Echolocation in Whirligig Beetles Using Surface Waves: An Unsubstantiated Conjecture

Abstract

Understanding the physics of signal transmission in insect vibratory communication is nearly impossible for most natural substrates on which insect communicates, except for the water surface, on which the propagation of linear capillary surface waves is reasonably well mastered. The water surface is an environment where several insect species live and exploit its static and dynamic deformations. Whirligig beetles use surface waves in a wide range of situations, and it has been hypothesized that they could use the echo of their own waves to scan the water surface. These insects were also suspected to perceive menisci, i.e., static deformations of the water surface. However, no manipulative experimental studies, based on quantitative predictions of the surface deformation, have been conducted to determine whether echolocation or meniscus perception is used. The aim of this work was to test the hypothesis that whirligig beetles detect the meniscus around immobile objects. If the perception of an immobile object on the water surface can be explained by resorting to the meniscus only, then the hypothesis of echolocation should be reconsidered. We used cylinders of varying diameter and wettability crossing the water surface to experimentally modify the meniscus profiles. Contacts between beetles and cylinders were recorded using a high-speed camera. Loops in trajectories before or after a contact, as well as unfolding of forelegs before a contact, were used as criteria for the distance at which cylinders were perceived. Based on a theoretical modeling of the meniscus profiles, we predicted the distance at which one type of cylinder was detected based on the meniscus amplitude corresponding to the distance of perception of another type of cylinder. Both diameter size and wettability affected cylinder perception. Our predictions were unfortunately contradictory, and the unfolding of forelegs could not be explained by meniscus perception only. Complementary experiments are needed to eliminate any influence of vision. We obtained conflicting evidence about meniscus perception, with the statistically most powerful of our tests being strongly in favor of it. Thus, echolocation by whirligig beetles should still be considered as a conjecture, at best.