Reversible Underwater Adhesion: The Unique C-shaped Suckers of Net-winged Midge Larvae (Blepharicera sp.).

Aquatic insects living in fast-flowing streams have developed various types of attachment systems to resist being carried away by strong currents. Combinations of various attachment devices offer aquatic insects advantages in underwater adhesion on substrates with different surface properties. In this study, the net-winged midge (Blepharicera sp.) larvae were investigated to understand micro-/nano-structural attachment mechanisms. The hierarchical structure of insect adhesive surfaces was characterized using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Centrifugal measurements were also conducted to measure the critical rotational velocity at which the larvae of Blepharicera sp. can adhere to substrates with varying roughness. Commercial suckers require smooth substrate surface to maintain a pressure that is lower than the surrounding pressure for adhesion under the sucker cup while the suckers of net-winged midge larvae possess hierarchical micro-/nano-structures, which attach closely to rough surfaces underwater. Furthermore, the functions of microstructures observed on the sucker, including wrinkled surface, inward setae, outer fibers, and nick were explored and may contribute to underwater adhesion. The aligned C-shaped suckers can attach and detach effectively by closing or opening the gap. The unique microstructure and adhesion capability of such suckers could shed light on the design and synthesis of novel bio-inspired devices for reversible underwater adhesion.

Climbing upstream: Multi-scale structural characterization and underwater adhesion of the Pulin river loach (Sinogastromyzon puliensis).

Hillstream fishes live in the current with high flow velocity and strong water impact. To survive in such environments, they evolved various adhesive apparatus and attachment mechanisms. In this study, we investigated the Pulin river loach (Sinogatromyzon puliensis) known for its ability to climb vertically against rapid torrent. The whole body forms a suction system, consisting of lips, overlapped pectoral and fused pelvic fins. The hierarchical structure of lips and fins was characterized by Scanning Electron Microscopy. Polygonal pad-like protrusions, called unculi, were found on the lips and micro-sized, needle-like unculi (setae) covered both pectoral and pelvic fin rays. Pull-off stress was measured to evaluate the adhesion capability of dead river loach to substrates with varying surface roughness and compared with commercial suction cups. Results showed that river loaches can adhere equally well to surfaces with varying roughness while suction cups fail to adhere to rough surfaces. Underwater adhesion was accomplished by three possible mechanisms: (1) unculi enhance friction force on substrates, (2) setae interlock with irregularities, and (3) setae seal the edge and prevent leakage. The specialized adhesion apparatus and capability of Pulin river loach could provide inspirations and design strategies for novel underwater adhesives and devices.