Cacoxenus indagator

Cacoxenus indagator, a drosophilid fly, is a parasite of solitary bees. It particularly favours nests of the Red Mason Bee - Osmia bicornis (=Osmia rufa). Like many other solitary bees, the Mason Bee provisions her nest hole with pollen and nectar stores, contained in individual brood cells. Each cell is separated by a wall of mud and a single egg is laid on each food store. Cacoxenus indagator females use these brood cells to lay around eight eggs. The fly larvae consume the food store, depriving the bee larva of the food. The following Spring when the adult bees and flies start to emerge, some of the flies have a problem. The larger bees can break through the mud walls easily and this may make an exit hole for the flies too. Where this hasn’t happened, the flies break the barrier by pushing their heads into crevices in the mud and then inflating them. This is done by pumping haemopymph (the insect equivalent of blood) into their head. Even though other flies can use a similar mechanism for escaping their pupae, in Cacoxenus indagator, this is a much more powerful process.