|17-year cicada adult|
(Photo courtesy of USDA)
There are several broods of cicadas that emerge in different years, because I distinctly remember seeing them in the Chicago area as a child -- but when I trace back this brood (Brood II) in increments of 17, I get 1996-1979-1962-1945. I was born in 1947 and was in high school in 1962, so the brood I remember from childhood must be a different one. I remember being in grade school at the time.
According to this chart from the University of Michigan, in Illinois there were emergences of brood X in 1956 and brood XII in 1957, which is about the time I remember seeing them. There were thousands of empty nymph cases clinging to the bark of trees after the adults had emerged, each one shaped exactly like a bug. We kids used to stick them on the backs of people as a joke -- they really do cling well to clothes! (Boys will be boys...)
Not long after that, we moved to Philadelphia, where I encountered the 1962 emergence that was of this same brood II coming out this year. At the time, we called them "17-year locusts," but they aren't really a locust, which is a type of grasshopper. I suppose people called them that because they emerged in such huge numbers, reminiscent of the biblical plagues. But these aren't as harmful. They make a lot of noise and can be a nuisance (only the males buzz, but there are a LOT of them!), but they don't strip the trees or eat all the crops like real locusts.
They do naturally prune trees, because they lay their eggs in little slits they make in the branches. When the eggs hatch, the little nymphs drop to the ground and burrow in, where they spend the next 17 years underground, feeding on roots, until the time comes to dig out and become an adult. Nobody really knows how they can tel how many years have passed -- but they do. One of nature's amazing mysteries.