Abstract
Insects from the order Embioptera (webspinners) spin silk fibres which are less than 200 nm in diameter. In this work, we characterized and compared the diameters of single silk fibres from nine species—Antipaluria urichi, Pararhagadochir trinitatis, Saussurembia calypso, Diradius vandykei, Aposthonia ceylonica, Haploembia solieri, H. tarsalis, Oligotoma nigra and O. saundersii. Silk from seven of these species have not been previously quantified. Our studies cover five of the 10 named taxonomic families and represent about one third of the known taxonomic family-level diversity in the order Embioptera. Naturally spun silk varied in diameter from 43.6 ± 1.7 nm for D. vandykei to 122.4 ± 3.2 nm for An. urichi. Mean fibre diameter did not correlate with adult female body length. Fibre diameter is more similar in closely related species than in more distantly related species. Field observations indicated that silk appears shiny and smooth when exposed to rainwater. We therefore measured contact angles to learn more about interactions between silk and water. Higher contact angles were measured for silks with wider fibre diameter and higher quantity of hydrophobic amino acids. High static contact angles (ranging up to 122° ± 3° for An. urichi) indicated that silken sheets spun by four arboreal, webspinner species were hydrophobic. A second contact angle measurement made on a previously wetted patch of silk resulted in a lower contact angle (average difference was greater than 27°) for all four species. Our studies suggest that silk fibres which had been previously exposed to water exhibited irreversible changes in hydrophobicity and water adhesion properties. Our results are in alignment with the ‘super-pinning’ site hypothesis by Yarger and co-workers to describe the hydrophobic, yet water adhesive, properties exhibited by webspinner silk fibres. The physical and chemical insights gained here may inform the synthesis and development of smaller diameter silk fibres with unique water adhesion properties.
Highlights
New technologies are being developed to synthesize artificial fibres with unique physical, mechanical or water-interaction properties based on genetically modified silk proteins from spiders and silkworms [1]
We may assume that placing natural silk under vacuum has no significant effect on fibre diameter because significant swelling in our samples due to additional water would suggest that our samples should be larger compared to the twice ‘dehydrated’ samples
Our study made progress along three avenues of investigation into webspinner silk diversity: (i) developing methods for imaging and quantification, (ii) testing whether fibre diameter differences were correlated with insect size or evolutionary history, and (iii) exploring interactions between naturally spun silk and water, inspired by observations of natural colonies in the field
Summary
New technologies are being developed to synthesize artificial fibres with unique physical, mechanical or water-interaction properties based on genetically modified silk proteins from spiders and silkworms [1]. To inform the synthesis of finer silk fibres, developers may take inspiration from insects of the order Embioptera (commonly called webspinners or embiids), which spin nature’s ‘finest known insect silk fibres’ [4]. With stereotypical stepping patterns [11], they create protective silken domiciles on trees (figure 1c) in humid climes. In dry regions, they live in leaf litter or in underground burrows, which they line with silk. Our survey included representatives from about one third of known families of webspinners [12]
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