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Periodical cicadas live only in eastern North America. Of the seven species, three of them have a life cycle of 17 years, the other four have 13 year life cycles. They spend most of this cycle living as nymphs (larvae) underground, where they suck juices from plant roots. In the spring of their last year, they construct tunnels for themselves to the surface and emerge with precisely synchronized timing to molt into their adult form, mate (which involves loud, species-specific choruses by the males to attract females), and lay eggs. Their emergence is thought to be triggered by soil temperature.
In different geographical areas the populations are not synchronized with each other; overall there are 15 “year classes” (or “broods”) of cicadas that predictably emerge in different years from each other, so in any one year there are adults of at least one of the seven species in some part of the country. There are three 13-year broods, and 12 17-year broods, named with roman numerals. Most broods contain all the species within that particular life-cycle (i.e. all the 13-year or all the 17-year cicadas), all at the same phase of their life cycle, however, a few broods do not (for example, brood VII contains only M. septendecim. The mapping of these broods and their occurrences is the subject of study, and is of interest for understanding mechanisms for speciation (see the mapping project homepage: http://www.magicicada.org/map_project/maps.php).
During an emergence, adults may be present in very large densities, sometimes as high as 1.5 million individuals/acre. Their occurrences are usually much denser than non-periodic cicada species. It is hypothesized that these high densities satiate the many predators of these cicadas before impacting their population sizes, and the long (prime-number) lifecycles make it impossible for predators to evolve specialized strategies to predictably utilize periodic cicadas as a food source. Adult cicadas feed on plant juices using piercing and sucking mouthparts (that are of no harm to humans or other animals). Females lay up to about 600 eggs, ovipositing them into eggnests under the bark of twigs. Adults die before the eggs hatch. The eggs hatch after 6-10 weeks, and then the nymphs drop to the ground to begin a new 13- or 17-year long development to adults.
Below is a list of Magicicada species (these are most easily distinguished by the very specific male call). Each species has as its closest relative a species with the alternate lifecycle. While it may be that in fact these are not actually seven distinct species, they are considered as such until more data comes in to resolve this question.
13-year cicada species:
• Magicicada tredecim (Walsh and Riley 1868)
• Magicicada neotredecim (Marshall and Cooley 2000)
• Magicicada tredecassini (Alexander and Moore 1962)
• Magicicada tredecula (Alexander and Moore 1962)
17-year cicada species:
• Magicicada septendecim (L. 1758)
• Magicicada cassini (Fisher 1851)
• Magicicada septendecula (Alexander and Moore 1962)
(Simon 2011; Hill and Marshall, 2011; Cooley 2011; Cooley and Marshall 2011)
For an extensive list of references on periodical cicada literature current through Oct 12, 2000, see Cooley J. and Marshall, D.C. 2000. http://insects.ummz.lsa.umich.edu/fauna/Michigan_Cicadas/Periodical/magilit.html (retrieved September 19, 2011).