Figure 1.
SEM of fungal matrix degraded feathers.
Gallus gallus. (A) Cortex, dorsal surface, slight fungal degradation exposing surfaces of syncitial barbules. Insets show detail of degraded surface of rachidial cortex and partially exposed syncitial barbules (identified by characteristic thickness ∼5.0 µm; white semicircles). Upper and lower insets are details marked by right and left arrows respectively in (A). (B) Lateral tangential section of the epicortex of the rachis just below the barb base (marked x in (A)) showing cross-fibre structure. (C) Boundary between cortex and epicortex (approx. area in black rectangle in (A); arrows show the boundary where the cortex overlaps the epicortex, with a degraded area between arrows showing underlying epicortex with cross-fibres). Fungi can be seen at bottom of image.
Figure 2.
SEM of feathers of Gallus gallus and Falco peregrinus.
(A) Gallus gallus. Fungal degraded. Rachis epicortex between successive barbs. Patches of cuticular microvilli (middle, right; also see Fig. 2B) indicates section is at the surface. Successive layers of cross-fibres form a meshwork.The rippling effect represents fibre loss of tension (possibly from wear) nearest surface (see text; left to right = long axis of rachis). Lower inset rectangle (detail from Fig. 1C) shows two geodesic cross-fibre layers from a section in which fibres are under tension (straight). (B) Falco peregrinus. Native (non-biodegraded). Rachis epicortex adjacent to barb. Transverse section of entire depth of epicortex, cut at acute angle, shows numerous fibre layers as they naturally occur with matrix intact. Section shows approximately 16 layers, each comprised of a two-ply of oppositely oriented fibres (see rectangle). Top left shows epicortex surface with villus cuticle intact while bottom right tapers sharply to near tangential plane to union with barb (also see details in Fig. S4A, B and S5).
Figure 3.
SEM of feather rachis and barb epicortex.
(A) Rachis epicortex. Native (non-biodegraded). Otus leucotis. Tangential section (resin embedded and etched) in the left lateral wall of the rachis between adjacent barbs and very close to the medulloid pith layer below. Relief impressions of the medulloid pith cells can be seen on the overlying epicortical layer (dissected cells can be seen below). Inset, detail showing how epicortical fibre angles may change under pressure (also see Figure S1E). (B) Barb epicortex. Native (non-biodegraded). Bubo africanus. Tangential section (parallel to surface) showing at least three fibre layers with different fibre orientations (small arrows; top, long arrow = rachis long axis).
Figure 4.
SEM of barb cortical and epicortical microstructure.
Fungal selective matrix disassembly of Gallus gallus (A, D, E). (A) Dorso-lateral view of fungal action lifting a syncitial barbule from the left dorso-lateral surface of the barb cortex (see Fig. S1A). The arrowed hemi-circle indicates the cortex. Below it is the epicortex (right). (B) Native (non-biodegraded) feather. Falco peregrinus. Longitudinally sectioned barb cortex showing syncitial barbules (SI Fig.2E and inset). (C) Native (non-biodegraded) feather. Otus leucotis. Layers of barb epicortex fibres close to medulloid pith, closely packed with only traces of underlying layer (within oval) detectable. Impressions from the medullary cells underneath can be seen (elongated, bright areas). (D, E) Epicortex. (D) Cross-fibres of epicortex below and around barbules. (E) Several alternating layers of oppositely oriented fibres just below barbules (some outermost fibres are parallel with the barb long axis, arrow 1).
Figure 5.
SEM of fungal selective matrix disassembly of barb epicortex in Gallus gallus.
(A) Alternating cross-fibre structure of epicortex in section just above a barbule and below the cortex. Fungus in bottom right corner shows papulose apical tip of hypha. Arrow = long axis of barb. (B) Longitudinal view of barb from mid-length of rachis and at half-to three-quarters barb length reveals 3-D view of fibril bundles in epicortex in situ (∼0.8 µm thick). Fibrils, left and right (latter mostly degraded) oriented at ∼48.24° to the long axis (mean, Table S1). Inset shows detail of fibrils.
Figure 6.
A new microstructural fibre model of feather rachis and barbs and classic engineering analogues.
(A) An exploded view of three fibre divisions of the rachidial cortex and one of the barb cortex (both in dorsal and ventral walls). The cortex is identified by the thick syncitial barbules cells (∼6–8 µm in diameter [4]). The lateral walls of the rachis and barbs, the epicortex, are characterized by a crossed-fibre structure and absence of syncitial barbules cells. One barb shows cortex removed to expose the medullary pith cells. (B) Diagrammatic view of rachis and barb as an I-beam (here, a cantilever) in which most material is concentrated in the upper (tension) and lower (compression) surfaces to resist maximum stresses –with the “web” in the middle to resist shearing forces at ±45°. (C) Diagrammatic view of barb as a thin-walled pressure cylinder. Slice in latter shows circumferential stress is twice the longitudinal stress i.e. S2 = rp/t. S = stress, t = thickness, p = pressure, r = radius. Double-headed arrow = long axis of cylinder (modified after Lingham-Soliar [17]).