Abstract
The silks of certain orb weaving spiders are emerging as high-quality optical materials. This motivates study of the optical properties of such silk and particularly the comparative optical properties of the silks of different species. Any differences in optical properties may impart biological advantage for a spider species and make the silks interesting for biomimetic prospecting as optical materials. A prior study of the reflectance of spider silks from 18 species reported results for three species of modern orb weaving spiders (Nephila clavipes, Argiope argentata and Micrathena Schreibersi) as having reduced reflectance in the UV range. (Modern in the context used here means more recently derived.) The reduced UV reflectance was interpreted as an adaptive advantage in making the silks less visible to insects. Herein, a standard, experimental technique for measuring the reflectance spectrum of diffuse surfaces, using commercially available equipment, has been applied to samples of the silks of four modern species of orb weaving spiders: Phonognatha graeffei, Eriophora transmarina, Nephila plumipes and Argiope keyserlingi. This is a different technique than used in the previous study. Three of the four silks measured have a reduced signal in the UV. By taking the form of the silks as optical elements into account, it is shown that this is attributable to a combination of wavelength-dependent absorption and scattering by the silks rather than differences in reflectance for the different silks. Phonognatha graeffei dragline silk emerges as a very interesting spider silk with a flat ‘reflectance'/scattering spectrum which may indicate it is a low UV absorbing dielectric micro-fibre. Overall the measurement emerges as having the potential to compare the large numbers of silks from different species to prospect for those which have desirable optical properties.
Highlights
The silks of certain orb weaving spiders are emerging as highquality optical materials
Given that the PX-2 pulsed xenon light source has an intensity at wavelengths of approximately 300 nm and approximately 700 nm that is only about 15% of the maximum value, the signal to noise at these wavelengths is 0.15 what it is at maximum intensity
The modest rise in normalized reflectance for the P. graeffei silk in the wavelength range 350–300 nm could be due to low signal to noise but it was observed reproducibly
Summary
The silks are not flat, transparent reflectors and so the effect of the shape of the silk on the effective reflected (backscattered) light has to be considered. A single, transparent cylinder of refractive index approximately 1.536 has been described as a first-order model for ecribellate radial silk [13] This is a standard, light scattering problem that has been solved [29,30]. The primary approach taken in this work is to study what results are obtained using a simpler, but essentially analogous, experiment to that reported in [19], as supported by current, commercially available photoreflectance measurement equipment. Such equipment has been used to measure the reflectance spectra of butterfly wings [31,32] and spider silk from the species P. clavis [33]. Before introducing the experiment and the results, the following information introduces the background optics relevant to informing the experimental design and the interpretation of the results
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