A shadow-detection skill known as skototaxy helps cicadas find trees to molt on
When periodical cicadas surface after years underground, they don’t blindly seek out trees. They head towards the shadowsreport the researchers on March 20 in the American naturalist.
A detailed analysis of Brood XIII cicadas – which spend 17 years developing underground tunnels before emerging all at once – we discovered that newly arrived wingless nymphs use darkness cues to move with striking precision towards tree trunks.
In dozens of recorded trajectories, the insects deviated only slightly from the most direct route. “They just zoomed in, walking toward the trees,” says Martha Weiss, an evolutionary ecologist at Georgetown University in Washington, DC.
This near-direct movement, Weiss and his colleagues discovered, relies on cicadas’ ability to detect dark shapes against paler backgrounds in dim evening light. This signal guides nymphs to vertical surfaces that they must climb to become winged adults.
While working on the lush green grounds of Lake Forest College in northern Illinois in 2024, Weiss’ team temporarily painted over the compound eyes and simpler light-sensing organs of newly emerged nymphs. Without the contrast between light and dark to guide them, most immature cicadas wandered aimlessly and never reached a trunk. In contrast, control nymphs with clear vision moved quickly and directly toward neighboring trees.
The researchers then put the nymphs through a visual preference test, giving them a simple choice between lighter and darker targets. Just like a person instinctively moving toward the dark outline of a door in a dark room, 28 of the 32 insects crawled toward the darker surface. Only 4 of them opted for the lighter option.
The result confirmed that it was indeed darkness that was guiding the insects, a behavior known as skototaxy.
Looking back, Gene Kritsky, a cicada expert and entomologist at Mount St. Joseph University in Cincinnati, says he also witnessed this darkness-seeking instinct. But the possibility of a formal investigation “didn’t cross my mind,” Kritsky said. The new study “fills a void with experimental evidence for a behavior so common that it usually goes unnoticed.”
Skotaxis exist in the insect world: cicadas join crickets, beetles, ants, flatbed bugs and even swimming bees.
Earlier this year, entomologist Zach Huang of Michigan State University in East Lansing and his colleagues reported that honey bees and mason bees stranded on water swim to darker areasusing differences in brightness to head towards dry land.
Like Weiss, Huang says he didn’t know about skototaxy until he studied its behavior. “I didn’t even know that word existed.” But after reading the new research, Huang suspects that skototaxy might be much more widespread than researchers imagined.
Many plants and animals, it seems, have learned the same simple lesson: When survival is at stake, following the shadows can be a brilliant idea.
