The 17-year cicada is here! A trillion-dollar "zirwa" swept across the United States. If you miss it, you'll have to wait another 200 years!

Before reading the article, a reminder

The pictures in this article may trigger both insect phobia and claustrophobia

This month, the famous seventeen-year cicada is about to emerge again, and the United States is about to usher in a large-scale "Zi'erwa Zi'erwa" scene.

Male cicadas use their chirping to attract females (it’s noisy enough just looking at the picture) | Pixabay

The "Seventeen-Year Cicada" cicada is just as its name suggests: they hibernate underground for 17 years, and when the time is up, billions of them break out of the ground together - covering the sky and earth, making a lot of noise, mating and leaving offspring, then dying and remaining silent. The eggs they lay hatch into nymphs underground, waiting for another 17 years.

Cicada colonies that appear in the same area in different years are given their own numbers, and the group that broke out this year is called " Brood XIII ."

This year is also the year of the outbreak of "periodic cicadas". In addition to the 17-year cicada Brood XIII, the 13-year cicada Brood XIX also broke out this year. The number is expected to reach one trillion. They can be seen in half of the United States, and the scene will be very spectacular.

Both 17-year and 13-year cicadas will erupt in 2024 | US Forest Service /Wikimedia Commons

Moreover, since 13 and 17 are both prime numbers, these two types of cicadas can only appear at the same time once every 221 years. The last time this happened was in 1803 , and the next time will be in 2245.

So how spectacular is it to see trillions of cicadas chirping together? Is this a sight that only happens once every 17 or 13 years? Why do they spend so long underground?

Hidden for many years, suddenly emerged

Children who grew up in cities or rural areas are probably familiar with cicadas. The long or annoying cicada chirps on the big banyan tree in the schoolyard and on the weeping willows beside the classrooms ushered in the beginning of each summer. If you were a kid with older children, you would have probably used bamboo poles wrapped with spider silk to "catch" cicadas, or looked for cicada shells near the roots of big trees in the early morning.

Like many other incomplete metamorphosis insects, the life of cicadas is divided into three stages: egg, nymph and adult. Before the final molting, they live in the dark underground for a long time, often more than a year. Most cicadas do not have cycles . Their nymphs absorb enough nutrients from the underground roots of plants or stems close to the ground, and then they will climb onto the tree trunks in spring and summer to molt and emerge; then they will mate, lay eggs, die, and live out their lives. This is the life of most cicadas we have seen.

The more common one around us is the red-veined bear cicada (Cryptotympana atrata), also known as the black grasshopper cicada | Will Liang / Flickr

In North America, there is a type of cicada whose nymphs live underground in a very regular manner for 13 or 17 years . Then, as if they were called upon, tens of millions of individuals break out of the ground together on several weeks of summer nights, completing the final stage of their lives amid a collective roar of tsunamis. Then they die and disappear, until they appear again after the next 13 or 17 years.

Because of these characteristics, they are called "periodic cicadas."

Periodical cicada | David Gumbart / TNC

The rebirth after years of dormancy is like a metaphor of life; and the periodic cicada truly interprets this fable-like story to the extreme.

Seventeen Years

Because of the lack of historical data, we have no way of knowing how the North American natives viewed these mysterious creatures that appeared in large numbers and then disappeared for more than a decade. Europeans who arrived in North America did not begin to pay attention to periodical cicadas until the 17th century. Many people have investigated and studied the life cycle of these cicadas, and even Benjamin Franklin and Thomas Jefferson, two of the founding fathers of the United States, have made relevant records.

Because of the impressive numbers they appear in, the 17-year cicadas are referred to in some sources as " noisy locusts ."

It's about 100 decibels louder than a lawn mower | CNN

When the Swedish naturalist Pea Kalm visited the United States in 1749, he observed and recorded in detail the outbreak of periodic cicadas at that time: "There is sufficient and accurate evidence to show that these amazing insects appear in Pennsylvania every 17 years. This also means that except for that strange summer, they stay underground for the rest of the time."

These discoveries attracted more attention and curiosity in Europe. In 1785, Carl Linnaeus, the master of biological taxonomy, named this cicada for the first time in his 10th edition of Systema Naturae, Cicada septendecim, and the specific epithet means "seventeen years" .

A female 17-year cicada from the 1930 book How Insects Survive | RE Snodgrass

Over the next 100 years, more periodic cicadas were discovered across North America, mostly in the eastern and southern United States, all the way to the Mississippi River Valley and the Great Plains. People also discovered some patterns in these periodic cicadas:

1) The outbreak cycle is either 17 years or 13 years, but there is no other number;

2) Periodic cicadas in a region basically strictly follow the cycle law, occasionally a few individuals appear early or in the second year of the outbreak, and there is no trace in other years;

3) Almost every year, there is an outbreak of periodic cicadas in one or more places across North America.

After tracking and studying in the 19th century, entomologists summarized 30 breeding groups (broods) , which vary in size. The same group always breaks out in the same year and is distributed in a similar area. Perhaps due to climate change or habitat destruction, two groups have never appeared since 1870 and 1953, while the others still break out in specific areas of North America as scheduled.

Further morphological studies have found that these periodic cicadas can be basically divided into three species based on characteristics such as size, wing veins, reproductive methods and cicada sounds. Entomologists have named them in addition to their scientific names: Decim, Cassini and Decula. The same morphological species has 13 or 17 year types, and almost every breeding group that breaks out in the same year and place contains these three species - surprisingly, these three species rarely hybridize.

