The Tachinidae are parasitic flies, the female laying her eggs on other living insect larva. The fly larvae then develops inside the living host, devouring it and eventually killing it. Its main hosts are the large hairy Lepidopteran caterpillars, particularly the Oak eggar moth (Lasiocampa quercus).
Tachina grossa below, now identified from my previous piece, resembles a bumble bee in flight. At 15 - 19mm it is one of the largest species of fly throughout much of its range and is the largest Tachinid in Europe. It is very distinctive, being hairy and having a black thorax and abdomen and a bright yellow head. It is found in woods and on heathland.