Fig. 4

Scanning electron microscopy summary of leg microstructures in Rhagovelia distincta. (A) Hindleg: (A1) a row of spoon setae, Sp, on proximal ventral tarsomere 2; (A2) anterio-ventral view of the tarsal joint (between tarsomeres 1 and 2), showing overlapping Sp setae with flattened tips, flanked by macrosetae 2, M2; (A3–A5) anterio-ventral views from proximal tarsomere 2 to the tarsal tip, showing a progressively beam-like structure formed by overlapping Sp setae on the ventral side; (A6) a row of spoon setae, Sp, on distal tarsomere 2; (A7) posterior-lateral view of distal tarsomere 2 with a row of spoon setae, Sp, along the water-interacting ventral side (yellow shading in A2–A7). (B) Midleg: (B1) schematic of the pretarsal swimming fan used for hydrodynamics-based thrust; (B2) anterio-ventral view of tarsomere 3 showing the fan and anterior claw extending from the cleft between two lobes (posterior claw not visible); (B3) protracted fan in water showing hierarchical structure of setae and setulae; (B4) anterior view of folded fan, highlighting relative thickness of setae and setulae; (B5) “board-like” cross-sectional shape of fan; (B6) cross-section of a fan seta showing internal layers, hollow core with pillars, and outer layers; (B7) surface of the anterior claw extruding from the cleft surrounded by hook setae (anterior lobe`s H1 setae visible); (B8) cross-section of the claw; (B9) Ventral edges of the cleft with rows of H1 (anterior lobe) and H2 (posterior lobe) setae; (B10) anterior view of the distal portion of tarsomere 3; (B11) close-up of the anterior lobe tip showing long, flattened modified H1 setae resembling spoon setae, Sp, and internal cleft wall lined with H2; tip of anterior claw also visible (posterior lobe removed); (B12) lateral view of tarsomere 2; (B13) ventral view of tarsomere 2 showing orderly rows of Sp and H1 setae. (C) Foreleg: (C1) posterior view showing sparse cuspidate setae, C, dorsal macrosetae 2, M2, and ventral hook setae, H1; (C2) anterio-ventral view showing microsetae, m, ventral hook setae, H1, and a row of spoon setae, Sp along the ventral water-interacting surface; (C3) close-up of spoon setae, Sp, near the claw base; (C4) array of spoon setae, Sp, at the tarsal tip. Color-shaded regions in SEM panels denote ventral (water-interacting) leg surfaces. Additional SEMs are provided for hindlegs in Figure S19, midlegs in Figures S11–S17, and forelegs in Figure S19.