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Periodical cicadas, a type of homopterans with the longest known insect life cycle, include the 17-year cicadas. Every 17 years, the largest brood appears in the northeastern portion of the United States, as if on cue.
Why Do Some Cicadas Appear Only Every 17 Years?
A 17-year cicada nymph burrows into the earth shortly after hatching from its egg, where it spends the first 17 years of its existence, as its name indicates. It only survives four to six weeks after emerging from the earth, only long enough to mate, fertilize, or deposit eggs, and repeat the cycle.
Periodical cicadas do not spend their years underground in hibernation, contrary to common belief. In their wingless nymph phases, they are awake and active, constructing tunnels and eating on the sap from tree roots.
The temperature determines when the cicada nymphs emerge from their subterranean nests. After their 17 years are over, the cicadas wait for the ideal temperature before venturing to the surface when the soil around 8 inches (20 cm) below the surface reaches 64 degrees Fahrenheit (18 degrees Celsius).
This implies that cicadas arrive at various times in different sections of the country: the soil in Virginia may reach the appropriate temperature before the dirt in Illinois. However, once that temperature is reached, all of the cicadas in the region will be aware of it and will all make the trek to the surface at the same time.
Many scientists believe that periodical cicadas have an internal molecular clock that tracks the passage of time using environmental signals, yet no one explanation has been verified.
The composition of a tree’s sap varies as it goes through its seasonal cycle of losing and sprouting leaves. And when cicada nymphs eat that sap, they are likely to pick up on time markers. The nymphs’ last hint comes from the 17th repetition of the trees’ yearly cycle: it’s time to emerge.
The cicadas have not yet reached adulthood when they emerge. They’re still nymphs, and they will stay that way until they finally molt. They take to the trees with their newfound wings once their fresh exoskeletons have solidified, and the males start making loud mating cries.
We have come full circle. Cicada nymphs fall or crawl down from the trees where they hatched, burrowing into the earth for the next 17 years.