This Weird, Spiky Caterpillar Is So Good at Imitating Ants That Even AI Can't Tell the Difference

Homodes bracteigutta, a caterpillar that mimics weaver ants. Franz Anthony

When nature artist Franz Anthony found a caterpillar on a leaf rearing both its ends up at him, he thought it kind of looked two weaver ants. The color, the seemingly segmented body, the bulbs and butt-spots that look like ant eyes, and the leg-like protrusions from its body made it look like a pair of ants. But despite the effective mimicry, Anthony knew it was one long animal, and not an ant at all.

Anthony took a picture and uploaded it to iNaturalist, a website where you can report the different animals that you see, with a date and a location, to contribute to an archive of animal data. If you don't know the species, the website will use image recognition software to guess what it is.

Proving just how effective the animal is at pretending to be two ants, the insect even fooled the artificial intelligence, or AI, the website was using. iNaturalist responded to the uploaded image with "We're pretty sure it's in this superfamily: Ants and Wasps, Vespoidea." The top species suggestion was a green tree ant.

After some help from human Twitter followers, Anthony learned that the animal was actually the caterpillar form of Homodes bracteigutta. After its larval stage and before its moth stage, the H. bracteigutta mimics weaver ants. Predators are less likely to mess with weaver ants than caterpillars because weaver ants taste bad and have a stinging bite.

This caterpillar is one of three animals that mimic the weaver ant, along with two species of spider. Globally, at least several hundred spider species mimic other ant species.

Actual weaver ants. Troup Dresser on Flickr

Another good reason to mimic the weaver ant is to keep it from biting you. If a foreign animal walks among them, they might gang up and attack the larger insect. But you can enjoy the plants that the weaver ants live on if you blend in.

Newsweek tested different animals on iNaturalist to see how often it got animals right. The results were impressive for a computer program, but not quite perfect. Of 24 random animal images that happened to be on the writer's computer, the website correctly guessed the species, or at least the genus, of 17 of them.

The AI program correctly identified common animals like house cats and cattle, as well as obscure animals like an eyed click beetle and a by-the-wind sailor. However, the program misidentified a beluga as a stingray and a flying fox (bat) as a member of the Canis genus, with dogs.

iNaturalist did have some pictures of H. bracteigutta in its system to cross-reference, but the submission photos all depicted the animal in its moth stage. When we put another picture of the caterpillar in the system, it suggested the subfamily Alydinae, with the third species guess as weaver ants.

Image recognition software, or "computer vision," is a field still in development but advancing quickly. This caterpillar can only hope that Indonesian predators don't learn to use computers.