Underwater Attachment of the Water-Lily Leaf Beetle Galerucella nymphaeae (Coleoptera, Chrysomelidae)

While the reversible attachment of artificial structures underwater has moved into the focus of many recent publications, the ability of organisms to walk on and attach to surfaces underwater remains almost unstudied. Here, we describe the behaviour of the water-lily leaf beetle Galerucella nymphaeae when it adheres to surfaces underwater and compare its attachment properties on hydrophilic and hydrophobic surfaces underwater and in the air. The beetles remained attached to horizontal leaves underwater for a few minutes and then detached. When the leaf was inclined, the beetles started to move upward immediately. There was no difference in the size of the tarsal air bubble visible beneath the beetles' tarsi underwater, between a hydrophilic (54 degrees contact angle of water) and a hydrophobic (99 degrees) surface. The beetles gained the highest traction forces on a hydrophilic surface in the air, the lowest on a hydrophobic surface in air, and intermediate traction on both surfaces underwater. The forces measured on both surfaces underwater did not differ significantly. We discuss factors responsible for the observed effects and conclude that capillary forces on the tarsal air bubble might play a major role in the adhesion to the studied surfaces.

Automated summary

This study describes the behavior of the water-lily leaf beetle when it adheres to surfaces underwater and compares its attachment properties on hydrophilic and hydrophobic surfaces underwater and in the air. The results show that the beetles can adhere to underwater surfaces for a few minutes and move upward when the leaf is inclined. The size of the tarsal air bubble does not differ between hydrophilic and hydrophobic surfaces. The beetles achieve the highest traction forces on a hydrophilic surface in the air, the lowest on a hydrophobic surface in air, and intermediate traction on both surfaces underwater. Capillary forces on the tarsal air bubble may play a major role in the adhesion to the studied surfaces.