Huge yellow-headed fly comes under scrutiny

With a large hairy black body, yellow head and fan-like leg ruffs, it was discovered to be a Tachina grossa, or yellow-faced fly.

Swiftly identified by the Guernsey Biological Records Centre, its gruesome habits were revealed.

‘This beautiful hairy fly is the giant tachinid fly. We call tachinids parasitic flies, but really they are parasitoids. Parasites don’t kill the host creature, but parasitoids usually cause the death of the host in some way,’ said Elizabeth Sweet, GBRC manager.

‘In the case of Tachina grossa this fascinating creature is both a pollinator and parasite.

‘The adults feed on nectar and pollen, but when it comes to reproduction, its habits are somewhat gruesome.’

Eggs are laid into lappet moth caterpillars for larvae to develop, giving a continuous fresh food supply.

Tachina grossa are native to the British Isles.

‘We have records going back nearly 20 years, although this species is under-reported. Due to their size, they have been mistaken for Asian hornets before.’

Cheryl Meerveld found the fly on Tuesday.

‘I spotted a huge, unusual fly in my garden in Torteval yesterday. I’ve never seen anything like it over here,’ she said.

Found buzzing against her window, it was roughly two to 2.5cm long.

‘I caught it in a glass and took photos, but released it soon after as it seemed to be suffering from the heat.’

Later she showed her partner, who had never seen one either.

‘He thought they might be migrating or have come up on warm winds. By coincidence, he spotted one as we sat on the beach at Saints Bay, it was flying over the rocks and landing every few feet.’

Intrigue surrounding the insect has been fanned following identification.

‘It’s very interesting to have it identified and learn of its behaviours. I thought only wasps were gruesome enough to lay their eggs in living host insects.’