The curious case of the ant and the Large Blue butterfly

This edited article about the Large Blue butterfly originally appeared in Look and Learn issue number 709 published on 16 August 1975.

The ants’ hidden nursery for the Large Blue butterfly by R B Davis

The little caterpillar, not much more than 1/16in. (1.5mm) long and dirty white in colour, might have been mistaken for a maggot as it made its way across the downland turf. It had just left a clump of wild thyme on which it had been feeding ever since it hatched from an egg some three weeks earlier.

It appeared to wander aimlessly until a passing ant spotted it and darted excitedly towards it. The ant caressed the tiny larva with its antennae and the caterpillar responded by exuding a sweet liquid like honey from a gland on its back. The ant consumed this liquid greedily.

After a time the caterpillar hunched itself and this seemed to be a signal for the ant to seize it in its jaws and carry it off to the communal nest below ground.

An observer might have thought that now the caterpillar’s doom was sealed but astonishingly, by an instinctive agreement, it was allowed to feed on the small grubs (larvae) of the ants in return for the regular ‘milking’ it provided. The caterpillar’s strange diet was not so remarkable as it might appear, for it had been a cannibal when very young – sometimes varying its diet by making a meal of its smaller and weaker brothers and sisters.

It grew rapidly on the unlimited supply of food which surrounded it until the approach of winter, when it went into hibernation.

With the coming of spring the caterpillar again began feeding on more ant larvae until mid-summer. Then it stopped eating and the fully-grown caterpillar became transformed into a shiny, brown chrysalis.

Three weeks later a rather crumpled creature broke out from the chrysalis case and made its way from the pitch darkness of the nest, through narrow underground passages, to the bright light of day. Here, in the sun, its wings expanded to nearly 2in. (5cm) across and it was revealed as a beautiful butterfly, called the Large Blue.

The mystery of this astonishing life history was solved soon after the First World War when a small ant colony was induced to inhabit the bottom half of a walnut shell, with a caterpillar as its guest. By raising the top half of the shell at intervals the behaviour of its inhabitants could be observed.

The men responsible for this discovery were the naturalists F. W. Frohawk and E. B. Purefoy, who for the first time, bred the Large Blue in captivity.