The chemical con trick used by Alcon blue butterfly larvae to get ants to take care of them has been detailed by scientists working in Denmark.
The Alcon blue grubs are well known for deceiving the ants into feeding them while letting their own brood starve.
David Nash and colleagues describe in Science Magazine how larval odours mimic those of the ants' own young.
The closer the hydrocarbon chemistry, the more successful the butterfly is in attracting the ants, they report.
"The caterpillars first start developing on a food plant but after they reach a certain stage, they leave the food plant and wait on the ground to be discovered by these ants," explained Dr Nash.
"They mimic the surface chemicals that the ants have on their own brood, and we've been able to show that the closer that mimicry, the faster they get picked up by ants and taken back to the ant nest and put amongst their own brood," he told the magazine's podcast.
"Once they are there, they become highly virulent parasites: they eat some of the brood and they will also get fed by the worker ants, and get fed in preference to the ants' own brood."
The researchers studied several sites in Denmark where the Alcon (Maculinea alcon) caterpillars develop on a marsh plant before Myrmica rubra and Myrmica ruginodis ants bring them into their nests.
By looking at the patterns of infection/resistance and the genetics of different populations, the team was able to describe how the separate chemistries of the butterflies and the ants co-evolve in what amounts to an ongoing "arms race" - giving each animal periods and locations of dominance in their relationship.
Jutland, and the island Laeso in particular, are among the last European strongholds of the Alcon blue. The scientists say a fuller understanding of the parasite-host relationship is required to help conservation efforts.
The study was led from the Centre for Social Evolution (CSE) at the University of Copenhagen.