Demystifying the 17-year cicada

In the United States, there is a magical cicada, they can hibernate underground for 17 years, the day they break the ground, is the United States' "disaster" time.

What kind of cicada is it? Why does it hibernate underground for 17 years? What's so great about it?

About Periodic Cicadas

Cicadas that follow a certain cycle and appear intensively are periodic cicadas, and the 17-year cicadas mentioned in this article are periodic cicadas.

The 17-year cicada is the one with the longest life cycle and is also the leader of the insect life form. Other cicadas generally have a cycle of 1~3 years, while this cicada appears and then disappears collectively until it reappears after 17 years.

Photo: The hole left by cicadas breaking out of the ground

The vast majority of the cicada's life is spent in darkness. After the female cicada lays her eggs, the eggs lie dormant underground and continue to feed by sucking on plant roots. When the time is ripe, the cicadas break out of the ground in the form of a worm and climb trees or plant roots and branches, etc. to continue growing.

From the time the cicada breaks the ground, its life enters a countdown. Within a limited time of 4~6 weeks, the cicada has to fledge into an adult, to mate and lay eggs, and finally, the male cicada dies from the exhaustion of sperm "cicada" and the female cicada dies from egg-laying consumption.

Sometimes, you hear the cicadas chirping hard, disturbing people, hate to put it in the dust, but in fact, this is the male cicadas in the race for life, it is only the most cheerful, the largest and the most vigorous, to attract the female cicadas over to incite the wings of mating.

Why is it called the 17-year-old cicada?

The 17-year cicada is found only in the United States, and to this day, it has swept through at least 15 states in the eastern United States, including Washington. Their raucous voices only appear en masse every 17 years.

Some people jokingly say, when I was a child, they came to play music, when I got married, they came to say goodbye, when my children were in junior high school, they came to help, my children and grandchildren in retirement, they came to congratulate, when I stepped into the twilight, they came to send the end. It is a great blessing for people to hear it 5-6 times in a lifetime.

Through research, scientists have roughly discovered two mysteries of this cicada.

First: the cicada's life force is pushed to the limit under the screening of harsh environment

Periodic cicadas originated roughly 1.8 million years ago when the earth's crustal plates moved and there were multiple ice ages. Cicadas generally emerged from the ground in summer, when ice ages caused cold summers, and some cicadas froze to death upon emergence, while some tried to hibernate underground to escape the cold.

Cold summers occurred irregularly, and C.E. Carlton of the University of Arkansas and Randel T. Cox of the University of Memphis calculated the survival rate of cicadas in the cycle at that time. Assuming that cold summers occurred every 50 years for 1500 years, the survival rate of cicadas that emerged every 7 years was only 7%, the survival rate of cicadas that emerged every 11 years was 51%, and the survival rate of cicadas that emerged every 17 The survival rate of cicadas that emerge every 17 years is as high as 96%.

During the long evolution, some cicadas with short life cycles may have all been frozen to death, and only those with long cycles barely survived. Maybe there were also 20-year cicadas and 25-year cicadas at that time. After all, they are insects, so I guess they couldn't survive the long years and died before they came out of the ground.

So, some people claim that 17 years of cicada is the maximum limit to balance out between harsh climate and life cycle.

The reason why it is 17 years is that it is estimated that the life limit of cicadas, with evolution, has not reached 18 years and above, so why not 14, 15, or 16 years? This brings us to the second mystery of the 17-year-old cicada.

Secondly, under the predation of natural enemies, the 17-year cicada found the law of natural enemies

In the United States, there is another species of 13-year cicada that, like the 17-year cicada, is the best choice after natural evolution.

Let's take the example of the periodic cicadas that were unearthed in Maryland in 2016 when the cicadas roamed the land and could have delighted the local cicada-eating birds, snakes, frogs, arthropods, and small mammals. The natural predators ate as much as they could, and when they were full, they reproduced to their heart's content, greatly increasing the number of natural predators of cicadas.

And it is completely impossible to keep the population thriving for a long time with these flooded natural enemies and food to keep up. By then, the cicadas burrowed underground, they can not eat, and they will starve to death part if the cicadas hibernate longer, some natural enemies of life can not carry, and die of the old part, by then, the number of natural enemies back to the previous level.

Natural enemies are also concerned about the regular cycle of reproduction, such as birds, which generally take 3 or 6 years to become sexually mature. Taking 3 years as an example, during the 3, 6, 9, 12, 15, and 18 years when the cicadas are dormant, the birds have a large number of offspring produced, and if the cicadas come out of the ground in these years, it will cause a large number of deaths, which is not conducive to the reproduction of the population. The same is true with a 6-year breeding period.

Then there is the case of animals whose breeding period is a double year. If cicadas emerge in the double year, they may also be eaten by natural predators in large numbers, which is also unfavorable to the continuation of the cicada population.

At this time, cicadas figured out the law of natural enemies through natural evolution, and gradually formed the law of emergence in their prime years to avoid them as much as possible. It is well known that prime numbers are only divisible by 1 and itself, and this law substantially avoids the breeding period of natural enemies.

Some natural enemies reproduce every year, and it takes the 17th generation to eat 17-year cicadas, even if the 17th generation reproduces in large numbers after satiation, the 18th generation and later will have to wait until 34 years to eat 17-year cicadas. Many animals do not live that long, and populations always fall back to normal or reduced levels. For 17-year cicadas, appearing every 17 years is tens of billions, and even if they are partially eaten by their natural predators, they do not hurt.

Figure: Birds don't live that long and can't wait for the second or N batch of 17-year cicadas

Taking birds as an example again, a joint paper was published in 2012 by Walt Koenig, a behavioral ecologist at Cornell University, and Andrew Liebhold, an ecologist with the USDA Forest Service. The paper stated that after 17 years of cicadas appearing in large numbers to be fed on by birds, it was probably the 12th year that the number of birds began to decline, and by the 17th year, the number of birds dropped to a minimum, and the 17th year was the year that the next generation of 17-year cicadas broke ground. It means that the second generation of 17-year cicadas, which are less eaten by the birds, i.e. ensure the population continuation and also promotes the birds' population to rebound.

Therefore, in prime numbers from 1 to 20, 2, 3, 5, 7, 11, 13, 17, and 19 prime years, cicadas are vulnerable to a large number of natural predators when they emerge in years less than 13, and 19 years too long for the cicadas to persist. And from the interval of the rebound of the number of birds above, the only prime number years of 13 and 17 are the most suitable, after which, even if there is an overlap with the breeding period of natural predators, the probability is also very low for 17-year cicadas, as long as most of them can reproduce. Therefore, there are rare 13-year cicadas and 17-year cicadas in the United States.

17-year-old cicada "captured" the U.S. White House and Congress

The 17-year cicadas have been dormant for 17 years and then emerged en masse, each time in numbers of tens of billions. In 2021, for example, about 30 billion of them will appear on the land of the United States, so dense that they have nowhere to land.

Don't think they are only in the jungle, they have even "captured" the U.S. Congress and the White House.

On the building of the White House, on the parking lot, on the tree trunks, and on the lawn, 17-year-old cicadas forgot to chirp as news reporters and tourists took pictures. Outside the White House, cicada corpses are everywhere, even people walking their dogs, have nowhere to land, while worrying about their dogs eating non-stop.

Many people are very helpless about this.