Spiders spinning electrically charged nano-fibres

Abstract

Most spider threads are on the micrometre and sub-micrometre scale. Yet, there are some spiders that spin true nano-scale fibres such as the cribellate orb spider, Uloborus plumipes. Here, we analyse the highly specialized capture silk-spinning system of this spider and compare it with the silk extrusion systems of the more standard spider dragline threads. The cribellar silk extrusion system consists of tiny, morphologically basic glands each terminating through exceptionally long and narrow ducts in uniquely shaped silk outlets. Depending on spider size, hundreds to thousands of these outlet spigots cover the cribellum, a phylogenetically ancient spinning plate. We present details on the unique functional design of the cribellate gland-duct-spigot system and discuss design requirements for its specialist fibrils. The spinning of fibres on the nano-scale seems to have been facilitated by the evolution of a highly specialist way of direct spinning, which differs from the aqua-melt silk extrusion set-up more typical for other spiders.

Nanofibre production in spiders without electric charge

“…This kind of nanofibre production involving only low electric charge has attracted the interest of researchers, as technical nanofibres are typically produced by electrospinning, involving high tension (Teo and Ramakrishna, 2006;Kronenberger and Vollrath, 2015). From the biological perspective, evidence for a charging of cribellate fibres by the action of the calamistrum is missing so far.…”

“…Another type of capture thread, the cribellate capture thread, is even suggested to employ electrostatic forces not for the attraction of prey but for the formation of its own structure, using the repulsion between two identically charged objects (Opell, 1995a;Joel et al, 2015;Kronenberger and Vollrath, 2015). Prey is captured here by a combination of hygroscopic forces, van der Waals' forces and an entanglement of the prey in a wool-like mat of nanofibres (cribellate fibres) surrounding two larger stabilizing axial fibres (Opell, 1994;Hawthorn and Opell, 2003).…”

“…1) (Peters, 1984;Opell, 1989;Joel et al, 2015). It is commonly assumed that the adhesive cribellate fibres are kept separate within the puff by being charged uniformly during their extraction, leading to a repulsion of the fibres (Peters, 1984;Sahni et al, 2011;Kronenberger and Vollrath, 2015). 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).…”

“…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).…”

Adhesion of dry and wet electrostatic capture silk of uloborid spider

“…Blackledge and Hayashi 2006] that the cribellate system would fundamentally depend on its original, non--wetted, highly 'puffed out' configuration state for the nano--fibrils to retain their function. Indeed, the very spinning mechanism of the cribellum fibre composite is specially adapted to an electrostatic spinning process that leads to the configuration of hackled puffs of dry silk nano--filaments astride core carrier threads [Kronenberger and Vollrath 2015]. One must argue that the puffs, in turn, would rely on dryness for their continued function, if indeed electrostatic forces and nano--adhesion sites are key to their functionality.…”