Cricket to Cricket: Cricket Calling Songs and Neuronal Behavior
Thousands of crickets have lived and died in the Andrews University Department of Biology. If one were to stand still in Price Hall and hear the high chirp of a cricket, it is probably not a stray, outdoor cricket lost in the maze of biology classrooms and offices. In all likelihood, it is a lab cricket that has ventured outside of the plastic bin where it was raised.
Every Tuesday, a new shipment of Acheta domesticus, also known as the house cricket, arrives from Fluker’s Cricket Farm in Louisiana. The crickets are placed in a temperature-and light-controlled chamber until they have molted. A student, or “cricketeer” as the undergraduate cricket researchers call themselves, then examines the newly-molted insects and picks out the females, which are easy to spot because of the long ovipositor, or egg-laying organ, that extends from their abdomen. The females are placed in separate bins until the researchers are ready to begin an experiment.
As an Andrews biology professor, John Stout first started researching cricket behavior in the 1970s. In the 1980s, the research focus was expanded to include the neurophysiology of the crickets. The research continued to grow as faculty members Gordon Atkins and David Mbungu became interested in the project.
More than 30 journal articles have been published by the research group in journals such as the Journal of Comparative Physiology A, Physiological Entomology and the Journal of Experimental Zoology. Since Stout began his research, the main focus has been to understand the auditory system of crickets. While his research has been primarily concerned with female crickets, David Mbungu has begun working with male crickets as well, further expanding the research possibilities.
Undergraduate and graduate students have also been involved in the project. Over 50 honors theses and 26 biology masters theses have come from this project. One student, Benjamin Navia, came back to Andrews after pursuing graduate work and picked up where he left off with his six-legged friends. When John Stout retired, Navia joined the biology faculty and has continued conducting research on crickets. He also mentors undergraduate biology majors Re’jeanne Greene and Karis Kang on how to conduct the experiments and collect data.
According to Navia, there are a number of reasons why crickets are important. “When we do science,” he says, “we attempt to explain natural phenomena. In the process of looking for answers, we must have a model we can use to test our hypothesis.” Enter the crickets.
Crickets exhibit acoustic communication. In other words, male crickets chirp a specific kind of calling song to attract females. The song has a set of syllables for each chirp and females respond to the song by moving in the direction of the sound in what is called “phonotactic behavior.” The fact that they are able to respond to sound (phonotaxis) means that they have a nervous system. The nervous system is simple, nothing as complex as the human nervous system which contains somewhere around 85 billion neurons. Whereas a single human neuron cannot easily be examined, researchers can study a single cricket auditory neuron. This “enables us to perform meaningful experiments when attempting to study and describe a nervous system,” says Navia.
The goal of Navia’s research is to understand the underlying neuronal mechanisms that allow female crickets to respond to a specific calling song. A female responds to a male calling song by walking towards it, which is observable. However, not all females respond positively, indicating that there is a preference in the quality of the song.
Crickets’ “ears,” or tympanic membranes, are on their two front legs. They walk toward a sound in a zigzag motion, rather than in a straight line, depending on which leg picks up more sound. Navia, and the undergraduate students who work with him, can track the movement by placing a cricket on a specially made “cricket treadmill.” The cricket’s back is attached to a small rod with a bit of wax, leaving its legs free to move. It is then placed on a foam ball that rotates as the cricket moves. Sensors detect the movement and track it on a computer program. A speaker in front of the ball plays a computerized calling song composed of three chirps, which the female cricket then responds to.
If the female responds by walking toward the sound, she is then placed tummy up on a block of wax. Using a microscope, Navia or a student researcher finds the auditory nerve and attaches a suction electrode to it, close to the neuron. The calling song is played again and a computer records the neuronal response.
“It was initially thought that cricket phonotaxis was a fixed behavior,” says Navia, “in other words, that the female cricket would always respond the same way to the calling song of a male.” But there is variability in the behavior and recent studies have shown that female crickets do not always respond in the same manner. “Some females are very selective in terms of what call they respond to within that range of variation, while others tend to be more unselective and may respond to a wider spectrum of calls. A third group may only respond to calls that fall on either end of the range.” This variation is called “phonotactic plasticity” and is likely regulated hormonally and neuronally. A 2010 article by Stout, Navia, Atkins and others in Physiological Entomology concluded that age contributes to a reduction in syllable period selectiveness.
Navia is currently studying the role of the L3 auditory neuron in cricket phonotaxis. His experiments so far have demonstrated that the neuronal response is more positive when the three-chirp calling song has a period of 50–70 milliseconds between syllables. While the crickets still respond when the period time is lengthened or shortened, the specific range of 50–70 ms provokes the most significant response.
Navia tracks the responses in histogram software, which enables him to see where the greatest number of spikes in neuronal activity occurs. If the syllable period is longer or shorter than 50–70 ms, the number of spikes remains the same for all three syllables within the chirp. If the syllable period is 50–70 ms, the first syllable elicits the greatest number of spikes and steadily decreases for each following syllable within that chirp.
The results will soon be submitted for publication in a manuscript that Navia is working on titled “Selective Processing by the L3 Prothoracic Auditory Interneuron in Response to Model Calls in the Cricket Acheta domesticus.”
Photo: Benjamin Navia explains the "cricket traedmill" to a student.