It’s hard to believe it’s happening again so soon, but this week we began seeing the 17-year “periodical cicadas” emerging from their underground chambers. With each passing day, we’re seeing more and more of the newly emerged cicadas on leaves, bark, grass, and buildings. It rained last night, and it must have rained cicadas because they are everywhere.
The periodical cicada species are so named because, in any one location, all of the members of the population are developmentally synchronized—they emerge as adults all at once in the same year. All other cicada species (about 6,000 worldwide) are not synchronized, so some adults mature each summer and emerge while the rest of the population continues to develop underground. 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.
Magicicada spp. spend much of their 17-year life cycle underground feeding on xylem fluids from the roots of deciduous trees. In the spring of their 17th year, a few weeks before emerging, the cicada nymphs construct exit tunnels to the surface. These tunnels are visible as approximately ½-inch diameter holes, or as chimney-like mud “turrets” which the nymphs sometimes construct over their holes.
Then on some spring evening when the soil temperature at roughly an 8-inch depth is above 17 °C (63 °F), the nymphs come out of their tunnels and climb to a suitable place on nearby vegetation to molt one last time and complete their transformation into an adult cicada.
After molting, cicadas spend about six days hidden in the leaves waiting for their exoskeleton to harden completely. During this time, they are easy prey for birds, reptiles, squirrels, cats, and other large and small mammals. So it would appear that overwhelming predators by their sheer numbers, ensuring the survival of most of the individuals, is an advantageous survival mechanism for the cicadas.
Despite their prolonged developmental phase, the adults are only active for a few weeks. Soon after molting, the males gather in the trees and “sing” to attract the females. Within two months of their appearance (about mid-July in Virginia), the females will lay their eggs and then the adults will die and the cicadas will be gone for another 17 years.
Nature never hurries. Atom by atom, little by little, she achieves her work. Ralph Waldo Emerson