http://en.wikipedia.org/wiki/Magicicada
Nearly all cicadas spend multiple years growing underground as juveniles, before emerging aboveground for a short adult stage of several weeks to a few months. The seven periodical cicada species are so-named because, in any one location, all of the members of the population are developmentally synchronized – they emerge as adults all at once in the same year. This periodicity is especially amazing because their life cycles are so extremely long – 13 or 17 years. Cicadas of all other species (perhaps 3000 worldwide) are not synchronized, so some adults mature each summer and emerge while the rest of the population continues to develop underground. Many people refer to these non-periodical species as annual cicadas since some are seen in every summer. The life cycles of most annual species range from two to ten years, although some could be longer.
Periodical cicadas are average-sized for cicadas, but they are slightly smaller than the annual cicada species found in the same regions of the United States. Imagines (or adults) have a size of 2.5 to 3 cm (1 to 1.2 inches). They are black, with red eyes and yellow or orange stripes on the underside. The wings are translucent and have orange veins.
They are harmless insects; they neither bite nor sting. They are not venomous, and there is no evidence that they transmit diseases. They generally do not pose a threat to vegetation, but young plants may be damaged by excessive feeding or egg laying. It is thus advised not to plant new trees or shrubs just before an emergence of the periodical cicadas.
Mature plants usually do not suffer lasting damage even by a mass-emergence.