A comparison of the cuticular properties of the female ticks Ixodes pacificus and Amblyomma hebraeum (Acari: Ixodidae) throughout the feeding period
The mechanical properties of the cuticle of Ixodes pacificus (Ip) are compared to those of Amblyomma hebraeum (Ah) from our earlier work. The 10-fold size difference between the species is expected to lead to significant differences in mechanical properties, because cuticular stretch depends on high internal hydrostatic pressure during the rapid phase of engorgement. We demonstrate here: (1) The cuticle of partially fed Ip is less stiff and viscous than that of Ah. (2) A stretch-recoil cycle in both ticks consists of recoverable deformation (ESv) and permanent deformation (ESp); ESp is higher in Ip, and increases sharply during the slow phase of engorgement, but not in Ah. (3) Injected dopamine (DA) increases ESp and reduces all measures of stiffness and viscosity, suggesting that a catecholaminergic neurotransmitter plays a fundamental role in modulating mechanical properties of the cuticle. However, unlike Ah, DA’s effect was not different from that of the control (1.2% NaCl). Mere insertion of the needle may have punctured the gut, causing the release of perhaps a catecholamine that increases ESp, an hypothesis supported by the fact that inserting a needle without any injection also caused an increase in ESp. (4) Stretch reduces ESp, but subjecting loops to pH 6.5 in vitro restores it. (5) Despite the smaller size of Ip, later onset of the rapid phase of engorgement, a thinner cuticle and different mechanical properties all reduce the internal pressure needed for stretch.
KeywordsIxodes pacificus Amblyomma hebraeum Ixodid tick Cuticle Young’s modulus Dopamine
Following retirement in 2012, WRK moved from Edmonton Alberta to Salt Spring Island, British Columbia. Dr. Justene Tedder of the Gulf Island Veterinary Clinic (GIVC) donated a room in her clinic to set up a laboratory for tick research. She and other members of GIVC have most generously provided ongoing support. We thank Prof. Michael Caldwell, Chair of the Department of Biological Sciences at the University of Alberta, who allowed us to take off campus the laboratory equipment and supplies that enabled us to continue our research. We also gratefully acknowledge the numerous residents of Salt Spring Island who provided us with ticks removed from their pets. Profs. Mike Williams, Phillip Choi and Morris Flynn of the University of Alberta and Prof. Julian Vincent of the University of Bath have provided helpful comments and advice throughout our research on the analysis of polymeric and biologic materials and the application of the Kelvin–Voight model.
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