van der Waals and hygroscopic forces of adhesion generated by spider capture threads

Cribellar thread is the most primitive type of sticky prey capture thread found in aerial spider webs. Its outer surface is formed of thousands of fine fibrils that issue from a cribellum spinning field. The fibrils of primitive cribellar thread are cylindrical, whereas those of derived threads have nodes. Cribellar threads snag on insect setae but also adhere to smooth surfaces. A previous study showed empirically that cylindrical fibrils use only van der Waals forces to stick to smooth surfaces, as their stickiness is the same under different humidity. By contrast, noded fibrils are stickier under high humidity, where they are presumed to adsorb atmospheric water and implement hygroscopic (capillary) adhesion. Here, we model thread stickiness according to these two adhesive mechanisms. These models equate stickiness with the force necessary to overcome the adhesion of fibril contact points in a narrow band along each edge of the contact surface and to initiate peeling of the thread from the surface. Modeled and measured thread stickiness values are similar, supporting the operation of the hypothesized adhesive forces and portraying an important transition in the evolution of spider threads. Cribellar threads initially relied only on van der Waals forces to stick to smooth surfaces. The appearance of fibril nodes introduced hydrophilic sites that implemented hygroscopic force and increased thread stickiness under intermediate and high humidity.