On any summer day in the Foret de Preuilly you could see quite a few impressive robberflies. Many of them are Dasypogon diademia. 20 years ago we knew next to nothing about this relatively common predatory fly. Then Fritz Geller-Grimm spent the summer of 1995 on a German military training area making notes on their behaviour and life cycle. What he learned is fascinating. His report is here if you want to read it in full, but here are the highlights:
The sexes look different, with the male being plain black and the females having a red patch on the abdomen. They can have either red or black legs. Everything about them points to them being predators - the conspicuous beard, which acts to protect the eyes from struggling prey; the stabbing proboscis, possibly used to inject a poison and certainly used to suck their prey dry; long legs covered in bristles and claws to hold and manipulate prey. And their prey is formidable - wasps and bees, sometimes bigger than the flies themselves and armed with deadly stings.
Females lay their eggs in clusters in the sand. The clusters of eggs are protected by a cocoon covered in grains of sand, like a little sand pill less than 5mm across. It is thought the purpose of these unusual egg repositories is to protect the eggs from drying out and from predators. When the larvae hatch they remain in the ground, preying on beetle and bee larvae in the soil.