Cicadas Use Darkness to Climb Trees | Skototaxis Explained

Newly emerged cicadas rely on shadows to guide them to trees, a behavior known as skototaxis, according to research published in the American Naturalist on March 20, 2026. The discovery sheds light on how these insects, which spend years underground as nymphs, successfully locate suitable habitats after emerging.

Researchers at Georgetown University, led by evolutionary ecologist Martha Weiss, conducted a detailed analysis of Brood XIII cicadas – a group that emerges every 17 years – in northern Illinois during 2024. They found that the wingless nymphs consistently moved with remarkable precision toward tree trunks, utilizing darkness cues to navigate.

“They just zoomed in, marching toward the trees,” said Weiss, highlighting the directness of the nymphs’ movements. The study revealed that the cicadas detect dark shapes against lighter backgrounds in dim evening light, enabling them to find the vertical surfaces necessary for climbing and molting into their winged adult form.

Skototaxis: A Light-Dependent Behavior

Skototaxis is a biological response where organisms move towards or remain in dark environments. While this behavior is observed in various species, the recent research provides a detailed understanding of its application in cicadas. The Georgetown University team demonstrated the importance of light and dark contrast by temporarily obscuring the compound eyes and light-sensing organs of newly emerged nymphs.

Nymphs with obstructed vision exhibited aimless wandering and failed to reach tree trunks, while control groups with unobstructed vision quickly and directly moved toward trees. This experiment confirmed that the ability to perceive differences in light levels is crucial for successful tree-finding.

Cicada Biology and Emergence

Cicadas are insects belonging to the superfamily Cicadoidea, within the order Hemiptera (true bugs). There are over 3,000 described species worldwide, with the majority being annual cicadas. However, some North American species, like the Magicicada genus, are periodical cicadas, emerging in mass numbers every 13 or 17 years.

These insects spend most of their lives as nymphs underground, feeding on xylem sap from tree roots. Upon reaching maturity, they emerge from the soil, climb trees, molt into their adult form, and reproduce. The timing of emergence is synchronized within a brood, creating a spectacular natural event.

Implications of the Research

Understanding how cicadas navigate after emergence has implications for broader ecological studies. The research highlights the importance of visual cues in insect behavior and provides insights into how insects adapt to their environment. The study also underscores the complex sensory mechanisms that guide insect movements, even in relatively simple tasks like finding a tree.

The findings could also contribute to a better understanding of insect responses to environmental changes, such as light pollution or habitat fragmentation. Alterations in light levels could potentially disrupt the skototaxis behavior, impacting cicada populations and their role in forest ecosystems.

Further research is needed to investigate how other environmental factors, such as temperature and humidity, might influence cicada navigation. Exploring whether similar skototaxis behaviors are present in other insect species could reveal broader patterns in insect sensory ecology.

The study, published in the American Naturalist, provides a fascinating glimpse into the sensory world of cicadas and the remarkable adaptations that allow them to thrive. By utilizing shadows as a guide, these insects demonstrate a sophisticated navigational strategy that ensures their successful transition from underground nymphs to winged adults.