Eye on nature

Each autumn, at the end of October, we find caterpillars of the large white butterfly climbing the walls of our house until they find a niche in an overhang such as a window sill or behind a facia. This year one of the caterpillars stopped on a window as if it had run out of puff for the climb. It finally made it to the window frame where it stayed for two days. Then, on the third morning, we found that several dozen little "maggots" were emerging along the length of the caterpillar. By the following morning they had spun themselves little fluffy yellow cocoons and were arranged in rows around the corpse. I take it they were the larvae of an ichneumon wasp. Out of seven caterpillars we found six were parasitised, a very stringent control, and there must be other parasites and predators.

Ciaran O'Keeffe, Glenveagh National Park, Co Donegal

They were the larvae of the ichneumon fly, Apantales glomeratus, which lays its eggs in the body of the caterpillar of the large white butterfly. However, a smaller ichneumon fly, Hemiteles nanus, lays its eggs through the skin of the caterpillar and parasitises the larvae of Apanteles. Hemiteles, in turn also has a parasite, a small chalcid wasp . . . "and so ad infinitum". A lepidopterist has calculated that out of every 10,000 caterpillars, only 32 butterflies survive.

Edited by Michael Viney, who welcomes observations sent to him at Thallabawn, Carrowniskey PO, Westport, Co Mayo. e-mail: viney@anu.ie