Ultrastructure of spider thread anchorages

Abstract

Spiders attach silken threads to substrates by means of glue‐coated nanofibers (piriform silk), spun into disc‐like structures. The organization and ultrastructure of this nano‐composite silk are largely unknown, despite their implications for the biomechanical function and material properties of thread anchorages. In this work, the ultrastructure of silken attachment discs was studied in representatives of four spider families with Transmission Electron Microscopy to facilitate a mechanistic understanding of piriform silk function across spiders. Based on previous findings from comparative studies of piriform silk gland morphology, we hypothesized that the fibre‐glue proportion of piriform silk differs in different spiders, while the composition of fibre and glue fractions is consistent. Results confirmed large differences in the relative proportion of glue with low amounts in the orb weaver Nephila senegalensis (Araneidae) and the hunting spider Cupiennius salei (Ctenidae), larger amounts in the cobweb spider Parasteatoda tepidariorum (Theridiidae) and a complete reduction of the fibrous component in the haplogyne spider Pholcus phalangioides (Pholcidae). We rejected our hypothesis that glue ultrastructure is consistent. The glue is a colloid with polymeric and fluid fractions that strongly differ in proportions and assembly. We further confirmed that in all species studied both dragline and piriform silk fibres do not make contact with the environmental substrate. Instead, adhesion is established by a thin dense skin layer of the piriform glue. These results advance our understanding of piriform silk function and the interspecific variation of its properties, which is significant for spider biology, web function and the bioengineering of silk.

Strong and Tough Silk for Resilient Attachment Discs: The Mechanical Properties of Piriform Silk in the Spider Cupiennius salei (Keyserling, 1877)

“…Based on the observation of Wirth et al (2019) that in C. salei the glue fraction of piriform silk is much smaller (i.e., mean thickness of ∼20 nm) than the fiber fraction (thickness of ∼0.5-1.5 µm), we neglected the presence of the glue (third phase) in the calculation of the volume. We assume that the density of the piriform silk is similar to that of major ampullate silk (ρ p ∼ ρ silk ∼ 1.3 g cm 3 = ρ).…”

“…The sectioning of attachment discs revealed that the relative proportion of the cement-like glue and the fiber content of the attachment discs differs between spiders of different ecology and evolutionary history; and that the piriform fibers and the dragline are not in contact with the substrate (Denny, 1976;Wirth et al, 2019). Accordingly, the fibers are not directly involved in the attachment disc's adhesion, yet they may modulate the mechanical properties of the surrounding glue layer.…”

“…Accordingly, the fibers are not directly involved in the attachment disc's adhesion, yet they may modulate the mechanical properties of the surrounding glue layer. Whereas the ultrastructure of the piriform fibers is rather constant across species, the ultrastructure of the glue is very different (Wirth et al, 2019).…”

“…All these aspects, together with the structural hierarchical analysis that has been done on the piriform attachment discs (Wolff et al, 2015;Wirth et al, 2019), could be used for the development and the design of bioinspired adhesives with superior mechanical performances (Jain et al, 2014).…”

Evolution of aerial spider webs coincided with repeated structural optimization of silk anchorages

“…; Wirth et al. ). The openings of the numerous small piriform glands are grouped on the tip of the second pair of spinnerets (the “anterior lateral spinnerets”) around one or few openings of the large “major ampullate” glands, which produce the silk thread (“dragline”) used to build the web scaffold (Eberhard ).…”