The complexity of insect behaviours is a frontier we have barely explored. As conspicuous, charismatic creatures, butterflies get more attention than most, and yet there is still so much to discover.

Adult butterflies usually feed on nectar, although some, such as male purple emperors, enjoy the minerals from muddy puddles and animal turds. But milkweed butterflies in North Sulawesi, Indonesia, have been discovered to feed upon live, dead and dying caterpillars.

To produce mating pheromones that attract females, male butterflies may supplement their diet with other chemicals. Usually these are obtained from plants but the milkweeds have been observed scratching caterpillars and apparently imbibing their juices with their proboscis.

“The caterpillar larvae would contort their bodies rapidly in what appeared to be futile attempts to deter the scratching,” said Yi-Kai Tea from the University of Sydney, who observed this never-previously-reported behaviour with colleagues.

Even stranger is the fact that the milkweed butterflies are feeding off similar caterpillars to their offspring – not their own species but from their own subfamily.

Like all good scientific discoveries, this “kleptopharmacophagy” – chemical theft – only raises further questions.

Does the scratching actually kill caterpillars or were the carcasses already dead? And do only related species supply the relevant chemicals? It will be fascinating to find out.