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Every 17 (or 13) years across the United States millions of strange looking nymphs emerge from the ground in what resembles a scene from a science fiction movie. They climb trees, poles and buildings where their exoskeletons break open and a very eerie white creature emerges. In this molting process the newly emerged creature soon darkens and hardens into a winged insect up to 2 inches or longer with bulging, bright red eyes.
The hardening process may take 4 to 6 days after which the males begin to make a loud screeching sound, which might be described as creepy and certainly annoying. In 30 to 40 days the insects will have mated, each female will have laid hundreds of eggs in incisions made in the bark of trees, and they will have died. The wild invasion will be over. As the dead bodies of the cicadas are consumed by many kinds of creatures, the eggs hatch into tiny ant-like nymphs which drop from the trees and burrow into the ground where they will live on tree-root juice for the next 17 years. At that time they will emerge from the ground and the whole cycle is repeated.
It may sound like science fiction, but it isn’t. There are thousands of species of cicadas around the world, but the periodical cicadas, known as magicicadas, are found in eastern North America. They have either a 13 or 17-year life cycle. The 13-year species are mostly in the southern areas and the 17-year species in northern areas. In the United States there are 12 broods in different areas on a 17-year cycle and 3 broods on a 13-year cycle.
Anyone who has lived through one of these cicada invasions has seen and heard these creatures. Cicadas hold the record for being the world’s noisiest insects. A single cicada can be heard half a mile away. When you get thousands together the noise is overpowering, and in wooded areas there may be a million and a half of them per acre of land. 2008 is the year for the large 17-year Appalachian brood to emerge in more than a dozen states.
The nymphs live under ground (from just a few inches to several feet) until the spring of their 13th or 17th year. Then they begin to construct tunnels to the surface in preparation for their grand debut. On that special night in May they emerge from their tunnels at about sunset and by the thousands they find their places on nearby trees and shrubs where they will shed their juvenile exoskeletons.
There is an interesting design feature built into this cycle and it has a bearing on why there are such enormous numbers of these magicicadas. By coming out once every 17 years the periodic cicada avoids any serious predators. During their emergence they provide a feast for many animals including birds, squirrels, snakes and spiders, but there are so many of the cicadas that those predators do not significantly reduce their numbers. Magicicadas do not have a primary predator because of their life-cycle period. The numbers 13 and 17 are prime numbers, evenly divisible only by 1 or the number itself. For this reason predators cannot find a cycle which will match the cicada’s. Only if a predator emerged every 13 or 17 years could it severely affect the cicada. There are no other cycles like that in the animal kingdom
Do periodic cicadas do have a purpose in life? Yes, when they emerge for the culmination of their life cycle, they mulch and aerate the ground and during their brief time above ground they do provide food for all kinds of birds and small rodents, giving a little relief for corn and earthworms. These cicadas are part of the balance we see in creation.
Scientists do not understand the “clock” which tells the cicada nymphs to all emerge from the ground at the same time after 13 or 17 years. Who told them to do this? How do they know when it is time to come out? The cicada clock and life-cycle is certainly strange, but it shows evidence of design.
References-
“cicada,” “prime number,” Wikipedia
http://www.ivyhall.district96.k12.il.us/4th/KKHP/1insects/cicada.html
http://insects.ummz.lsa.umich.edu/fauna/Michigan_Cicadas/Periodical/Index.html
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