Brood XIX, or the Great Southern Brood, has left us and will not return for another 13 years. I will miss them. Is you want to see some great pictures and hear a recording of the cicadas check out the The Cicada Invasion by Kristin Oguntoyinbo from Snapshots from the Hill right here on There is something irresistibly fascinating about their 13 year life cycle. Imagine hanging on to the same root underground for 13 years and then finally coming above ground for just a couple of weeks before you die. I can’t decide if it’s heroic or sad. But I digress, this is a science blog. 
Significant research has been done over the years to determine why their life cycle is 13 years, the same being true for their brethren with 17 year cycles. Many theories have been investigated over the years, including an investigation of weather cycles during the Pleistocene era. In the end, the consensus is that evolution has slowly settled on cycle periods which are prime numbers. Give the world several million years to work something out and you can arrive at some surprising and innovative solutions.
Recall from math class that prime numbers, like 13 and 17, are only divisible by themselves and 1. If brood XIX had a 12 year life cycle, predators with life cycles of 2, 3, 4, and 6 years could have co-evolved to be accustomed to eat them. At 13 and 17 years a predator would have devise its own strategy to be largely dormant for 13 or 17 years. I like to think of the cicadas winning a marathon, methodically increasing their life cycles over the eons until they outlasted any predator. As an additional protection for the species, they all emerge from the ground at roughly the same time, a strategy biologists refer to as predator satiation. Basically, with millions of them emerging all at once, most of them will survive to reproduce.
My friend, Robert Newton, asked me “So how do they count to 13?” Well Robert, as always when you ask me a science question, I have been trying to work out the answer. Unfortunately, the answer is not particularly satisfying, nobody knows. Seriously, nobody knows. Sometime this spring millions upon millions of cicada’s bodies released a hormone that had not been released in the previous 12 years. Once this hormone was been released the Great Southern Brood just waited for the ground temperature to reach 63 oF,  and out they came to live out their short above-ground lives.
It may be an odd thing for the author of a science blog to say, but I like the idea that we don’t know how the cicadas count to 13. It preserves some mystery and wonder in the world and reminds us that there are still things we need to learn. See in you 2024 Brood XIX.

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