1 Natural History Museum Bern, Department of Invertebrates, Bern; 2 Institute of Ecology and Evolution, Division of Community Ecology, University of Bern, Bern; 3 Arvenweg 4, CH-3604 Thun, Switzerland An organic coating keeps orb-weaving spiders (Araneae, Araneoidea, Araneidae) from sticking to their own capture threads Christian Kropf 1,2 ,Dina Bauer 1,3 ,Thomas Schla ¨ ppi 1,2 and Alain Jacob 1 Abstract More than 95% of orb-weaving spider species ensure prey capture success by producing viscous threads equipped with gluey droplets. However, this trap may bear serious risks for the web-inhabiting spider as well. The obvious question, how a spider avoids getting stuck in its own capture spiral, has gained little scientific attention up till now. In 1905, the French naturalist Jean-Henry Fabre concluded from anecdotal observation that orb-weaving spiders protect themselves by a fatty surface coating. Here, we test this hypothesis by indirectly measuring the force necessary to detach an autotomized spiderÕs leg from the capture spiral of its own web (here called Ôindex of adhesionÕ, IOA). Three groups of legs, each of the species Araneus diadematus Clerck, 1757 and Larinioides sclopetarius (Clerck, 1757), were tested. One was left untreated, one was washed with distilled water (H 2 O), and one was washed with the organic solvent carbon disulphide (CS 2 ). In both species, we found a weak IOA between the spider leg and the gluey capture spiral in untreated and water-washed legs without significant differences between the two. The IOA approximately doubled, when spider legs had been washed with carbon disulphide prior to measurement, that is, the CS 2 -washed legs stuck significantly more strongly than the untreated and water-washed legs. These results provide indirect evidence for a protective anti-adhesive organic coating on the spiderÕs body surface and so support FabreÕs hypothesis. Key words: Araneae – Araneoidea – Araneidae – capture threads – sticky threads – glue – evolution – adhesion – anti-adhesive coating Introduction Modern orb-weaving spiders (Araneoidea) are a diverse group of spiders (e.g. Coddington and Levi 1991; Griswold et al. 1998; Jocque´ and Dippenaar-Schoeman 2007) comprising roughly 11650 species worldwide (Platnick 2011). Limits and interrelationships of Araneoidea are still under discussion (Schu¨tt 2000, 2003; Griswold et al. 2005; Blackledge et al. 2009). One key innovation in the evolution of araneoids occured at the latest in the early Cretaceous, and probably even earlier in the Jurassic (Eskov 1984; Selden 1989; Zschokke 2003; Pen˜alver et al. 2006; Penney and Ortun˜o 2006; Selden and Penney 2010): the replacement of cribellate capture threads coated with puffs of dry adhesive fibrils in the orb-webs of araneoid ancestors (and in recent Deinopoidea, the sister group of araneoids) by viscous capture threads equipped with elaborate aqueous glue droplets in the orb-webs and their derivatives of modern araneoids (e.g. Peters 1983, 1984, 1987, 1992; Coddington 1986 with references therein; Bond and Opell 1998; Opell 1999; Opell and Bond 2001). These gluey capture threads (especially their axial fibres) are significantly stretchier than those of cribellate spirals (Ko¨hler and Vollrath 1995; Opell and Bond 2000, 2001; Blackledge and Hayashi 2006; Opell and Hendricks 2007) in this way increasing the overall stickiness of the araneoid capture thread. Further- more, the stickiness per web area has been shown to be considerably higher in araneoids than in cribellate orb-weavers, and a unique suspension bridge mechanism that recruits multiple glue droplets to simultaneously resist detachment by dynamic interactions between the stretchy axial fibres and the viscoelastic glue droplets increases the efficiency of the araneoid orb-web as well (Opell 1999; Opell and Hendricks 2007, 2009; Opell et al. 2008; Agnarsson and Blackledge 2009; Opell and Schwend 2009; Sahni et al. 2010). Energetic costs of web building (especially metabolic costs) may be higher in cribellate orb-weavers (Lubin 1986) although material costs of cribellate thread stickiness are lower (Opell and Schwend 2009). Recently, it was suggested that one decisive advantage of Araneoidea is their ability to build sticky webs from the second instar on, while cribellates are not able to do so before the third instar (Szlep 1961; Yu and Coddington 1990; Opell et al. 2011a). Taken together, these and other facts may well elucidate the much higher diversity of araneoid as compared to cribellate orb-weavers: today, over 95% of all orb-weavers belong to the Araneoidea (Coddington and Levi 1991; Bond and Opell 1998). The glue droplets on the araneoid capture spiral are produced by the so-called aggregate glands and are composed of various organic and inorganic compounds. The droplets are complex in structure: glycoprotein nodules within each droplet make the capture thread sticky while a hydrophilic cover attracts moisture from the atmosphere and so maintains the threadÕs water content (Tillinghast and Townley 1987; Town- ley 1990; Vollrath et al. 1990; Townley et al. 1991; Vollrath and Tillinghast 1991; Edmonds and Vollrath 1992; Vollrath 1992; Tillinghast et al. 1993; Peters 1995). The araneoid capture thread with its gluey droplets appears as a highly complex and dynamic multifunctional system (Opell and Hendricks 2010). The adhesion of gluey threads is always less than the tensile strength of their supporting axial fibres (Agnarsson and Blackledge 2009). So, if a capture thread is loaded too heavily, droplets get pulled off without breaking or splitting, then reform and can adhere again; in this way, web damage is efficiently prevented (Sahni et al. 2010; Opell et al. 2011b). In addition, glue droplets contain a central granule probably anchoring the glycoprotein glue to the axial fibre of the capture thread, in this way preventing slipping of the droplet on the axial fibre, especially when the capture thread gets stretched (Opell and Hendricks 2010). Glue droplets even contain different Corresponding author: Christian Kropf (christian.kropf@iee.unibe.ch) Contributing authors: Dina Bauer (dinamun@hotmail.com), Thomas Schla¨ppi (thomasschlaeppi@students.unibe.ch), Alain Jacob (alain.jacob@nmbe.ch) Ó 2011 Blackwell Verlag GmbH Accepted on 24 October 2011 J Zool Syst Evol Res doi: 10.1111/j.1439-0469.2011.00648.x J Zool Syst Evol Res (2012) 50(1), 14–18