Yesterday, stunning photos of a semi-translucent, gelatinous caterpillar spread quickly across the Internet—probably setting a new speed record for larvae of all kinds. Scuba instructor and amateur wildlife photographer Gerardo Aizpuru spotted the creature in early April on a mangrove tree leaf near Cancun, Mexico. He submitted his pictures to Project Noah, a user-created database of geotagged wildlife photos, where various commenters identified the species as the larva of a fuzzy orange moth called Acraga coa.
Although it's not 100 percent certain that the "jewel caterpillar" Aizpuru photographed is Acraga coa, it almost definitely belongs to the same family of moths, known as Dalceridae. Scientists have identified around 84 different species of Dalceridae moths, whose larvae are sometimes called "slug caterpillars" because they are so gooey. If you search for "Dalceridae" in Google Images, you'll see different larvae with the same roly poly bug shape and gumdrop spines, but different colors and patterns. Dalceridae larvae reminded me immediately of nudibranchs, a group of strikingly colored mollusks whose appearance is perhaps best summarized as "trippy."
Nudibranchs and many other animals—including many caterpillars—use vivid pigments to advertise their toxicity and keep predators away. But so far biologists have not figured out why some Dalceridae larvae are so colorful. Daniel Janzen, a biologist at the University of Pennsylvania, has raised Dalceridae in captivity. He says that although the larvae tend to be bright and conspicuous and spend a lot of time walking on the tops of leaves—as though they did not fear birds and other predators—he has no evidence that Dalceridae are poisonous and he knows that they do not sting, unlike some of their cousins.
Biologists do have some ideas about the function of larvae's gumdrop spines, however. The glutinous cones break off extremely easily—one can gently tweeze them off or even pull them off by accident—suggestive of the way some lizards' tails snap off in a predator's mouth. Janzen says this trick might help the larvae escape from hungry insects and birds, but researchers have not yet confirmed this. In one telling experiment, however, Marc Epstein—an insect biosystematist at the California Department of Food & Agriculture—and his colleagues placed Dalcerides ingenita larvae in glass Petri dishes and introduced a few ants (Camponotus floridanus) to each dish. Many ant species devour caterpillars and other plump grub if they get the chance. Once inside the Petri dish, the ants inspected the larvae with their antennae, but most backed off without trying to take a bite. The few ants that chomped down got their mouths temporarily stuck in the larvae's jelly coat or pulled away quickly and cleaned the gunk off their mandibles. In subsequent tests, Epstein found no evidence of toxic chemicals in the larvae's goo, suggesting that it deters ants purely because of its stickiness.
Perhaps "jewel caterpillar" is an apt name after all—you can look, but you can't touch.