The physical properties of the stick insect pad secretion are independent of body size

Abstract

Many insects use adhesive organs to climb. The ability to cling to surfaces is advantageous but is increasingly challenged as animals grow, due to the associated reduction in surface-to-volume ratio. Previous work has demonstrated that some climbing animals overcome this scaling problem by systematically altering the maximum force per area that their adhesive pads can sustain; their adhesive organs become more efficient as they grow, an observation which is also of substantial relevance for the design of bioinspired adhesives. What is the origin of this change in efficiency? In insects, adhesive contact is mediated by a thin film of a liquid, thought to increase adhesive performance via capillary and viscous forces. Here, we use interference reflection microscopy and dewetting experiments to measure the contact angle and dewetting speed of the secretion of pre-tarsal adhesive pads of Indian stick insects, varying in mass by over two orders of magnitude. Neither contact angle nor dewetting speed change significantly with body mass, suggesting that the key physical properties of the pad secretion—its surface tension and viscosity—are size-invariant. Thus, the observed change in pad efficiency is unlikely to arise from systematic changes of the physical properties of the pad secretion; the functional role of the secretion remains unclear.

Characterization of Morphologically Distinct Components in the Tarsal Secretion of Medauroidea extradentata (Phasmatodea) Using Cryo-Scanning Electron Microscopy

“…Firstly, due to the high viscosity, it was not possible to collect a large amount of tarsal fluid using self-pulled glass needles and a micromanipulator, and secondly, the tarsal fluid was distributed over the surface in numerous fine droplets, and thus, there was no sufficient initial volume (own observations). Nevertheless, some results regarding the physical properties of the tarsal secretion of Phasmatodea were recently provided in [ 35 ]. They measured the contact angle (°) and dewetting speed (µm/s) of the tarsal secretion of phasmids with different body sizes and showed that the surface tension and viscosity of the fluid on glass are independent of the body size [ 35 ].…”

“…It is then released through channels onto the substrate [ 25 , 26 ]. The chemical compositions and functions of the tarsal fluids of insects have been investigated in several studies during the past decades (e.g., [ 2 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]). A chemical analysis of the tarsal secretion was conducted for representatives of different insect groups, such as Diptera [ 36 ], Hymenoptera [ 37 , 38 ], Coleoptera [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ], Orthoptera [ 28 , 47 ], and Blattodea [ 34 ].…”

Attachment Performance of Stick Insects (Phasmatodea) on Plant Leaves with Different Surface Characteristics

“…The hydrophilic properties of the leaves enhanced adhesion of Phasmatodea species studied [ 49 ]. Phasmatodea have adhesive secretions [ 50 , 51 , 52 , 53 ] that could be positively affected by the hydrophilicity of the substrate [ 49 ].…”

Studying Stickiness: Methods, Trade-Offs, and Perspectives in Measuring Reversible Biological Adhesion and Friction

“…Surface tension is another important physical property of a bioadhesive fluid, as the capillary forces associated with it can be dominant at small spatial scales. However, to our knowledge, this property so far has been measured only indirectly through contact angle measurements [ 58 , 99 , 100 ]. Contact angle, or the angle between the substrate and fluid meniscus, quantifies the ‘wettability’ of a fluid on a substrate.…”

“…Contact angle, or the angle between the substrate and fluid meniscus, quantifies the ‘wettability’ of a fluid on a substrate. For insects and tree frogs, this contact angle has been found to be quite small (~10°) on a wide variety of substrates, so the adhesive fluid appears to be highly wetting regardless of substrate chemistry [ 58 , 99 , 100 ]. Recent studies of insects have made assumptions of the surface tension of the fluid given that it is comprised of hydrocarbons [ 101 , 102 ].…”