Cicadas are flying, plant-sucking insects whose closest relatives are leafhoppers, treehoppers, and fulgoroids. Adult cicadas tend to be large (most are 25-50 mm), with prominent wide-set eyes, short antennae, and clear wings held roof-like over the abdomen. Cicadas are probably best known for their conspicuous acoustic signals or "songs," which the males make using special structures called tymbals, found on the abdomen. There may be as many as 3,000 different cicada species worldwide.
All but a few cicada species have multiple-year life cycles, most commonly 2-8 years (de Boer and Duffels 1996). In most cicada species, adults can be found every year because the population is not developmentally synchronized; these are often called "annual" cicada species. In contrast, populations of the periodical cicada species are synchronized, so that almost all of them mature into adults in the same year. The fact that periodical cicadas remain locked together in time is made even more amazing by their extremely long life-cycles of 13 or 17 years.
Periodical cicadas are found in eastern North America. There are seven species -- four with 13-year life cycles (including one new species described in 2000), and three with 17-year cycles. The three 17-year species are generally northern in distribution, while the 13-year species are generally southern and midwestern. Magicicada are so well-synchronized developmentally that they are nearly absent as adults in the 12 or 16 years between emergences. When they do emerge after their long juvenile periods, they do so in huge numbers, forming much denser aggregations than those usually achieved by cicadas. Many people know periodical cicadas by the name "17-year locusts" or "13-year locusts," but they are not true locusts, which are a type of grasshopper.