A Cassini cicada | Fontaine K, Cooley J, Simon C (2007)

Some entomologists have further divided the three cicadas into 6 or 7 species based on their 13- or 17-year outbreak cycles. Other studies have shown that cicadas of the same species also switch between 13- and 17-year cycles, which is likely controlled by a single gene or a few gene loci.

Why are they prime number years like 17 and 13?

After learning about the seventeen-year cicada, the first question that may come to every curious person's mind is: Why does it choose a cycle of seventeen years?

All life on Earth is inevitably connected to other life. Cicadas are a type of insect that lacks defenses , and therefore become the target of many predators. Mammals such as raccoons, coyotes, opossums, and shrews, and birds such as woodpeckers, blue jays, blackbirds, and cuckoos all feed on periodical cicadas. Even tortoises, snakes, wasps, and spiders will prey on seventeen-year cicadas. In addition, humans also use cicadas as food.

Humans also eat cicadas | Ngguls / Wikimedia Commons

For prey, synchronous group appearance is a strategy favored by evolution - gathering into large groups not only increases reproductive success and provides collective early warning and defense, but also triggers a "predator saturation effect."

After all, a predator in a certain place can only catch a limited amount of food, and appearing in groups means that the probability of each individual being preyed on will be greatly reduced . This is not a rare phenomenon in nature. Sardines in the sea gather to face predatory sharks, snow geese flock in millions to resist golden eagles, and the explosive appearance and reproduction of periodic cicadas is also easy to understand.

Cicadas are most vulnerable when they just shed their shells. It takes 30 minutes for their wings to harden. | David Gumbart/TNC

But what is driving a 13- or 17-year cycle of eruptions? Why don't they occur in 2, 3, or 4 years?

We may never know the exact answers to all the questions, but by using molecular biology and computer simulation technology within the framework of evolutionary biology, we can largely speculate and even review the reasons for the formation of these amazing lives - 13 or 17 are relatively large prime numbers.

A Brood X in 2004 | Pmjacoby / Wikimedia Commons

The explosive emergence of periodic cicadas will inevitably trigger fluctuations in predators or parasites. If a predator appears in the breeding grounds of periodic cicadas, it will use this abundant food source as a basis to form a survival advantage, expand reproduction, and increase the population. If the cicadas and predators have overlapping cycles, the offspring of the cicadas will encounter a considerable number of predator offspring in the next round, which is likely to be a devastating blow to them. Therefore, evolution will prefer periodic cicadas whose cycles do not overlap with those of predators as much as possible.

If the period of a predator is a and the period of a periodic cicada is b, then the shortest time that the outbreaks of the two overlap is the least common multiple of a and b.

If the predator has a 2-year reproduction cycle and the periodic cicada has a 4-year cycle, they will overlap every 4 years at the shortest - there will be 25 such years in 100 years, which is undoubtedly a heavy blow to the 4-year cicada. If it is a 17-year cicada, the shortest time the two cycles overlap is 2*17=34 years - this is less than 3 times in 100 years .

Moreover, this overlap is superimposed. If the cycles of the two always overlap, the predator will have sufficient food and will be able to reproduce more, and the offspring of the periodic cicada will fall into the tragic situation of being preyed upon by more natural enemies. The cycle of generations will form a strong positive selection pressure. If the cycle of the periodic cicada is a large number, this positive enhancement will be weakened.

A pair of mating 17-year cicadas | Fontaine K, Cooley J, Simon C (2007)

A 2012 study showed that when the 17-year cicadas emerged, populations of several of their bird predators fell to their periodic lows ; this meant that any benefits that the previous outbreak of periodic cicadas brought to the predators had disappeared after a long 13 or 17 years.

Of course, the above may be the most well-known explanation, and there are several other convincing explanations. For example, since the Pleistocene (2.6 million to 11,700 years ago), the unusually cold climate has greatly extended the cycle of periodic cicadas, and nymphs can have a greater probability of staying in relatively warm underground to avoid the occasional devastating cold summer. For another example, such a large cycle can avoid hybridization depression between 13-year and 17-year groups - if there is no abnormal climate change, it will take 221 years for cicadas with 13-year and 17-year cycles to overlap.

Even the unarmed periodic cicada, which has almost no self-defense skills, is by no means a weakling in the race of evolution.

A cicada molting | Russkiypimp / Wikimedia Commons

In the summer, the 17-year cicada chooses to lay eggs on tender branches, which also allows it to avoid active predators on the surface. About 6 weeks later, the nymphs hatched from the eggs fall back to the ground, burrow into the ground, and look for young root tips to obtain a small amount of nutrition. When it grows bigger, it will burrow deeper into the ground, find and hold the main root, and pierce its mouthparts into the wood to suck sap.

Until that destined summer comes again, trillions of cicadas break out of the ground, once again using their sounds and numbers enough to block traffic to shock everyone who has forgotten them or is seeing them for the first time.

Swipe left to feel the arrival of the 17-year cicada | Chip Somodevilla (2021)

And this month, the summer belonging to the seventeen-year cicadas is coming.

Author: Uncle Zhong

Editor: Jiang Beau, Calendar Girl