The caterpillar that is fed on ant larvae
Watch how the ants follow the butterfly larva licking the irrestible juice off its abdomen, before carrying it off to their nest where they rear it on their own babies until it is ready to form a pupa.
Large Blues pairing. One of the UK's rarest butterflies.
It became extinct in 1979, but has been successfully re-introduced into the west country.
Large Blue female
The Large Blue female lays its eggs on the flowers of Wild Thyme, and on hatching a week or two later, the tiny caterpillars feed on the pollen and the seed of the plant. They develop quickly, passing through all the moult stages but do not put on any weight. At the last larval moult, still very tiny (weighing one milligram!), they develop a minute honey-gland so that they can produce a sweet liquid to attract ants, then flick themselves off the plant flower onto the ground where they are found by a particular type of red ant.
When an ant finds a larva, it drums on its body with its antennae, which is a signal for the larva to start producing honey. The ant gets very excited and goes off to find its friends. These come back to the larva and continue to milk it, licking and prodding its body with their antennae and jaws. But then they disappear, leaving the larva with the original ant which continues to milk it.
After a few hours of this, an extraordinary thing happens – the larva rears up which is a signal to the ant to take the larva into its nest. The larva already looks quite like an ant larva – it is the right size, has the right amount of hairiness – and it produces a chemical scent that the ants recognize on their own larvae. The butterfly larva tricks the ant into taking it into the brood chamber of the ant’s nest. Once it is in there, it feeds on ant larvae, eventually becoming much larger, so that when it is ready to pupate in May, it is 100 times bigger than it was when it originally arrived in the ant nest and may have eaten 1200 ant grubs! The Large Blue larvae even communicate with the ants by making squeaking sounds.
Listen to the call of a Large Blue larva inside an ants' nest, recorded by Chris Watson 2
Listen to the call of a Large Blue larva inside an ants' nest, recorded by Chris Watson.
When the adult butterfly emerges from its pupa in the ants’ nest in June/July, the ants continue to pamper it and protect it against predators as it makes its way out of the ant nest and up a plant stem to pump up its wings before flying away.
This very complicated life cycle makes that the Large Blue very dependent on one species of ant – if it ends up in the wrong ant nest, it will die!
If the ants that it relies on become reduced because of habitat change, then the Large Blue will suffer as well. When rabbit numbers crashed in the 1950’s as a result of myxomatosis, the thyme plants that the butterflies lay their eggs on became suffocated by rank grasses and the ants could not survive in the long turf because the sun could not reach their nests to keep them warm.
Changes in agriculture since World War 2 have led to many areas of grassland being ploughed up to create rye-grass pastures. The last colony of the Large Blue in the UK disappeared in 1979.
And now for the good news! The Large Blue has been successfully re-introduced into the UK from colonies in Sweden and is so far doing well on several west country sites.