I do not usually make an effort to identify the various flies that I encounter when looking for other insects. However, the other day I came across a large and rather distinctive fly, settled on a hawthorn. A quick flick with my butterfly net and the creature was soon potted up for closer examination at home, where I would be able to use my reference books to attempt an identification. My guess that the fly might prove readily identifiable turned out to be correct and, thanks to an old and rather battered copy of Colyer & Hammond’s “Flies of the British Isles”, I discovered that it went by the Latin name of Tachina fera and belonged to a group of flies with some rather unsavoury habits. Colyer & Hammond describe this group as containing some of the most handsome flies but with their hairy, almost bloated, abdomens I feel that the use of the word “handsome” gives rather more credit than is deserved.
Tachina fera is a rather common fly and may be abundant around water mint during late summer. The species, like other members its family, is parasitic upon the caterpillars of various moths and butterflies and has been recorded using autumnal rustic, pine beauty, black arches and large white, amongst others. Adult flies deposit their eggs in places frequented by suitable hosts and the larvae that hatch from these eggs search out and bore into a host. The parasite remains attached to the body wall of the caterpillar by its rear spiracles. These are the tubes through which it breathes. So, even though it is living within its host (and feeding on its body fluids) it still draws oxygen in from the outside. Other members of this group of flies may enter their host as eggs, being ingested as the host feeds on plant material. They then hatch within the intestine and ‘migrate’ to their final position. The presence of the parasite does not usually cause fatal damage straightaway. In fact, the host will typically continue to grow and develop as normal, with the parasite “creaming off” some of the resources being laid down. Ultimately, however, the parasite will need to pupate and emerge from its host and it is at this stage that death occurs.
While the discovery of such unpleasant habits was a little unsettling, Colyer & Hammond provided an unexpected twist to this tale of parasite and host. It turns out that adult flies in this family may be subject to parasitism themselves, typically by small wasps, and sometimes the fly larva, living inside its caterpillar host, may itself have a parasite living within it – a parasite within a parasite, a bit like a Russian doll.