May 13, 2016
By Bob Merz
Zoological Manager of Invertebrates
There is a soundtrack of natural sounds to all of our lives. It plays in the background, often unnoticed, yet affecting our moods and memories. Sometimes it is silent; other times it is as loud as a rock concert.
It has been nearly 30 years since I've been in school, yet one sound, as reliable as the seasons, still whips up a twinge of melancholy that the long, warm, leisurely days of summer are coming to an end, and it is time to start preparing for winter. This sound is the droning buzz of Dog Day Cicadas on a hot, muggy August day.
I like to say that invertebrates are the group of animals we humans have the most interaction with in our lives, but know the least about. So, because just the sound of them calling has such an impact on my emotions, I thought I'd share what know about cicadas.
First and foremost, I grew up calling them "locusts." This is incorrect. Locusts are, in fact, the winged migratory phase of certain grasshopper species. To most people, "a bug is a bug," but to entomologists, there is a huge difference. So, just please remember, every time you call a cicada a locust....an entomologist somewhere cries.
Now, to complicate things, let me add that there's more than just one type of cicada. In fact, there are somewhere around 3,390 species worldwide –nearly 200 in North America. Missouri has somewhere between 20 and 30 reported species. And they all have different calls. A nice website to hear some of the calls is: cicadamania.com/audio/ .
So what's all the noise about? For the most part, it's about finding a mate. While cicadas do have alarm calls and courting calls (after a mate is found), the large majority of what we hear is simply, the boys letting the girls know that they are available and where they are located. Another interesting thing about their calls is that so not to damage their own hearing, males will cover their "ears"—a hearing organ called a tympanum—with a sort of "sound muffling" apparatus, a flap called an operculum.
Now, you know how you can dent a plastic soda bottle and when it pops back into form it makes a nice snapping noise? That is, in a simplistic way, how cicadas make their calls. Using muscles in their abdomen, they flex these drum-like structures called tymbals located just behind their last pair of legs. And each time they flex, the tymbals make a snap. But, cicadas do this really-really quickly so that it sounds like a droning whir.
Next, there is a lot of confusion about why so many cicadas emerge in certain years and not in others. This confusion exists, mainly, because it is kind of confusing. To simplify, know that some species of cicada emerge every year. These are called "annual cicadas." Other species emerge every 13 years. And some other species emerge every 17 years. The infrequent "emergers" are called "periodic cicadas."
What really gets confusing is that there are "broods" of both the 13-year and 17-year species that emerge in different years. So, in other words, not every single 17-year cicada emerges in the same year. Some groups are on a different 17-year cycle—the same for the 13-year species. So, some years, in certain places, a 17-year emergence will coincide with a 13-year emergence.