth where necessary. A strange case of a spider being a fly’s best friend. 

Vanna Bartlett.

My thanks to Dr Tony Irwin for identifying the Jackal Flies and supplying information on their fascinating behaviour.