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adhesion of spider setea

adhesion of spider setea - 2733 The Journal of Experimental...

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Feet are one of the primary points of contact between an animal and its environment. Consequently, they feature morphological adaptations required for survival of an animal in its natural habitat. Larger animals often minimise friction between the feet and the substratum during locomotion with hard-walled hooves and claws. In comparison, smaller animals maximise friction, and a variety of attachment devices has evolved accordingly. A broad diversity of coupling and clamp- like structures that ensure attachment to rough surfaces can be found (e.g. Nachtigall, 1974; Stork, 1980a; Gorb, 2001; Scherge and Gorb, 2001); other devices enable attachment to comparably smooth surfaces. These latter systems usually involve small- scale structural modifications and can be categorised into wet adhesion and dry adhesion systems. The former is present in frogs (e.g. Emerson and Diehl, 1980; Green, 1981; Hanna and Barnes, 1991) and many insects. Flies, for example, make use of an adhesive fluid that is secreted from specialised glands and pores (e.g. Hasenfuss, 1977; Bauchhenß, 1979; Walker et al., 1985; Gorb, 1998). Dry adhesion, on the other hand, is used by geckoes (e.g. Gecko gecko ). The great adhesive capacity of these reptiles relies on ultrastructural specialisations of their foot pads (e.g. Ruibal and Ernst, 1965; Hiller, 1968; Williams and Peterson, 1982; Stork, 1983; Autumn et al., 2000). An analogous ultrastructure is found in spiders. In addition to the tarsal claws, which are present on the tarsus of all spiders, adhesive hairs can be distinguished in many species. These adhesive hairs are either distributed over the entire tarsus, as for example in Lycosid spiders (Rovner, 1978), or concentrated on the pretarsus as a tuft (scopula) lying ventral to the claws (Hill, 1977), as also found in the jumping spider Evarcha arcuata (Salticidae), where a scopula is found on each pretarsus. So far, the effectiveness of these attachment structures has not been analysed. In the present study, the adhesive force ( F a ) of the cuticular scopula was analysed via atomic force microscopy. This method permits highly localised measurements of mechanical surface parameters (Binning et al., 1986; Radmacher et al., 1994) and, thus, for the first time, the determination of the adhesive characteristics of the tiny terminal ends (setules) that supply the initial contact area with the substrate (Fig. · 1). Materials and methods Animals Spiders of the species Evarcha arcuata Clerck (Salticidae) were collected near Saarbrücken (south-western Germany), weighed and kept frozen until scanning electron microscopy (SEM) preparation. Atomic force microscopy (AFM) analyses were carried out on recently captured, untreated individuals. Scanning electron microscopy Prior to SEM studies, individuals were dehydrated in ascending acetone concentrations (70%, 80%, 90%, 100%), cleaned with ultrasound, critical-point-dried (CPD 030 Critical Point Dryer; Bal-Tec, Witten/Ruhr, Germany) and sputter- coated with gold (SCD 005 Sputter Coater; Bal-Tec).
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