Morphological adaptation of the calamistrum to the cribellate spinning process in Deinopoidae (Uloboridae, Deinopidae)

Abstract

Spiders are famous for their silk with fascinating mechanical properties. However, some can further produce, process and handle nano fibres, which are used as capture threads. These ‘cribellate spiders’ bear a specialized setae comb on their metatarsus (calamistrum), which modifies cribellate nano fibres to assemble a puffy structure within the capture thread. Among different species, the calamistrum morphology can differ remarkably. Although a model of thread production has been established for Uloborus plumipes, it is not resolved if/how different shaped calamistra influence the production process. We were able to transfer the model without restrictions to spiders with different shaped calamistra. Fibres are not locked between setae but are passing across a rather smooth surface-like area on the calamistrum. This area can be relocated, explaining the first morphological difference between calamistra, without changing the influence of the calamistrum on fibres. By performing an elongated leg movement, contact between fibres and calamistrum could be adjusted after finishing thread production. This movement has to bring the thread in contact with the second morphological peculiarity: cribellate teeth. We suggest these teeth are used to handle the thread independently of the spinnerets, a feature only necessary for spiders, which do not move during web construction.

Spidroin profiling of cribellate spiders provides insight into the evolution of spider prey capture strategies

“…The cribellate silk is generated by a small acinous-shaped cribellate silk gland 16 , 17 . Typical cribellate spiders, such as deinopids and uloborids, use a pseudoflagelliform silk produced by pseudoflagelliform spidroin (Pflag) instead of flagelliform silk produced by flagelliform spidroin (Flag) for core fibres 18 – 20 , which are coated with finely brushed cribellate silks 17 , 21 , 22 as is found in ecribellate orb-weavers. The cribellate capture threads are composed of materials with different mechanical properties, the core fibre is stiff, and the cribellate nanofibrils is extensible 23 .…”

Nanofibre production in spiders without electric charge

“…Because neither a compensatory behaviour nor an attraction between spiders and their capture threads was observed, we conclude that cribellate spiders cannot employ permanent larger charge through triboelectrification during capture thread production. Although we calculated that extremely low charges are sufficient to repel two nanofibres, the simulation of their arrangement within the thread only as a result of repulsion does not fit the characterized structure of the cribellate capture thread (Joel et al, 2015(Joel et al, , 2016. Hence, the employment of any electrostatic charge to establish the thread's structure can be excluded.…”

“…Our further mathematical calculations revealed, however, that nanofibres with such a low charge do accumulate in a denser outer layer as observed in reality ( Figs S1 and S2); however, single fibres are also uniformly distributed within the cross-section of the capture thread. But this does not fit the observed structural assembly of the capture thread, building a hollow structure around the two axial fibres (Joel et al, 2015(Joel et al, , 2016. Any employment of Coulomb forces to generate the structure of the cribellate thread is therefore very unlikely.…”

“…During the production of the thread, the nanofibres are extracted from the cribellum, a spinning plate with up to 40,000 spigots anterior to the spinnerets (Bertkau, 1882;Foelix and Jung, 1978). To assemble all these fibres to one functional thread, the spiders deploy a highly sophisticated movement of the spinnerets as well as the fourth pair of legs, finally wrapping the sheet of nanofibres around the axial fibres (Joel et al, 2015(Joel et al, , 2016. On the metatarsus of the fourth leg, cribellate spiders bear a comb-like row of specialized setae, the calamistrum.…”

“…It is assumed that the charge is transferred to the cribellate fibres by triboelectrification when they are brushed over a continuous area composed of the specialized setae of the calamistrum (Fig. 1B) (Peters, 1984;Kronenberger and Vollrath, 2015;Joel et al, 2016).…”