Silk is a key innovation in spiders, fascinating both biologists and material scientists. However, to fulfil their biological function silken threads must be strongly fastened to substrates or other threads. The majority of modern spiders produce a unique and rather unexplored bio-adhesive: the two-compound pyriform secretion, which is spun into elaborate patterns (so called attachment discs) and used to anchor silken threads to substrates. Strong adhesion is achieved on a high variety of surfaces with a minimum of material consumption. Pyriform threads polymerize under ambient conditions, become functional within less than a second and can remain stable for years. They are biodegradable, biocompatible and highly versatile - the adhesion and the overall toughness of the attachment disc can be controlled by spinneret movements on a macroscopic level (ref. 1: V. Sahni et al., Nat. Commun., 2012, 3, 1106, DOI: 10.1038/ncomms2099). We found that the pyriform thread is a silk fibre that is coated with glue-like cement consisting of aligned nanofibrils, lipid enclosures and a dense, isotropic boundary layer. The threads are spun in a meshwork pattern that promotes stress distribution and crack arresting. Our results demonstrate, that hierarchical organization and fibre embedding may explain the high adhesive strength and flaw tolerance of a structure made by the same, rather simple type of silk glands.

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.