Effect of shear forces and ageing on the compliance of adhesive pads in adult cockroaches

Abstract

The flexibility of insect adhesive pads is crucial for their ability to attach on rough surfaces. Here, we used transparent substrates with micropillars to test in adult cockroaches (Nauphoeta cinerea) whether and how the stiffness of smooth adhesive pads changes when shear forces are applied, and whether the insect's age has any influence. We found that during pulls towards the body, the pad's ability to conform to the surface microstructures was improved in comparison to a contact without shear, suggesting that shear forces make the pad more compliant. The mechanism underlying this shear-dependent increase in compliance is still unclear. The effect was not explained by viscoelastic creep, changes in normal pressure, or shear-induced pad rolling, which brings new areas of cuticle into surface contact. Adhesive pads were significantly stiffer in older cockroaches. Stiffness increased most rapidly in cockroaches aged between 2.5 and 4 months. This increase is probably based on wear and repair of the delicate adhesive cuticle. Recent wear (visualised by Methylene Blue staining) was not age dependent, whereas permanent damage (visible as brown scars) accumulated with age, reducing the pads' flexibility.

Adhesion and friction of the smooth attachment system of the cockroachGromphadorhina portentosaand the influence of the application of fluid adhesives

“…This result indicates the significance of the compliance of the pad cuticle, which is important for maximizing the real contact area and thus the adhesion-induced friction, namely in the push direction. Hence, with respect to the findings of Zhou et al (2015), the proximal euplantulae of cockroaches might show the opposite behaviour to that of the arolium, enhancing their compliance during distally directed shear stresses. In the hind tarsus, almost the same pattern has been observed as in the fore tarsus (all friction values reduced compared with the non-manipulated tarsus).…”

“…For the increased shear forces established in the arolia of N. cinerea , Zhou et al (2015) suggest some form of change in the cuticular pad material possibly related to the internal fibre structure. It is possible that in G. portentosa the observed anisotropy of the arolium is attributable to its unfolding in the preferential direction causing an enlargement of its contact area.…”

“…In this case, the authors have attributed this directionality to the interlocking of the surface irregularities with the ‘friction ridges’ established on the euplantula surface. In the same cockroach species, Zhou et al (2015) have found that pulling shear forces directed towards the body improve the compliance of the arolium and, thus, improve their capabilities on surface microstructures. On consideration of our results attained on fixed tarsi, we conclude that, in G. portentosa on microrough surfaces, the cuticular pad material itself together with its tarsal adhesion-mediating secretion is able to generate higher friction forces in the push direction (Fig.…”

“…of G. portentosa ( n =20) of about the same age and with no visible (age-related) damage of the adhesive pads (cf. Zhou et al, 2015) were taken from a laboratory colony and kept in individual plastic cages (20 cm×10 cm×6 cm) with moistened plaster of Paris (mixed with activated charcoal to prevent mould formation) as the ground material and a paper towel (Zewa, Mannheim, Baden-Württemberg, Germany) covering the plaster of Paris. Under anaesthetization with CO 2 , the cuticular claws of the cockroach were removed with a scalpel to prevent their interference with the attachment of the euplantulae and arolia.…”

Jumping without slipping: leafhoppers (Hemiptera: Cicadellidae) possess special tarsal structures for jumping from smooth surfaces

“…This mechanism ensures good grip during the acceleration phase when the legs are pushed and easy detachment when the legs are pulled at take-off. The engagement of platellae only during the contact phase of a jump may also lessen damage and wear, which would reduce attachment performance [48]. …”