Adult Periodical Cicada Magicicada septendecim with oviposition scars on twig.
Drawing by Mr. L. H. Joutel (1905) via http://alpha.fdu.edu

An interesting article on Salon.com looks at the upcoming emergence of periodical cicadas in the United States, the only place they exist. Periodical cicadas are the seven species of the Magicicada genus of Cicadidae. While all cicadas have long life cycles, usually 2-8 years, Magicicada have synchronized life cycles of either 13 or 17 years. This means that all the Magicicada of one species in a given area (a group known as a Brood) begin as eggs at the same time, feed underground on tree roots at the same time, and then emerge as a genuine force of nature after either 13 or 17 years of prime-number maturation to have a massive mating session of a few weeks during a single season before they lay eggs and die. They are among the longest-lived of all insects.

They don’t bite, they don’t sting, they don’t even eat when they are above ground. What they do is mate. Mate, and sing. Loudly.

I’ve only experienced cicadas once over a prolonged period, during a summer I spent driving around Japan. I can’t think of any of the sights I saw without the soundtrack of the 100-decibel cicada thrum playing in the background. It made the air feel even hotter than it was.

Here‘s a collection of recordings from the 2004 emergence of Magicicada Septendecim Brood X in West Virginia, a group which won’t re-emerge until 2021. Turn your volume up to 100%, preferably with headphones, to get the full effect.

During the years they emerge from the ground to mate, predators can eat their fill and still leave so many cicadas that propagation is ensured. The dead bodies of millions (billions?) of cicadas provide a mat of fertilizer for vegetation, and in the year previous to an emergence, populations of ground-dwelling animals such as moles are recorded as thriving on the growing cicada nymphs. Historically, cicada emergences have been known to save human populations from starvation. Likewise, trees during the final year of maturation are sometimes seen to falter a bit from underground nymph populations feeding on their roots.

The nymphs emerge on a spring evening when the soil temperature at about 20 cm (8 in) depth is above 17 °C (63 °F). In most years, nymphs emerge en masse on a single evening, when the soil temperature at a depth of 8 inches (20 cm) rises above 63°F (17°C). Depending on the region’s latitude, this can be anywhere from late April to early June. It’s not easy to get data on periodical cicadas for the simple reason that the time spans are so long and the insects spend so much of it underground. But there is an overall impression that, in spite of the swarms that emerge, the broods as a whole are in decline. Some have been listed as extinct.

Climate change may be the culprit in shrinking distribution. According to the Salon.com article, scientists will be gathering information on the Brood 2 emergence in an attempt to establish better cicada data baselines.

More:

Salon.com article by Richard Schiffman – ‘Cicadas prepare to invade by the billions’

Wikipedia article

Cicada Central website – Lots of information, and a place to report sightings.