When periodical cicadas surface after years underground, they don’t grope blindly for trees. They head for the shadows, researchers report March 20 in the American Naturalist.
A detailed analysis of Brood XIII cicadas — which spend 17 years developing in subterranean tunnels before emerging all at once — found that newly arrived, wingless nymphs use darkness cues to move with striking precision toward tree trunks.
Across dozens of recorded trajectories, the insects deviated only slightly from the most direct route. “They just zoomed in, marching toward the trees,” says Martha Weiss, an evolutionary ecologist at Georgetown University in Washington, D.C.
This near-direct movement, Weiss and her colleagues found, hinges on the cicadas’ ability to detect dark shapes against paler backgrounds in the dim evening light. That cue guides the nymphs to the vertical surfaces they must climb to become winged adults.
While working on the leafy grounds of Lake Forest College in northern Illinois in 2024, Weiss’s team temporarily painted over the compound eyes and simpler light-sensing organs of newly emerged nymphs. Without the contrast between light and darkness to guide them, most of the immature cicadas wandered aimlessly and never reached a trunk. In contrast, control nymphs with unobscured vision moved quickly and directly toward the nearby trees.
The researchers then put the nymphs through a visual-preference test, presenting them with a simple choice between lighter and darker targets. Much like a person instinctively heading for the dark outline of a doorway in a dim room, 28 of 32 insects crawled toward the darker surface. Only 4 moved toward the lighter option.
The result confirmed that it was indeed darkness guiding the insects, a behavior known as skototaxis.
In hindsight, cicada expert Gene Kritsky, an entomologist at Mount St. Joseph University in Cincinnati, says he has witnessed this darkness-seeking instinct as well. But the possibility of formally investigating it “didn’t dawn on me,” Kritsky says. The new study “fills in a blank with experimental evidence about a behavior that is so common that it usually goes unnoticed.”
Skototaxis exists across the insect world: Cicadas join crickets, beetles, ants, flat bugs and even swimming bees.
Earlier this year, entomologist Zach Huang from Michigan State University in East Lansing and colleagues reported that honeybees and mason bees stranded on water swim toward darker areas, using brightness differences to direct themselves toward dry land.
Like Weiss, Huang says he didn’t know about skototaxis until he studied the behavior. “I didn’t even know that the word existed.” But after reading the new research, Huang suspects skototaxis may be far more widespread than researchers have appreciated.
Many plants and animals, it seems, have learned the same simple lesson: When survival is on the line, following the shadows can be a bright idea.
