Cicadas: Dr. Randy Hunt explores the science behind the Brood X buzz

3rd June 2021

By Steven Krolak

(NEW ALBANY, Ind.)–Buzzy, squirty and occasionally crunchy underfoot–the periodical cicadas of Brood X have begun to emerge in what will be Biblical bazillions for their once-every-seventeen-years frolic of mating, egg-laying and hatching.

Like the bugs themselves, human reactions to them are all over the map, ranging from “Cool!” to “Gross!” to “SHUT UP ALREADY!!”

For Dr, Randy Hunt, professor of biology, the emergence of Brood X is a chance to resume research he began during the insects’ last sojourn above ground, in 2004.

The primary focus of Hunt’s research over the years has been leafhoppers and treehoppers, which belong to the same large, diverse group of insects as cicadas. These insects communicate via inaudible (to humans) vibrations that they transmit through plants–like playing bongos on the branches with their strong legs. Despite belonging to the same group, cicadas are audible, over considerable distances. They do this through the tymbal, a pair of membranes that pop when compressed by the tymbal muscles. These pops resonate in the bug’s large abdominal sac, which acts like a megaphone, broadcasting the male’s intentions to the world.

The question that intrigues Hunt is whether cicadas also use inaudible vibrations as part of their mating behavior, once the males and females are sitting on the same branch.

“From a purely scientific perspective, the possible use of vibrations has bothered me for a long time, and it requires an answer,” Hunt said. “Either the ancestors of modern cicadas evolved sound communication and lost the use of vibrations, or modern cicadas might use vibrations along with sound.”

 Back in ’04, Hunt tried to get to the bottom of this question, but ran into some logistical obstacles.

“The cicadas proved very difficult to maintain and work on in the laboratory,” Hunt said. “But I learned a few things that might help this time.”

For one thing, he has used Nextdoor, an online community bulletin board, to recruit people who allow him to collect cicadas on their property.

“Collecting just as they emerged out of the ground was important so that I could separate and house males and females,” Hunt said. “It’s a great example of the citizen-scientist approach.”

For another thing, he has reworked the devices used to maintain the cicadas he captures–such as sleeve cages that loosely fit over the ends of the small tree branches where cicadas perch.

“In 2004 the cicadas wandered off the branches and frequently got stuck in the fabric,” Hunt said. “Mortality was a problem, so I redesigned the cages to make that less likely.”

He has upgraded all the technology used to record and play back their sounds.

Professor Randy Hunt places captured cicadas in sleeve cages at his home.

Once in the lab, the cicadas enter the world of the experiment. Hunt places a male on a twig that is wired to vibrate, simulating the presence of a female. Hunt plays an audio recording of cicada vocalizations he has made, to give the courtship a nudge. Any non-audible vibrations coming from the male will be recorded by a state-of-the-art laser Doppler vibrometer trained on the twig, sent to Hunt’s computer and converted to an audio file that will provide information about this aspect of cicada communication.

There are over 80,000 species in order hemiptera, which includes leafhoppers, Hunt explains. But there are only 400 species of cicadas, including the periodical group. Though small as a percentage of species within the order, the periodical cicadas are gargantuan in size compared to other relatives such as leafhoppers. And they are adept at flying.

Unlike leafhoppers that shake their abdomen to produce minute vibrations that travel through their legs and into a plant, the muscles attached to the tymbal of male cicadas allow them to produce powerful audible sound. Female cicadas, on the other hand, only make quiet clicking and fluttering sounds by flicking their wings after male lands nearby.  Some suspect that females use these sounds  at close range during courtship with males.

These factors are all interrelated.

“The evolution of loud acoustic signals has the advantage of increasing the chances of locating a distant mate,” Hunt said. “Evolving this ability can only occur when selection favors large size, since it is physically impossible for tiny insects to produce loud signals.”

So why are they so big?

“Some have suggested that selection for larger size in cicadas improves survivorship after the life cycle became longer,” Hunt said. 

If Hunt’s hypothesis is correct, it would suggest that vibrational communication complements airborne sound in the cicada’s mating system.  While the male’s loud sounds work at a distance, they may become less effective once a female lands nearby in the cluttered environment of a tree, Hunt explains.

“At this stage, directional, vibrational exchanges that allow a male to search for the now stationary female on a branch would be more effective than sound,” Hunt said.

The insects we see and hear are living out just a few days of their long life span, the rest of which, divided into five developmental stages, unfolds underground. We know a fair amount about cicadas, but far from everything. What determines their lifespan? What determines the cycles of their emergence–13 and 17 years? What do they really eat, and how does it sustain them exactly? Do they communicate with one another underground? Or is sex in the trees the only thing they talk about? These are just a few of the mysteries that still surround these creatures.

All of which adds up to one of the most epic spectacles you are likely to experience.

“Periodical cicadas interest me for the same reasons that they fascinate everyone,” Hunt said. “Their long-life cycle, massive emergence and incredibly loud singing represent one of the most remarkable phenomena in nature.”

Homepage photo: A periodic cicada perches on the finger of Dr. Randy Hunt, professor of biology, in his IU Southeast laboratory.

TAGS: , , , ,