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# Prime numbered cicadas

By Murray Bourne, 30 Apr 2013

One of the most remarkable events in the insect world is due to take place over the next few weeks in the NE states of the USA, from Massachusetts to Georgia.

Millions of Magicicada cicadas will emerge after 17 years of living underground. They will fly around for 3 weeks eating a lot of leaves, then mate, lay their eggs and die. (So you could say only around 0.3% of their lives would be called "fun".)

Magicicada [Image source: Patch (no longer available)]

There's another group of cicadas that have a 13-year life cyle, and these ones tend to live in the southern US states. (The next appearance of these little guys will be 2014.)

The cicadas provide a feast for birds, fish and many animals. Some people eat them, too!

## Why 17 years? Why 13?

It's intriguing these cicadas have life expectancies that are prime numbers.

Some scientists believe this may be due to an evolutionary adaptation that secured their continued existence.

Even though the cicadas emerge in huge numbers, it's possible they could be wiped out by a parasite.

Let's consider the life cycle of this alleged parasite. One possibility is the parasite could have a one-year life cycle and eventually after waiting some years, they would find some cicadas to infest. The trouble with this is they wouldn't have anything to feed on for many years so would probably die out first.

If the parasite has a 2-year life cycle, it will very rarely coincide with the cicada's. Similarly for 3-year, 4-year and any number of years up to 17.

Compare this to the case of a 12-year cicada. It would coincide with parasite that had 1-year, 2-year, 3-year, 4-year, 6-year and 12-year cycles, so would be very vulnerable.

So for our prime-numbered life-cycle cicadas, the parasite would need to have a life cycle that exactly coincided with the cicadas'. No such parasite has ever been found, so it appears to have been a successful strategy.

## The next appearance

Of course, like many plants and animals, the number of these cicadas is dwindling due to habitat loss, soil disturbance and abundant use of insecticides.

In 2030, when the Magicicada are due to emerge again, what kind of world will they see?

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