TAMBOPATA, PeruWhile wandering around the Amazon jungle during a rainstorm, entomologist Aaron Pomerantz observed a most unusual little drama: On a rain-splattered tree trunk, cryptic caterpillars were feasting on peculiar yellow bulbs, accompanied by ants that would periodically tap on the caterpillars’ rumps.
“That looks weird,” Pomerantz thought. Nothing about the scene made much sense. The bulbs, which he thought might be a fungus or fruit, resembled meandering pimply pinstripes on the tree’s bark. And the caterpillars, when thrummed by the ants, produced a droplet of liquid that the ants would then slurp.
Pomerantz snagged some of the bulbs and caterpillars and brought them to the Tambopata Research Center in Peru. He tried to rear the caterpillars into adults, but they all died. Then he tried contacting botanists and asking whether anyone knew what the strange yellow bulbs were.
“Most of their responses were, ‘I don’t know what that is,’” Pomerantz says.
He has now mostly solved the mystery, with the help of some colleagues. It turns out to be one of those classic relationships that appear when species evolve in tandem. The caterpillars, thought to be the species Terenthina terentia, have a special gland near their butts that secretes a nutritious nectar. “It’s this cocktail of good stuff that the ants like,” Pomerantz says, such as sugars and amino acids.
The ants, in turn, protect their barkeep from parasitic wasps and other critters that might enjoy a nice caterpillar feast.
It’s a relationship that’s quite common among the family these caterpillars belong to. “Their survival in nature without ant guards is very difficult,” says lepidopterist Andy Warren, senior collections manager at the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History.
But the yellow bulbs? Those are definitely strange. They’re neither fungus nor fruit, but a rare type of parasitic plant that lives inside particular trees. Once each year, the parasite Apodanthes caseariae emerges from hiding and pushes its yellow bulbs through the tree’s bark—perhaps just in time to feed the caterpillars.
“What this suggests to us is that there’s an evolutionary relationship between this butterfly, its caterpillars, and the parasitic plant,” Pomerantz says. And, of course, the ants.
Understanding those complex relationships is key for species conservation, Warren says, noting that one effort in England failed because “the ants got inadvertently snuffed out.” The butterflies disappeared, he says, while the food plants remained.
There’s one more chapter to the story in Peru: When Pomerantz first observed the yellow bulbs, he noticed a butterfly hovering around the pimpled tree trunk. If those hungry, hungry caterpillars are indeed T. terentia—which further genetic work should definitively answer—then when they emerge from their cocoons, they’ll have beautifully intricate wings bearing a bright yellow spot, perfect for blending in with the yellow bulbs.