Paleotrickery: A lengthy lineage for leaf-mimicking insects

For at least 47 million years, some insects have escaped predators by looking like foliage and moving like swaying leaves, a new fossil find suggests.

Many creatures elude predators by blending into their surroundings. But the 3,000 or so species in an insect group called the phasmids take camouflage to an extreme, says Sonja Wedmann of the Institute for Paleontology in Bonn, Germany.

Most modern-day phasmids have bodies and legs that look like sticks and twigs, but at least 37 known species are shaped like the tree leaves that they eat or frequent during daylight hours, she notes. To complete the deception, phasmids occasionally move back and forth to mimic the motion of a leaf or twig in the breeze.

Paleontologists have found precious few phasmid fossils, and they had never previously unearthed one of a leaf insect. Wedmann and her colleagues describe their discovery of such a fossil in a report posted online Dec. 29, 2006 and published in the Jan. 9, 2007 Proceedings of the National Academy of Sciences.

The 6-centimeter-long, almost-complete leaf insect was preserved in fine-grained sediments. They were laid down in a broad, shallow lake that formed about 47 million years ago inside a volcanic crater in what is now Germany. Portions of the creature’s antennae and legs are missing, but its abdomen is the size and shape of some fossil leaves retrieved from the same strata, says Wedmann. The genitalia of the fossil insect are almost identical to those of modern leaf insects, a sign that subsequent species changed little in the millions of years that followed.

“This creature has all the features you’d expect of a primitive leaf insect,” says Conrad C. Labandeira, a paleontologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C.

Unlike some modern leaf mimics, the newly discovered Eophyllium messelensis didn’t have flattened projections on its front legs that made them look like small bits of leaves. The legs, like those of most living phasmids, were slightly curved where they joined the body. When the modern insects aren’t on the move, they often extend their front legs forward, hold them together, and tuck their heads down into a position that helps them blend into their environment.

Many scientists suspect that ancient animals behaved quite differently than their modern-day relatives do, says Michael S. Engel, an entomologist at the University of Kansas in Lawrence. However, “fossils such as Eophyllium provide direct evidence to the contrary,” he notes.

Other fossils unearthed from the same rocks include remains of fish, birds, bats, and small primates called lorises. The shape and variety of fossil leaves collected suggest that the lake was surrounded by a rich tropical ecosystem—the same type of environment in which leaf insects are found today, says Wedmann.