Emergent Periodical Cicada Nymphs Use Skototaxis to Navigate to Trees.

AbstractAfter 13 or 17 years underground, periodical cicada nymphs come aboveground to molt into adulthood. Observers have long noted the strikingly directional orientation of nymphs toward trees on which they molt and have speculated on possible mechanisms to account for these movements. Through a combination of observational, experimental, and modeling studies, we demonstrate that cicada nymphs use skototaxis, or movement toward darkness, to reach their targets. Emergent nymphs walked directly toward isolated trees, regardless of initial compass direction or time of day, traveling only 15% farther than the minimum distance necessary. Nymphs whose vision had been temporarily obscured wandered randomly and rarely reached the trees. Indoor trials confirmed that cicada nymphs preferentially orient toward dark objects. This study adds to a growing list of organisms, including tropical plant seedlings, that move or grow toward darkness.