Downchirp Echo From a Stationary Water Surface Wave Field Accompanying Whirligig Beetle (Gyrinidae).

To investigate the hypothesis that the whirligig beetle utilizes a water surface-wave analog of echolocation, we analyzed the stationary capillary-gravity wave field generated during its high-speed swimming (>23 cm/s) at the air-water interface. Our wave dynamics analysis reveals that as the beetle swims past a static object, the object broadcasts a reflected wave. When the object is in the path of beetle's swimming, a single frequency signal is generated. On the other hand, an object off the swimming path reflects a time-dependent frequency that pitches down, creating a distinct downchirp signal. Furthermore, the Doppler effect shifts and widens the frequency range of the perceived signal. The echo arrives with enough time for the beetle to respond to obstacles, allowing it to detect upstream obstacles before physical contact. This suggests that this wave field has the potential to significantly extend the beetle's sensory range. When integrated with its specialized split-vision eyes, this ripple-based echolocation would complete an essential, multimodal sensory system, granting the whirligig beetle spatial awareness across all three fluid domains.