It’s that time of year again: when cicada songs flood our warm summer nights, announcing their presence as they attract mates. As a kid I was told they only came out every 7 years. I was confused because I heard them every year. It turns out whomever told me they emerge every 7 years was wrong on two counts: some species of cicadas emerge every year, and some emerge every 13 or 17 years.
Here in Northern Illinois, we have two primary species of cicada: the Magicicada which emerges every 17 years, and the Dog-day cicada which comes out every year. The latter probably sounds more familiar to you. But in a couple of years, you might hear them both!
These different species use their different sounds to help distinguish one another, and to prevent cross-species mating.
So why do the Magicicadas wait for so long before they emerge? Flooding the area at once gives their species the best survival chance possible. The first couple thousand to emerge provide fodder to predators who eat as many as they want because the cicadas are so plentiful. Eventually, the predators get sick and tired of gorging on cicadas, which allows the remaining population to gather and breed with very few interruptions.
The Magicicada will emerge en masse in 2024, but a portion of their population straddle this date, emerging about 4 years too soon or 4 years too late.
If enough emerge early or late -- and survive long enough to reproduce -- they can potentially form a new group of cicadas. It’s like when a group is on a pub crawl and some folks leave a bit early for the next bar; then the original group and new group move onto different bars. It’s a new kind of pub crawl.
Continuing our pub crawl analogy, cicadas of all types chirp powerfully to attract mates and fellow cicadas. Being born by the tens or hundreds of thousands, cicadas biologically need to be loud to call their species together. They make good use of an air sac in their abdomen and two flexible diaphragms to get really loud. They rapidly push and pull these diaphragms to create pressure waves of about 4000 hertz. Sound familiar? Yup, they’re basically tiny Hemholtz resonators!