When the relationship starts out, it seems to be more of a two-way street. Multiple ant species - even those known as bullet ants, which deliver the world's most painful sting - offer bodyguard duty while the caterpillars give the ants a nutritious "protein shake" of amino acids and sugars through a specialized body part called the tentacle nectary organ.
Caterpillars from the same family, called Riodinidae, even lure ants by singing to them with a special vibratory organ. (The caterpillar songs are too quiet for humans to hear them without specialized equipment.)
But as adults, the butterflies become freeloaders. The butterflies sport bright-red dots on their wings - a pattern that mimics stinging ants - allowing them to disguise themselves as ants and avoid predators, Pomerantz said.
"The butterflies aren't all that skittish; they just hang out in the open, and that's uncommon for a lot of butterflies," Pomerantz said.
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Even worse, the butterflies physically block the ants from feeding on the bamboo sap, hoarding all the goo for themselves. Meanwhile, researchers are unsure whether the ants get anything out of the relationship.
It's not clear why the adult ants tolerate this thievery, but one possibility is that the ants simply can't figure out what's going on. Ants have poor eyesight and typically communicate with each other via chemicals such as pheromones.
"Over evolutionary time, a lot of critters have figured out how to hack their chemistry so they can hang out with them," Pomerantz said, referring to ants.
Therefore, it's possible that the caterpillars continue to release "come hither" friendly pheromones even as they mature into adults, tricking the ants into tolerance, the researchers speculated.
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