Functional diversity and behavioural use of fibre-producing silk glands in Pholcus phalangioides

Spiders possess multiple types of silk glands producing silks with contrasting properties. The behavioural use of the different glands is poorly known, limiting our understanding of the function-property correlation in spider silks. Here, we investigated the involvement of three fibre-producing silk glands (major ampullate, minor ampullate and aciniform glands) in locomotory, predatory, web construction and reproductive behaviours by the cosmopolitan cellar spider, Pholcus phalangioides (Pholcidae). To generate a comprehensive understanding of the function of different silks, a multidisciplinary approach was deployed, encompassing microscopy, cryo-fixation during behaviours, high-speed video recordings and mechanical measurements (tensile testing) of different silk products. The results challenge the one-gland-one-function paradigm, revealing that multiple silk types are often employed in combination and gland activation patterns are variable. Notably, of all investigated silk products, only the aerial dispersal lines uniquely relied on only one type of silk, underscoring its specialisation. Tensile testing uncovered significant variation in mechanical properties between different silk products, with walking draglines exhibiting the highest load and stress-strain values. Toughness values of the mixed silk bundles were comparable to those of the high-performing major ampullate silks of araneid spiders, demonstrating that independent pathways may lead to the evolution of high-performance biological materials. These findings emphasise the versatility of silk uses in spiders, offering new perspectives on the interplay between diversified silk properties and behavioural ecology in spiders.