BGSU biologist Moira van Staaden examines a South African bladder grasshopper. The insects' impressive vocalizations may tell us something about the evolutionary process.

Loud South African grasshopper may tell us something about evolution

IIf you think crickets seem loud on a still summer night, you’ve probably never heard bladder grasshoppers.

And that would be understandable, since they’re indigenous to Africa—particularly South Africa—and have no relatives closer than California and Mexico.

But the grasshoppers’ sounds have become familiar to Moira van Staaden, biological sciences. She has studied the peculiar insects for nearly nine years, including the last two and a half with National Science Foundation funding, collecting them in her native South Africa to look for answers to questions about the interaction among their physiology, behavior and evolution.

“They clearly can do things they shouldn’t be able to do” considering their size, says van Staaden about the grasshoppers, which date from the Jurassic period.

For one, they make a lot of noise—almost 100 decibels if measured one meter away, compared to about 75 decibels produced by the familiar cricket. “It’s pretty loud,” she says, explaining that the relative racket results from the physical feature that puts the “bladder” in the grasshoppers’ name.

Roughly two inches long, the male of the biological family acquires an inflated, balloon-like abdomen at the final molt of his roughly four-month life. This fixed feature is an air-filled sac that acts as a resonating cavity. When the insect rubs two files together, the vibrations radiate on the abdominal surface, amplifier-like, to produce a loud signal.

Not only is it loud, but it’s also low-pitched and can be heard two kilometers (more than one mile) away, according to van Staaden. “There’s no other insect that can transmit an acoustic signal that far,” she says, noting that it flies in the face of the general rule of smaller beings—humans as well as insects—emitting higher-pitched sounds at lower-intensity frequencies.

Then there’s the matter of how the grasshoppers hear each other’s signals.

Van Staaden was introduced to them more than 20 years ago as an undergraduate at the University of Natal in South Africa. Asked in a laboratory exercise to locate the grasshoppers’ ears, she couldn’t. Rather than having to find only two ears, as most insects have, she learned that she was looking for 12, one pair for each abdominal segment.

In addition, there are two distinct kinds of ears, which are “incredibly sensitive” to sound, she says. The main ear has roughly 2,000 cells, compared to about 70 in a “normal” grasshopper. The other ears have only 11 or so cells, but those cells have acquired the function of ears, van Staaden adds, pointing as proof to experiments in which females have responded to a male’s signal even with the main ear disabled. In the mating ritual, males, which can fly, randomly call at night, and if a female responds, they duet so the male can track his non-flying partner, she explains.

How they can localize the source of the sound despite its low frequency is among the questions being addressed, she continues. Humans use the time lapse between a sound’s arrival at each ear to determine where the sound is coming from, she notes, but insects would have little time to do likewise. A process called scanning laser vibrometry—already done with one species of the grasshoppers—uses a laser beam to stimulate an animal with sound and measure its movement, enabling study of how the animal localizes the sound.

Also being studied is how different environments, such as forest and savannah, affect the quality and intensity of the grasshoppers’ signal. Sound transmits better in some habitats than others, van Staaden says, pointing out, for example, that low-frequency signals will go farther in the forest than those of high frequency, which are absorbed by vegetation and scattered.

She described the grasshoppers’ two kinds of ears in an article that was published in the journal Nature in 1998; an expanded version was published last October in the Journal of Comparative Neurology. Funded by the NSF, her current, $310,000 study of the creatures’ “acoustic lessons” continues through July 2005.

Van Staaden, who received her Ph.D. in zoology from Texas Tech University in 1989, joined the BGSU faculty full time in 2000. What brought her to Bowling Green, she says, was its J.P. Scott Center for Neuroscience, Mind and Behavior, where researchers study the relationships between animals’ nervous systems and behavior. That focus on behavior is what makes the center unique, says the associate professor, who has also done post-doctoral work at Harvard and taught at the University of Graz, Austria.

Moira van Staaden records a bladder grasshopper's sounds using this "biological microphone" for analysis in her research.