Questions about Cicadas!?

Question

The cicadas have returned to Louisville, KY! But they were just here four years ago, I thought they came every seven years? They've been here for a day or two already, so when are they going to be all gone? I was also wondering if they are out when it's dark? I had plans tonight but I am cancelling if the cicadas are going to be out.

Answer 1

There are about 1,500 species of cicadas known. There are those that appear yearly in midsummer, and there are also the so-called "periodic" cicadas. They appear at prime number intervals - 7 years, 13 years and 17 years.