Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

3.1 Introduction
The spectral properties of the ocular media, from the cornea to the photoreceptors of the retina, play a
significant role in tuning transmitted light as it transverses the optical tissues of the eye. Tissues may
filter out harmful ultraviolet (UV) radiation or increase the contrast of the retinal image, as with the
yellow lenses of squirrels (Walls 1931; Chou & Cullen 1984), some reptiles including snakes and
geckos (Walls 1942; Röll et al. 1996; Röll 2000) and some fishes (Walls & Judd 1933; Kennedy &
Milkman 1956; Muntz 1973), or the oil droplets in bird and reptile photoreceptors (Walls 1942;
Rodieck 1973). The spectral transmittance and absorption of the various ocular tissues and fluids have
been studied in all vertebrate taxa with some having received more attention than others. While they
have been well described for fish (review in Muntz 1972; Muntz 1973; Hawryshyn et al. 1985; Chou
& Hawryshyn 1987; Douglas & McGuigan 1989; Siebeck & Marshall 2001; Litherland et al. 2009)
and mammals (Pitts 1959; Boettner & Wolter 1962; Norren & Vos 1974; Chou & Cullen 1984),
comparatively few investigations have been done on individual media in birds (Emmerton et al. 1980;
Håstad et al. 2009) and very little on reptiles (sea snakes: Hart et al. 2012) and amphibia in which only
lens (Kennedy & Milkman 1956) and whole eye (Govardovskiĭ & Zueva 1974) measurements have
been published.
Snakes and geckos uniquely offer an extra layer to the visual apparatus in the form of the
spectacle, that corneous layer of integument that overlays their eyes. This spectacle may further tune
the spectrum of light by absorbing or reflecting wavelengths that are unnecessary for or deleterious to
an animal’s vision (due to chromatic aberration or scatter, Sivak 1982; Sivak & Mandelman 1982) or
that are potentially damaging to ocular tissues. But despite the potential of the spectacle in tuning
transmittance spectra, it has not been subject to much research in this area. The spectral transmittance
of the whole spectacle, including the scale, epidermis and dermis, has been published only for
hydrophiid sea snakes (Hart et al. 2012).
Reptilian spectacles, as described in Chapter 1, are composed of soft tissues (dermal stroma
and epithelia) and hard keratin. The dermis of the spectacle, presumably composed largely of collagen
53