It sounds like one of Aesop's fables but the story of the blue butterfly and the red ant is probably not one you would like to send your children off to sleep with. The caterpillar of the blue butterfly is "adopted" by the red ant which takes the butterfly larva back to its nest where it treats it like one of its own brood feeding it until it is old enough to turn into an adult butterfly.
The caterpillar, however, repays the ant's generous hospitality by greedily eating as much food as it can and gobbling up the ant's offspring as a tasty side dish.
All five species of large blue butterflies in Europe engage in this form of parasitism on red ants and now scientists have worked out the trick that allows them to do it the caterpillars cover themselves in a chemical that makes them smell like orphaned baby ants.
Researchers have found that the organic molecules secreted on the skin of the blue caterpillars closely match those on the skin of the red ant larvae. What is more, the closer the chemical cocktail is, the stronger the attraction of the ant to the caterpillar.
The findings should help conservationists in their attempts at reintroducing large blue butterflies which are endangered by making sure the chemistry of the caterpillars and the ants match each other as closely as possible.
David Nash of the University of Copenhagen, who led the study published in the journal Science, said the results were a vivid demonstration of the evolutionary "arms race" that can escalate between parasites and their hosts.
"Parasites are always trying to better adapt to their hosts to parasitise them whereas their hosts are experiencing selection pressures to avoid being parasitised," Dr Nash said.
"It means there can be an ongoing evolutionary arms race between the parasite and its host. There have been some previous studies on microscopic organisms showing this can occur in the laboratory but what we have here I think is one of the first cases where we have clear evidence that this has been happening out in the field."
The scientists studied dozens of red ant colonies on an island off the Danish coast and, in each nest they examined, they counted the number of caterpillars of the Alcon large blue butterfly that they found living there
Alcon blue butterflies lay their eggs on the marsh gentian plant and its caterpillars grow in the usual way by feeding on the plant's leaves. But, at the fourth stage of growth, the caterpillars gently lower themselves to the ground on silken threads.
"The caterpillars first start developing on a food plant but once they reach a certain stage they leave the food plant and wait on the ground to be discovered by one of these ants," Dr Nash said.
Often in nature, ants would make a meal of a caterpillar but in the case of the large blues, the passing ants pick them up gently and take them lovingly back to their nest the question was why?
"The butterfly gets into the ant nest by mimicking the surface hydrocarbons, the surface chemicals that the ants have on their own brood," Dr Nash said.
"They are producing this signal that says 'I'm an ant brood'. We've been able to show that the closer that mimicry is, the faster they are picked by the ants and taken back into the ant nest and put amongst the brood.
"Once they are there among the brood they become highly virulent parasites. They eat some of the brood and they also get fed by some of the worker ants, and they get fed in preference to the ants' own brood," he said.
Two species of Myrmica red ants were found to be routinely parasitised by the Alcon blue butterfly but the scientists also discovered that one of these species is far more heavily exploited by the blue's caterpillars than the other species of ant.
"We know that almost any ant within this Myrmica group of ants will pick up a caterpillar and take it back to the nest, but it is only within these two species that it will survive," Dr Nash said.
"We have one species where the ant is exploited at a relatively constant rate and we have another species where, when it is common, it is exploited even more than you'd expect but, when it is rare, it is hardly exploited at all," he said.
The difference in the exploitation rates between the two species of red ant helps the blue butterflies survive because if it relied on just one species there would be a danger of it overexploiting its only host. The second host species means there is a back-up in times of ant scarcity, Dr Nash said.
The large blue butterfly has a complex, parasitic relationship with red ants which involves its caterpillars being taken into the ant's nest and fed. The caterpillars manage this by mimicking the smell of the ants' own larvae
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