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First published online October 7, 2004
Journal of Experimental Biology 207, 3999-4009 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01240

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Blue integumentary structural colours in dragonflies (Odonata) are not produced by incoherent Tyndall scattering

Richard O. Prum^1,*, Jeff A. Cole² and Rodolfo H. Torres³

¹ Department of Ecology and Evolutionary Biology, Yale University, PO Box 208105, New Haven, CT 06520, USA
² Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA
³ Department of Mathematics, University of Kansas, Lawrence, KS 66045, USA

^* Author for correspondence (e-mail: richard.prum{at}yale.edu)

Accepted 12 August 2004

For nearly 80 years, the non-iridescent, blue, integumentary structural colours of dragonflies and damselflies (Odonata) have been attributed to incoherent Tyndall or Rayleigh scattering. We investigated the production of the integumentary structural colours of a damselfly – the familiar bluet, Enallagma civile (Coenagrionidae) – and a dragonfly – the common green darner, Anax junius (Aeshnidae) – using fibre optic spectrophotometry and transmission electron microscopy (TEM). The reflectance spectra of both species showed discrete reflectance peaks of 30% reflectance at 475 and 460 nm, respectively. These structural colours are produced by light scattering from closely packed arrays of spheres in the endoplasmic reticulum of box-shaped epidermal pigment cells underlying the cuticle. The observed reflectance spectra do not conform to the inverse fourth power relationship predicted for Tyndall/Rayleigh scattering. Two-dimensional (2-D) Fourier analysis of the TEM images of the colour-producing arrays reveals ring-shaped distributions of Fourier power at intermediate spatial frequencies, documenting a quasiordered nanostructure. The nanostructured Fourier power spectra falsify the assumption of spatial independence of scatterers that is required for incoherent scattering. Radial averages of the Fourier power spectrum indicate that the spheres are substantially nanostructured at the appropriate spatial scale to produce visible colours by coherent scattering. However, the spatial periodicity of the arrays is apparently too large to produce the observed colour by coherent scattering. The nanospheres could have expanded substantially (50%) during preparation for TEM. Alternatively, coherent light scattering could be occurring both from the surfaces and from structures at the centre of the spheres. These arrays of colour-producing spheres within pigment cells have convergently evolved at least 11–14 times independently within the Odonata. Structural colouration from arrays in living cells has also fostered the convergent evolution of temperature-dependent colour change in numerous odonate lineages.

Key words: structural colour, dragonfly, damselfly, coherent scattering, Tyndall scattering, Rayleigh scattering, pigment cells, Enallagma civile, Anax junius

This article has been cited by other articles:

				R. O. Prum, T. Quinn, and R. H. Torres Anatomically diverse butterfly scales all produce structural colours by coherent scattering J. Exp. Biol., February 15, 2006; 209(4): 748 - 765. [Abstract] [Full Text] [PDF]