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First published online February 12, 2007
Journal of Experimental Biology 210, 788-799 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02713

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Light habitats and the role of polarized iridescence in the sensory ecology of neotropical nymphalid butterflies (Lepidoptera: Nymphalidae)

Jonathan M. Douglas^1,*, Thomas W. Cronin², Tsyr-Huei Chiou² and Nathaniel J. Dominy³

¹ School of Life Sciences, Arizona State University, Tempe, AZ 85287-4601 USA
² Department of Biological Sciences, University of Maryland Baltimore County, Baltimore, MD 21250, USA
³ Department of Anthropology, University of California, Santa Cruz, CA 95064, USA

View larger version (43K): [in this window] [in a new window]   Fig. 1. Composite images of (A) non-polarized (Danaus erisimus) and (B) polarized (Memphis chaeronia) butterfly wings. Two pattern elements in B are polarized while none are polarized in A. The few instances where polarization exists in A are due to wear on the wings and the shine of the wing veins. False color imaging allowed transformation from a continuous to a binary polarized data set. Color reference keys: blue indicates no polarization while green, yellow, red and white indicate increased polarized reflectance (white=complete polarization).

View larger version (8K): [in this window] [in a new window]   Fig. 2. Graphical representation of the proportion of Costa Rican nymphalid species exhibiting polarized and non-polarized wing patterns in open and forest light environments. 68% of species dwelling within forests (N=104) with complex ambient light environments exhibit polarized reflectance patterns. 90% of species flying under open light conditions (N=40) display non-polarized reflectance patterns.

View larger version (96K): [in this window] [in a new window]   Fig. 3. Composite images of representative species from several nymphalid subfamilies (A) Prepona gnorima, (B) Baeotus baeotus, (C) Memphis xenocles and (D) Morpho cypris. A shows two differently colored localized bands of polarized iridescence while B and C exhibit prominent bands of polarized reflectance with an iridescent ground coloration that is polarized to a lesser extent. D presents a nearly uniform polarization pattern over the entire wing surface with the exception of the white diffuse reflecting patches along the midline of both wings. False-colored portions of the composite images are coded for intensity of polarization according to the color bar in the corner of each figure. False-color portions of the images are coded as in Fig. 1.

View larger version (89K): [in this window] [in a new window]   Fig. 4. Composite images of species from the nymphalid subfamily Heliconiinae, genus Heliconius. (A) H. melpomene, (B) H. charitonius, (C) H. cydno, (D) H. sapho, (E) H. hewitsoni, (F) H. cydno chioneus. In contrast to the depolarized patterns typical of open areas shown in A and B, C–F show a series of forest living co-mimics, each with polarized blue iridescent scales covering much of the wing surface. Variable white or yellow `windows' of pigmented scales reflect depolarized light. Heliconius cydno (C,F) is a close sister species with H. melpomene (A), showing the stark contrast that can arise between related butterflies living in dramatically different light environment. False-color portions of the images are coded as in Figs 1 and 3.

View larger version (62K): [in this window] [in a new window]   Fig. 5. Phylogenetic tree of the Nymphalidae compiled from published trees (Brower, 1994; Brower and Egan, 1997; Penz, 1999; Wahlberg and Zimmermann, 2000; Willmott et al., 2001; Wahlberg, 2001; Penz and DeVries, 2002; Blum et al., 2003; Wahlberg et al., 2003; Frietas and Brown, Jr, 2004; Murray and Prowell, 2005; Silva-Brandão et al., 2005; Wahlberg et al., 2005a; Wahlberg et al., 2005b) (A. V. Z. Brower, personal communication). Branch lengths are arbitrary. Black branches indicate a forest light environment, while white branches indicate open habitat. Ancestral branch states that could not be resolved by MacClade are hatched. Gains and losses of the polarized reflectance trait are shown as red and blue hatch marks, respectively. A concentrated changes test shows robust support for the correlated evolution of polarized reflectance patterns with life in forest environments (P0.008).