Predator foolhardiness and morphological evolution in 17-year cicadas (Magicicada spp.)

Abstract

Periodical cicadas in the genus Magicicada have an unusual life history that includes an exceptionally long life cycle and a massive, synchronized emergence. Considerable effort has been put into research aimed at understanding the evolutionary history of periodical cicadas, but surprisingly little attention has been given to their morphological evolution. Their slow flight and approachability have been described as ‘predator-foolhardy’ behaviour. We quantified flight speeds for M. cassini, M. septendecim, and Tibicen chloromera (a nonperiodical cicada species), and interpreted them in terms of thorax musculature, body proportions and wing size and shape in relation to body size. On average, T. chloromera flew three to four times faster than did the two Magicicada species. Using empirical relationships between flight speed and body length, body mass or wing loading, we determined M. cassini and M. septendecim to be unusually slow fliers for their body size, whereas T. chloromera was not. The relatively slow flight speeds of Magicicada species could be largely accounted for by relatively small thoracic muscle masses, as indicated by thorax length × width measurements, and low wing loadings. Aspect ratio differences were contributing factors. Male Magicicada and female Tibicen were more active in mate searching than was the opposite sex, and correspondingly had relatively large aspect ratios. We interpret the morphological traits responsible for the slow flight of Magicicada species as being adaptations to searching for mates in dense aggregations around the canopy of trees, relatively unconstrained by the per-capita risk of predation. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 1–13.