Emergent periodical cicada nymphs use skototaxis to navigate to trees

After 13 or 17 years underground, fifth-instar periodical cicada nymphs
must come aboveground to complete the final phase of their long life
cycle: molting into adulthood. A successful molt requires each nymph to
climb a vertical surface, typically at dusk or after dark, but it is
unclear how the cicadas locate such surfaces. Observers have long noted
the seemingly directional orientation of nymphs towards trees, and have
speculated on possible mechanisms, including negative phototaxis, the use
of topographic cues, random walks, or detection of mysterious ‘other
waves’ emanating from trees, to account for this directed movement. Taking
a hint from the behavior of tropical vine seedlings that grow towards tree
trunks in the forest, we demonstrate that cicada nymphs, once above
ground, quickly orient themselves towards the darkest sector of their
visual environment, a movement response termed skototaxis. Cicada nymphs
emerging from the rhizosphere of an isolated tree all walked more-or-less
directly towards the trunk, regardless of initial compass direction or
time of day, travelling a mean of only 15% further than the minimum
distance required to reach the tree. In contrast, nymphs whose eyes and
ocelli had been temporarily obscured wandered randomly and rarely reached
the tree. In an indoor experiment, cicada nymphs in a dark room were
significantly more likely to walk towards a dark target against a lighter
background than a light target against a darker background. Our results
demonstrate that cicada nymphs, like tropical vine seedlings, rely on
movement towards darkness to quickly orient and reach their proximate
destinations, a prerequisite to the well-studied adult phase of their life
cycle.