Data-driven frame-by-frame analysis of surface swimming and terrestrial walking in the coastal beetle Phaleria atriceps
Background. Inter-leg coordination of insects changes depending on environmental conditions. Although locomotor changes between terrestrial and aquatic environments have been examined in various insects, relatively little is known about coastal insects living at the boundary between these environments. We therefore compared the inter-leg coordination during terrestrial walking with that during surface swimming in the coastal beetle Phaleria atriceps. Methods. Surface swimming was recorded in 17 individuals and terrestrial walking in 10 individuals using high-speed video at 1,000 frames/s. The six leg tips and anterior and posterior body landmarks were tracked frame-by-frame using DeepLabCut, and leg movements were quantified in a body-centered coordinate system. Leg movement periods were estimated from sinusoidal fits to longitudinal leg-tip trajectories. Inter-leg phase relationships were calculated for all 15 leg pairs and integrated to reconstruct a representative six-leg phase organization for each locomotor mode. Results. Leg movement periods were significantly longer during surface swimming than during terrestrial walking for all six legs. During walking, the six-leg phase relationship closely matched an alternating tripod pattern, with a left foreleg (L1), a right middle leg (R2), and a left hind leg (L3) forming one phase group and a right foreleg (R1), a left middle leg (L2), and a right hind leg (R3) forming the opposing group. The same broad grouping was observed in the six-leg phase relationship during surface swimming, although significant differences in relative phase were detected for L1-L2, L1-R2, and R1-R2. Conclusions. Phaleria atriceps exhibited six-leg phase relationships broadly consistent with an alternating tripod pattern during both terrestrial walking and surface swimming, whereas leg movement periods and certain inter-leg phase relationships differed between the two locomotor modes. These findings provide a quantitative description of the similarities and differences in inter-leg coordination between terrestrial walking and surface swimming in a coastal beetle. Further mechanical and comparative studies will be needed to clarify the functional and evolutionary significance of these findings.