Why cicadas only come back every 17 years: a natural mystery that excites minds

(ORDO NEWS) — Cicadas make a lot of noise, but often their invasions happen with strict periodicity. But what happens to them during these 17 years they disappear?

Cicadas are admired for their life cycle. No insect spends 17 years underground in the form of a nymph, after which it grows wings and comes out

17-year-old cicadas are species of periodical cicadas of the order Homoptera with the longest known life cycle among insects. The largest brood appears every 17 years in the northeastern United States.

Shortly after the 17-year-old cicada nymph hatches from its egg, it burrows into the ground, where it spends, as the name suggests, the first 17 years of its life.

When she emerges from the ground, she lives another 4-6 weeks – long enough to mate, fertilize or lay eggs and start the cycle again.

Why do cicadas appear every 17 years?

Contrary to popular belief, periodic cicadas do not spend their years underground in hibernation. Rather, they are active in the form of nymphs, digging tunnels and feeding on the sap of tree roots.

Nymphs of cicadas come out of their underground homes for a reason – this process is primarily associated with temperature.

After their 17 years are up, cicadas wait for the ideal temperature before heading to the surface – they need soil about 20 cm deep to reach a temperature of at least 18°C.

This means that cicadas appear at different times in different parts of the United States, but once that temperature is reached, all the cicadas in the same area will sense it and come to the surface together.

But how do cicadas know when 17 years have passed? While none of the theories to explain this periodicity has been confirmed, many scientists suggest that periodic cicadas possess an internal molecular clock that keeps track of the number of years that have passed through environmental signals.

As trees go through their seasonal cycles, shedding and regrowing their leaves, the composition of their sap changes.

And when cicada nymphs feed on this sap, they probably pick up on these changes as well. The 17th iteration of the seasonal cycle of trees gives the nymphs one last signal: it’s time to come to the surface.


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