SUMMARYThe feet of the jumping spider Evarcha arcuata attach to rough substrates using tarsal claws. On smooth surfaces, however, attachment is achieved by means of a claw tuft, the scopula. All eight feet bear a tarsal scopula, which is equipped with setae, these again being covered by numerous setules. In E. arcuata, an estimated 624 000 setules, with a mean contact area of 1.7×105 nm2, are present. The spider's entire contact area thus totals 1.06×1011nm2. Adhesion to the substrate does not depend on the secretion of an adhesive fluid. Analysis via atomic force microscopy (AFM) shows that a single setule can produce an adhesive force (Fa) of 38.12 nN perpendicular to a surface. Consequently, at a total Fa of 2.38×10–2 N and a mean body mass of 15.1 mg, a safety factor (SF; Fa/Fm, where Fm is weight) of 160 is achieved. Tenacity (τn; Fa/A, where A is area of contact) amounts to 2.24×105 N m-2.

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During gliding, dragonfly wings can be interpreted as acting as ultra-light aerofoils which, for static reasons, have a well-defined cross-sectional corrugation. This corrugation forms profile valleys in which rotating vortices develop. The cross-sectional configuration varies greatly along the longitudinal axis of the wing. This produces different local aerodynamic characteristics. Analyses of the C(L)/C(D) characteristics, where C(L) and C(D) are the lift and drag coefficients, respectively (at Reynolds numbers Re of 7880 and 10 000), using a force balance system, have shown that all cross-sectional geometries have very low drag coefficients (C(D, min)<0.06) closely resembling those of flat plates. However, the wing profiles, depending upon their position along the span length, attain much higher lift values than flat plates. The orientation of the leading edge does not play an important role. The detectable lift forces can be compared with those of technical wing profiles for low Re numbers. Pressure measurements (at Re=9300) show that, because of rotating vortices along the chord length, not only is the effective profile form changed, but the pressure relationship on the profile is also changed. Irrespective of the side of the profile, negative pressure is produced in the profile valleys, and net negative pressure on the upper side of the profile is reached only at angles of attack greater than 0 degrees. These results demonstrate the importance of careful geometrical synchronisation as an answer to the static and aerodynamic demands placed upon the ultra-light aerofoils of a dragonfly.

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 × 105 nm2. The total area of contact in this species measured 1.06 × 1011 nm2. 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 Fa = 2.56 × 10−2 N and an average body mass of 15.1 mg, this species possesses a safety factor (adhesive force Fa/force for weight Fm) of 173. The tenacity σ (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.