Cattle had been sheltering in the lee of the hedge and had spattered the footpath with fresh cowpats. We had to be careful where we put our feet, but the male dung flies, Scathophaga stercoraria, sitting in the centre of these discs of ordure, had no such reservations; these were their courtship arenas. Each suitor, resplendent in golden hairs that glowed in the early morning sunlight, was waiting for the arrival of females – usually found to be in short supply.
Dung flies’ behaviour and their sex lives have intrigued evolutionary biologists. Males vary in size and females tend to choose the largest mates available on the cowpat, with multiple mating being the norm. It’s a situation where large males, which tend to be more fecund, should prosper.
The fly’s scientific name translates as “dung eater” but only its larvae feed in this way. Adults are agile predators, whose hunting prowess translates into rapid growth and a better chance of sexual success. We found some crouched on nettle leaves, darting out to catch small hoverflies in flight. When they returned they held their prey between their front legs, mantis-like, chewing them in the way we might eat hotdogs.
Whatever their size, hunting success or sexual prowess, most dung flies suffer the same fate. Ahead of us swallows were already skimming the grass, mouths agape, taking their fill of flies that rose from the cowpats and nettles.
The flies are also prone to the insect-eating fungus Entomophthora. We found several dung fly corpses stuck to the top of grass stems, their abdomens encased in this pathogen’s pink fungal hyphae.
These victims are death traps for other flies, which come to investigate and become contaminated with sticky spores that then germinate and penetrate the body of the new host.
As the pathogen proliferates and begins to digests the flies, they weaken, suffering a prolonged, gruesome, fate. In their final hours the flies become zombies as the fungus takes control of their nervous system and compels them to climb towards the light until they reach the top of grass stems – where they are conspicuously positioned for passing on the infection to another generation.
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