Spider Web Diffraction

January 19, 2005

Ragno_spettrofotometrista1 copy

Referred by: Claudio De Felice
Summary authors & editors: Claudio De Felice

The above image indicates how a spider-web, if made by exceptionally thin silk threads, can act as a light diffraction grating. This web exhibited unusual glittering in sunlight during a perfectly clear day in early September in Siena, Italy. A series of light diffraction spectra were taken with surprising results -- the visible dark bands being the consequence of wave interference. The observation suggested a silk thread diameter value comparable to the visible spectrum wavelength range (about 400-700 nanometers [nm]). Scanning electron microscopy (SEM) measurements confirmed this hypothesis, by evidencing an average thread diameter of 496 nm (extreme values range: 236-828 nm), a value significantly smaller than that observed in typical webs (i.e., average: 1,134 nm; extreme values range: 828-1660 nm). Whether diffraction may serve some behavioural purposes, such as attracting potential visually-oriented preys, in this particular spider species remains to be clarified.

Notes: SEM measurements on webs specimens were performed by Alessandro Gradi, and Giorgio Bianciardi, BS, MD, PhD. I wish to thank Prof. Mark E. Hauber, BS, PhD, School of Biological Sciences, University of Auckland, New Zealand, for inspiring suggestions and access to pre-printed scientific material.

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