Multimodal Aquatic Hexapods Use Hybrid Walking-Swimming Gaits in Shallow Water

Many animals use adaptive locomotor strategies to traverse the physical boundaries between aquatic, aerial, and terrestrial environments; these strategies can provide insight into not only evolutionary relationships but novel bio-inspired designs. Insects capable of locomotion both underwater and on land can offer a compelling window into the underlying mechanics of amphibious locomotion. This study focuses on understanding how aquatic hexapods (Dytiscidae and Notonectidae) transition their locomotion from dry land to shallow water. Using high-speed videography and kinematic tracking, we quantify different gaits at varying water depth (below, at, and above body height) as insects walk in a straight line. On dry land, diving beetles use the alternating tripod gait common to most terrestrial hexapods, while backswimmers primarily use their anterior legs to crawl at slower speeds. However, in shallow water, both insects exhibit novel "hybrid" gaits characterized by synchronous power/recovery strokes in the metathoracic legs (as in swimming) while the anterior legs maintain contact with the substrate (as in walking). At water levels at or above body height, both insects shift to full swimming, where there is no contact with the substrate and the body is propelled primarily or entirely by the bristled metathoracic legs. Our findings uncover how locomotion strategies can be adapted in response to competing forces (water resistance/fluid drag, buoyancy, and ground reaction) and demonstrate that limb coordination strategies can be tuned to transitional environments and across interfaces. These results provide insight into a previously understudied aspect of insect locomotion, and can fuel design inspiration for amphibious devices operating effectively across aquatic and terrestrial domains.