Proposal # IOS-0820533/0820435
COLLABORATIVE RESEARCH: Novel mechanisms of mate localization in plant-dwelling insects: an integration of behavior, neurobiology and biomechanics.
Plant-feeding insects are among the most abundant and diverse organisms on earth, with a major impact on both natural ecosystems and agriculture. For many of these insects, survival and reproduction depend on the detection of low-amplitude vibrations of the plant surface, generated by the activity of potential mates, competitors, or natural enemies. However, in spite of the importance of plant-borne vibrations for insect behavior and ecology, two fundamental questions remain unanswered. First, how can a small insect determine the direction of a vibration source elsewhere on the same plant? And second, can an insect use the complex motion of plant stems and leaves during vibration transmission to estimate its distance from the source? To answer these questions, an interdisciplinary team has been assembled with expertise in behavior, neurobiology, mechanical engineering, and computational modeling. The team will develop new research tools and use them to understand how insect behavior is guided by mechanical vibrations, using computational models to integrate the results of behavioral experiments, sensory neurophysiology, and biomechanical measurements. Previous research by this team has revealed surprising sources of information that insects may use in sensing; for example, the motion of an insect?s body, resting on its six legs, can be highly sensitive to the direction of travel of a plant-borne vibration. Results of the proposed research will transform the current understanding of how mechanical vibrations influence the behavior of one of the most ecologically and economically important groups of organisms. Because collaborative teamwork is increasingly important for scientific progress, a major goal of the project will be to train graduate students in the skills needed for a career in interdisciplinary research. Finally, insights gained during the study could lead to the design of directional vibration sensors that could have an impact on many industries.
The major goal of this project is to identify the mechanisms that a small insect uses to determine the source of a vibrational signal. The treehopper, Umbonia crassicornis uses vibrational signals to locate mates within plants with complicated networks of branches. Male Umbonia locate females by sending vibrational calls through the branches to which receptive females respond with their own characteristic calls. The two perform a "duet" as the male advances toward the female, locating her by her signals. The male treehopper is a relatively small insect (1 cm) and the vibration signals he uses to locate the female are of a relatively low frequency, around 200 Hz. The wavelength of the signal he senses to locate the female is roughly 20 times his own body length. This poses a significant challenge because a vibration wave that has such a long wavelength relative to the size of the insect will show almost no differences across the span of insect’s body. The project is interdisciplinary, combining the biology of the insect’s signal production and sensing mechanisms with an engineering analysis of the mechanics of how the insect’s body moves to allow it to detect the direction of a signal. The engineering component of the work also included designing devices that can precisely deliver vibrations of specific frequencies and frequency combinations that behave as waves travelling in different directions, in one or two dimensions. Such a device could be used by other researchers who may need to deliver precise vibrational stimuli. The project has included both undergraduate and graduate students, from engineering and biology, including three individuals that are considered members of underrepresented groups in science and engineering. Student participants have been exposed to fields they may not otherwise have been in their studies. They have also learned to collaborate with people with very different skill sets, and by doing so, perhaps have gained an appreciation for the approaches taken by a field other than their own. The study found that sensory neurons in the animal’s legs detect vibrational signals and are best at detecting signals that have the same frequencies as those sent by a female treehopper. When looking at how this small insect can localize a vibration source, this study found that a male treehopper’s body moves differently when vibrational signals originate behind him than when they come from the front. Sensory recordings from the front and middle legs of a male treehopper showed that for signals coming from behind, there is a time difference in the response of the sensory neurons in the two types of legs at the frequency most commonly found in the female’s signal. Taken together, these results indicate that the very minute timing difference of a vibration wave travelling through a branch from the middle to the front leg is expanded to a much larger difference that can be used by the nervous system to detect the signals separately, and thus determine that the signal originated behind the animal. This expansion must be due to the mechanics of how the treehopper’s body moves for the vibration wave. Understanding the mechanics of how the insects accomplish this could provide ideas for the design of small sensors that can be used to detect low frequency vibrations traveling through a substrate like the ground. This project also included as study of how a male treehopper generates his courtship signals; how he vibrates the branch he sits on, including what muscles are used and how his body moves while he is signaling. These results are of interest to people who want to understand the many ways that animals communicate, and add to the general pool of knowledge about these little-studied animals.
