Cicada cycles and prime numbers

Certain cicadas come back every 17 years. That seems pretty random. Or is it?

With 2,500 known cicada species in the world, North America boasts the Magicicada, which has the longest life cycle of any insect. What’s amazing about them is that every 17 years they all emerge at once, not to be heard from again until another 17 years elapse.

Some scientists believe that 17 being a prime number is no accident: if you're a predator with a one-, two-, four-, or even an eight-year life cycle, you won’t often have the opportunity to feast on cicadas that show up only once every 17 years.

For example, if a parasite has a life cycle of 4 years, the two species will only meet every 68 years, the least common multiple of 4 and 17. The periodical cicada thus escapes normal population control.

But why such a wait? Wouldn’t lower prime numbers like 5 or 7 be enough?

The answer may be climate. Cicadas like it hot. When they evolved over a million years ago, the eastern part of North America occasionally experienced very cold summers. Having a longer period lessens the chance that unusual cold will kill the whole brood.

Cicadas make up for their infrequent visits with high population density, which can reach 1.5 million cicadas per acre. There’s power in numbers. They usually emerge in late spring when soil temperature reaches 64 degrees Fahrenheit.

And do they make a racket! Cicadas can produce sounds up to 106 decibels, louder than a lawnmower and almost as loud as a car horn.

For some reason the cicada strategy has not been adopted by other insects or animals. But scientists have observed a 17 year cycle in the exposure times of the Earth to the interplanetary magnetic field. Maybe cicadas just have a magnetic personality.

Blue eyes

Paris Hilton’s eyes are really brown (she wears blue contact lenses). Frank Sinatra was known as Old blue eyes. What is it about blue eyes?

Blue eyes are basically a human trait. Blue-eyed black lemurs are the only non-human primates to have truly blue eyes. There are some cats with blue eyes: Here’s a strange fact: Regardless of breed, white cats with two blue eyes are usually deaf.

Lots of insects have blue eyes: the huge caterpillar of the oleander hawk moth for example. And the one in a million cicada found yesterday by a 6 year old in a Chicago suburb.

Here are the numbers for humans: Around 8% of the world's population has blue eyes, although nearly 90% of Icelanders have blue or green eyes. Thirty-four percent of non-Hispanic whites born in the US between 1936 and 1951 have blue eyes – it was 57% for those born between 1899 and 1905. Like UPS, the US is going brown.

Categorizing people by eye color is tough: Actress Kate Bosworth has one blue eye and one hazel eye. Actress Jane Seymour has one brown eye and one green eye. Contrary to appearances, David Bowie’s eyes have the same color. It’s just that the pupil of one eye is always dilated; making it look like it has a different iris color.

Most Caucasians are born with blue eyes, which may change to green, hazel, or brown as they grow older. Exposure to light after birth triggers the production of melanin in the iris so that by age three the eyes take on their permanent color, assuming no contacts of course.

Here’s some other blue trivia: There are only two animals with blue tongues, the Black Bear and the Chow dog. A robin's egg is blue, but after you put it in vinegar for thirty days it turns yellow. Blueberries are the only non-purple blue fruit or vegetable. I wonder if that’s on Paris’s jailhouse menu.

And can Paris Hilton wear blue contacts in jail? Or is she, too, going brown?