Getting a grip on spider attachment: an AFM approach to microstructure adhesion in arthropods

Although the spider exoskeleton, like those of all other arthropods (spiders, insects and crustaceans), consists of an extremely non-adhesive material known as cuticle, some spider species produce astonishingly high adhesive forces using cuticular appendages. Unlike other arthropods, they do not rely on sticky fluids but use a different strategy: the miniaturization and multiplication of contact elements. In this study the number of contact elements (setules) in the species Evarcha arcuata was determined at 624 000 with an average contact area of 1.7 x 10(5) nm(2). The total area of contact in this species measured 1.06 x 10(11) nm(2). By using atomic force microscopy it was shown that a single setule can produce an adhesive force of 41 nN perpendicular to a surface. Thus with a total adhesive force F-a = 2.56 x 10(-2) N and an average body mass of 15.1 mg, this species possesses a safety factor (adhesive force F-a/force for weight F-m) of 173. The tenacity a (ultimate tensile strength) amounts to 0.24 MPa. Due to the extreme miniaturization of the contact elements it is assumed that van der Waals forces are the underlying adhesive forces, although final evidence for this has yet to be provided. The present study was performed in order to clarify the fundamental basics of a biological attachment system and to supply potential input for the development of novel technical devices.