Mechanical properties of the cuticle of the tick, Amblyomma hebraeum (Acari: Ixodidae)

Abstract

Female ticks of the family Ixodidae increase their weight up to 100-fold during the 7-10 day feeding period. We determined the material properties of the alloscutal cuticle of female Amblyomma hebraeum from the time of moulting through full engorgement. The material properties of the cuticle were evaluated by a Kelvin-Voigt analysis of compliance determined from the stretch of loops of cuticle under stress. There was a 3-fold increase in cuticle dry weight during the first 3 weeks post moult, during which the ductility and stiffness of the cuticle increased substantially. Under stress, cuticle displayed time-dependent stretch, with a plastic (non-recoverable) and viscoelastic (recoverable) component. Plastic deformation was reasonably constant in the range of 10 to 15% over a wide range of induced stress above ∼0.6 MPa. The plastic component of tick alloscutal cuticle was about five to ten times higher than that of unsclerotized insect cuticle. Tick cuticle is also far more ductile than unsclerotized insect cuticle. Material properties of the cuticle did not change significantly as a function of cuticular water content over the normal range throughout the feeding cycle (13-37% wet weight). Injected dopamine (DA) reduced one measure of the viscosity of cuticle by 38%. Plastic deformability of cuticle was reduced by 70% after an in vitro stretch, but restored in fully engorged ticks, and in in-vitro stretched loops by treatment with DA and reduced pH. Thinning of cuticle by half during the rapid phase of engorgement requires plastic deformation (irreversible strain) in two orthogonal dimensions in excess of 40%. Treatment with DA increased plastic deformation and enabled extensibility (strain at the point of rupture) above 40%.