An anti-adhesive surface coating reduces adhesion during contact with cribellar threads in Pholcus phalangioides (Araneae, Pholcidae) but not in the web-owning spider Uloborus plumipes(Araneae, Uloboridae)

Cribellar threads are powerful tools for web spiders to catch and retain prey. Spiders encountering such threads, like cribellar web spiders or araneophagic spiders invading cribellar webs, should have a protective mechanism against the adhesion of these threads. We tested for an anti-adhesive surface coating in the web invader Pholcus phalangioides and the cribellate orb weaver Uloborus plumipes. We calculated an index of adhesion for differently treated legs of the two species in a cribellar U. plumipes capture thread, i.e. untreated legs, water-washed legs, and legs washed with the organic solvent n-hexane. The results show that legs of P. phalangioides stick significantly stronger when washed with n-hexane. Our interpretation is that P. phalangioides has an organic surface coating lowering the adhesive force of the cribellar thread. No such mechanism was found in U. plumipes.


Abstract
22 Cribellar threads are powerful tools for web spiders to catch and retain prey. Spiders encountering 23 such threads, like cribellar web spiders or araneophagic spiders invading cribellar webs, should have a 24 protective mechanism against the adhesion of these threads. We tested for an anti-adhesive surface 25 coating in the web invader Pholcus phalangioides and the cribellate orb weaver Uloborus plumipes. 26 We calculated an index of adhesion for differently treated legs of the two species in a cribellar U.
27 plumipes capture thread, i.e. untreated legs, water-washed legs, and legs washed with the organic 28 solvent n-hexane. The results show that legs of P. phalangioides stick significantly stronger when 29 washed with n-hexane. Our interpretation is that P. phalangioides has an organic surface coating 30 lowering the adhesive force of the cribellar thread. No such mechanism was found in U. plumipes. 31 32 33 Introduction 34 Many spiders construct webs and use adhesives to catch and retain prey. There are two mayor 35 types of adhesion mechanisms used: threads equipped with droplets of viscous glue (1-3) and 36 cribellar threads, composed of two axial threads surrounded by thousands of nano-fibrils 37 (4,5). Any of these threads adhere strongly to a large variety of surfaces, but never to the web 38 owning spider. This lack of adhesion is not purely due to the spiders behaviour, such as tip-39 toeing or avoidance of adhesive parts since it touches the capture spiral during construction as 40 well as when struggling with large prey (2,6-8). So, the spider seems to have a protection 41 against its own capture threads. Elaborate investigations in how this protection is formed were 42 missing from scientific literature for many years. Two studies published in 2012 resumed the 43 idea of the French naturalist Jean Henry Fabre from 1905, suggesting a putative lipid surface 44 coating to prevent araneoid spiders of being caught in their gluey capture threads (6,8-10). By 46 different organic solvents dissolving lipids, they found an increased adhesive force between 47 the spider's leg and its own capture thread. It was concluded that these spiders either have 48 chemical compounds on their epicuticle, which are washed off by the organic solvent, or 49 some structural surface layer got altered by the chemicals applied (6,8).

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Protective mechanisms against capture threads can be assumed in web-building 106 The boxes were moistened directly before the spider was placed in the box and again after the web had 107 been used for measurements. After a web was used for measurements, the spider was fed with fruit 108 flies (Drosophila melanogaster) and provided with water. 116 Assignment and application of treatments 117 Spiders were euthanized in the freezer, then legs were cut off between femur and trochanter using 118 forceps. In P. phalangioides legs I and II were used for measurements, in U. plumipes we used legs II 119 and IV.

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We applied 3 different treatments on the spider legs: 1) washing with n-hexane (h) for 2 121 minutes and drying it for 10 minutes under a fume hood, 2) leaving the leg untreated (u) but keeping it 6 123 under a fume hood. The treatments were randomly assigned to each of the 3 legs dissected from one 124 spider. Random samples were controlled under the stereomicroscope for complete drying. All 125 measurements were conducted using a blind testing procedure, meaning that the measuring person had 126 no knowledge on the respective treatment.

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Finding reduced mobility of P. phalangioides in webs of U. plumipes puts up the 246 question of how the web invader preys on cribellar spiders. In our laboratory as well as in 247 greenhouses, where both species co-occur, we observed, that the Pholcus' web often is woven 248 adjacent to cribellar webs, even appear to be fused, with P. phalangioides adjusting its own 249 silk lines on the cribellar web. This might suggest, that P. phalangioides does not enter the