The caterpillar needs the ants

The caterpillars are parasitized by braconid wasps and tachinid flies. Alone, they are almost defenceless against these lethal parasites; but the tending ants will fight off parasites from their caterpillars. Pierce and her collegues experimentally prevented ants from tending caterpillars, and measured the rates of parasitism in the experimentally unprotected and in normally protected (control) caterpillars.

Their results showed that ants reduce the rate of parasitism in Glaucopsyche lygdamus . The ants and caterpillars are therefore closely adapted to each other; the ants gain food, and the caterpillars gain protection. They form a kind of interspecific coadaptation.

In the image opposite the ant is defending the caterpillar against a parasitic braconid wasp: the ant has seized the wasp in its mandibles.