Abstract
Of the 7–8 silk fibers making up an orb-web only the hierarchical structural organization of semicrystalline radial fibers -composed of major ampullate silk- has been studied in detail, given its fascinating mechanical features. While major ampullate silk’s nanofibrillar morphology is well established, knowhow on mesoscale (> 50–100 nm) assembly and its contribution to mechanical performance is limited. Much less is known on the hierarchical structural organization of other, generally less crystalline fibers contributing to an orb-webs’ function. Here we show by scanning X-ray micro&nanodiffraction that two fully amorphous, fine silk fibers from the center of an orb-web have different mesoscale features. One of the fibers has a fibrillar composite structure resembling stiff egg case silk. The other fiber has a skin–core structure based on a nanofibrillar ribbon wound around a disordered core. A fraction of nanofibrils appears to have assembled into mesoscale fibrils. This fiber becomes readily attached to the coat of major ampullate silk fibers. We observe that a detached fiber has ripped out the glycoprotein skin-layer containing polyglycine II nanocrystallites. The anchoring of the fiber in the coat suggests that it could serve for strengthening the tension and cohesion of major ampullate silk fibers.
Highlights
Of the 7–8 silk fibers making up an orb-web only the hierarchical structural organization of semicrystalline radial fibers -composed of major ampullate silk- has been studied in detail, given its fascinating mechanical features
We generated composite images (CIs) of the hub fragment by scanning microXRD, corresponding to density projections based on X-ray scattering contrast
We identify in the WAXS CI (Fig. 2A) several orb-web features[1] such as two bunches of four radial threads and the hub-spiral as well as the mesh of decorating silk fibers
Summary
Of the 7–8 silk fibers making up an orb-web only the hierarchical structural organization of semicrystalline radial fibers -composed of major ampullate silk- has been studied in detail, given its fascinating mechanical features. The hierarchical structural organization of load-bearing radial or dragline fibers, composed principally of major ampullate gland silk (MaS) proteins, has been studied in most detail by scattering, imaging and spectroscopy techniques. There is, a lack of scattering or imaging evidence of higher-order (mesoscale: > 50–100 nm) assembly of nanofibrils in MaS fibers hierarchical network models based on nanofibrillar domains allow simulating mechanical data such as strain-hardening[21]. In the absence of network-reinforcing nodes discussed for MaS fibers[12,13], molecular or super-molecular reinforcement motifs (e.g. β-springs29) have been proposed contributing to the blend of toughness and extensibility of flag fibers[30] Enhancing these functional properties by mesoscale features could provide an evolutionary advantage for stopping larger preys impacting an orb-web while limiting the metabolic cost of making larger diameter fibers. A possible relation of the skin–core structure to the nature of prey caught in the web has been s uggested[27] but more flag fibers have to be studied across Araneoidea to establish a trend
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