Swarm of Cicadas Takes Aim at U.S.

wetshoe writes "'After 17 years of relative quiet, Mother Nature is bringing the noise. 'Periodical cicadas, a species of the grasshopper-like insects best known for the scratching, screeching "singing" of the males, will emerge this May, filling forests in more than a dozen states. Almost as abruptly as they arrive, they'll disappear underground for another 17 years.' The article also talks about areas in the Mid-West where 17-year June Bugs sometimes overlap with 13-year June Bugs. I remember as a child one such time, you literally couldn't walk anywhere without stepping on them, they were everywhere. Reminded me of a biblical plague."

Comments

Prime Cycles An Advantage? Yes, but not 4 all

[FreeUser]

To prevent a predator from evolving to depend on their "blooms" and wipe them out when they emerge, they only do it at prime number intervals. That way a predator would have to multiply in the same time interval and not some fraction of it. If it was 12 years a predator could multiply every 2,3,4, or 6 years and have a chance of feeding just when the cicadas were blooming. It's a lot less likely that a predator will multiply at only 17 years at the same time as the cicadas.

There are also species of bamboo that periodically produce tons of seeds to reproduce, but on the order of every 70 years. These too only do it on prime number years.

It's a neat theory, and it is probably true that species with life cycles which are a prime number of years have an evolutionary advantage over those whose cycles are evenly divisible, but the advantage is slight enough that his assertion there are only species with life cycles that are prime numbers is wrong.

Quoth the article:

Question: What is the life span of a cicada?

Answer: That depends on the Genus and species of the cicada. The Magicicada Genus of North America has a 17 or 13 year life cycle (the largest of any insect). Other Geniuses of cicadas have life cycles of a variety of years (never more than 17 and usually a primary number). The Tibicen or "dog day" cicada has a life cycle of only a couple of years and which is one of the reasons why we see them each year.

Most are prime number cycles (probably as a result of the advantage vis-a-vis cyclic predators you cite), but NOT ALL.

Re:Prime numbers

[SEE]

Who knows if he was right, but it is a cool theory.

Hmm. Interesting.

My only objection is that it explains why the times are prime, but not the coincidence of them all being 17 or 13 years.

Each of the three 17-year varieties' closest relative is a different one of the four 13-year varieties, not another 17-year one. We would therefore, evolutionarily, expect that the ancestral periodic cicadia first divided into at least three species, and then each of those three divided into 13 and 17 year varieties.

So why did each of the three branches evolve both 13 and 17 year terms, but none evolved the equally prime 11 or 19 year terms? 11 and 13, or 17 and 19, are both seemingly more likely pairs than 13 and 17. Why did it become 13 and 17 three times?