Cephalotes atratus ants infested with a parasitic roundworm develop a bright red gaster—the rear part of abdomen—and hold it up high over their body.

"It's really conspicuous," said Steve Yanoviak, an insect ecologist at the University of Arkansas in Little Rock.

Researchers suspect the parasitic nematodes thin the hard outer layer of the gaster, which makes the normally black covering appear amber. The parasite's eggs are yellowish.

"When you combine those two effects in the sunlight, you get a nice bright red ant rear end," Yanoviak said.

• More Photos in the News
• Today's Top 15 Most Popular Stories
• Free Email Newsletter: "Focus on Photography"