Newly emerged cicadas don’t stumble randomly toward trees after emerging from the ground. Instead, they rely on a simple yet effective strategy: following shadows. Researchers recently published in The American Naturalist that periodical cicadas use darkness cues to locate tree trunks with remarkable precision. This behavior, known as skototaxis, is far more common in the insect world than previously understood.
The 17-Year Cycle and Instinctive Navigation
Periodical cicadas, like Brood XIII, spend up to 17 years underground before emerging en masse. When they surface as wingless nymphs, they don’t wander aimlessly; they immediately head for the nearest tree.
Researchers observed that the insects moved directly toward tree trunks with minimal deviation, suggesting a pre-programmed navigational response. This is critical because the nymphs must climb trees to complete their development into winged adults.
Experimental Proof: Blocking Vision Impairs Navigation
To confirm that darkness cues drive this behavior, researchers at Lake Forest College temporarily blocked the compound eyes and light-sensing organs of newly emerged nymphs. Without the ability to detect contrast between light and dark, the insects wandered randomly and failed to reach trees.
In contrast, control groups with unobstructed vision moved quickly and directly toward nearby trunks. Further tests revealed that nymphs overwhelmingly chose darker surfaces over lighter ones, even when presented with a simple choice.
Of 32 nymphs tested, 28 crawled toward the darker option, confirming that skototaxis is the primary navigational mechanism.
Skototaxis: A Widespread Survival Strategy
This instinct isn’t unique to cicadas. Skototaxis is observed in a broad range of insects, including crickets, beetles, ants, and even aquatic bees.
Researchers at Michigan State University recently found that honeybees stranded on water also swim toward darker areas, using brightness differences to find dry land.
The prevalence of this behavior suggests it’s a fundamental survival strategy for insects across diverse environments.
Why This Matters: Filling a Knowledge Gap
While entomologists have observed this darkness-seeking behavior for years, the experimental evidence was lacking. Gene Kritsky, a cicada expert, admits that the idea of formally investigating skototaxis hadn’t occurred to him until now. The new study fills this gap, proving that following shadows is a bright idea when survival depends on it.
The research highlights how even seemingly simple behaviors can be deeply ingrained in animal instincts, driving survival without conscious thought.
This discovery underscores the power of basic instincts in the natural world. Skototaxis is a prime example of how evolution has shaped survival strategies in insects, enabling them to navigate effectively even in dim conditions.
