A Biological Model for Directional Sensing of Seismic Vibration

There are many applications for a compact device that is capable of indicating the direction of propagation of substrate vibration. In order to develop effective devices for this purpose, it can be helpful to examine biological systems that have evolved specialized sensory systems for finding a vibration source. We are studying an insect model of vibration localization that provides an approach to directional vibration sensing over very small spatial scales. The treehopper Umbonia crassicornis communicates using vibrational signals transmitted along plant stems in the form of bending waves. The insect detects these substrate vibrations using sensors in the legs. Because the legs in this small species span a distance along the stem of only 5 mm, the insect is faced with a difficult localization problem: time-of-arrival differences between receptors on different legs are in the microsecond range, and wavelengths are an order of magnitude larger than the insect’s own dimensions. To study this system we constructed a simulator that mimics the surface motion of propagating bending waves, then used the simulator to explore directional sensing mechanisms. Using laser vibrometry, we characterized the dynamic response of the insect’s body (analogous to a mass on a set of springs) when driven with vibration of the substrate. We found a remarkable mechanical directionality in the response of the insect’s body to substrate vibration, in which small time differences are converted to large amplitude differences across the insect's body. Preliminary evidence suggests that directionality results from the interaction of two modes of vibration in the insect’s body: one that responds to the spatial gradient of the vibrational signal, and one that responds to the spatial average of the signal over the region sampled. This system generates a marked directionality in the amplitude response of the insect’s body while sampling only 5 mm of a vibration gradient. We believe that a directional sensing system based on this insect model has direct applications to localization of other surface waves, such as those propagating along the surface of the ground.