Under the still-chilled dirt of northern Illinois, a brood of cicada nymphs is starting to stir. In less than a month, what is known as the Northern Illinois Brood of cicadas, will start to emerge.

There are at least three species of periodical cicadas (belonging to the genus Magicicada) and most are either on a 13-year or 17-year cycle. Scientists group the cicadas into 'broods' based on the year they emerge. Of all the known broods, the largest is believed to be the 17-year Northern Illinios Brood.

Scientists who studied the Northern Illinois Brood during the 1956 emergence, gathered detailed population information and calculated that in one forested floodplain outside of Chicago, an average of 311 cicada nymphs emerged in a single square yard of forested floodplain. That equated to 1-1/2 million cicadas per acre.

The cicada's life cycle is unique. The nymph form lives underground where it feeds on plant roots for 13 or 17 years (depending on brood and species). When ready to emerge, the nymph digs an exit tunnel to the surface. Once above ground, it seeks cover in vegetation and molts into the adult form of cicada complete with wings.

Adult cicadas live for a few short weeks during which time the males produce a raspy, loud mating song aimed at wooing a female. If he succeeds, they mate and soon after the male dies. The female survives just long enough to lay her eggs in the tender bark of young twigs.

The eggs hatch after about ten weeks and the nymphs plummet to the ground and dig their way down into the soil where they remain for another cycle.

Find out more:

• 17-Year Cicadas: From Nymph to Adult
• A Trill of a Lifetime (Illinois Natural History Survey)
• Magicicada (Wikipedia)
• Cicadas in Illinois (University of Illinois)
• Cicada Blog

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