780 INTRODUCTION Ommochrome pigments are a class of pigments that is widespread in insects and other arthropods. Their study has been mainly biochemical, and peaked in the ’70s and ’80s, just before the advent and rise of molecular biology (Linzen, 1974; Needham, 1974; Fuzeau-Braesch, 1972; Fuzeau-Braesch, 1985; Kayser, 1985). Scattered work has been carried out since then, related to the developmental biology of butterfly wing patterns (e.g. Koch, 1993; Nijhout, 1997; Reed and Nagy, 2005) or in the context of tryptophan metabolism (e.g. Han et al., 2007; Kato et al., 2006). The eye pigmentation pathway of Drosophila based on ommochrome synthesis and deposition has been extensively analyzed, mainly in the context of genetic and epigenetic work (e.g. Phillips and Forrest, 1980; Lloyd et al., 1998; Lloyd et al., 1999; Mackenzie et al., 2000). Ommochrome pigments are the main chromogenic class in the pathway from tryptophan. They range from gold (xanthommatin X) through red, purple and violet, up to black. The reduced form is red and the oxidised form usually yellow. The characteristic properties of ommochromes, i.e. redox behavior, absorption of ultraviolet and visible light, and low solubility, enable them not only to act as authentic functional pigments (eyes, integument), but also as an electron accepting or donating system and as metabolic end products (Needham, 1974). Ommochromes, principally xanthommatin, are widely distributed in arthropods as screening pigments in the accessory cells of the eyes and are also present in the retinula cells (Linzen, 1974). The functions of ommochromes are diverse and several complementary and non-exclusive hypotheses have been suggested for their common occurrence; these have been well reviewed in the general references cited above. Hypothesis 1: the ommochrome pathway is the main pathway for avoiding excess accumulation of highly toxic tryptophan. Insects and other spiralian phyla have never possessed or have lost the simpler vertebrate catabolism pathway for tryptophan based on the glutarate pathway. Supporting this hypothesis is the observation that ommochrome formation is strongly correlated with the massive breakdown of proteins at the onset of metamorphosis. This is the oldest and most popular view for the existence of ommochromes. Hypothesis 2: it is believed that the major function of ommochrome eye pigments is protection of photosensitive visual cells against excessive scattered light, and also to protect them against photodestruction by intense ultraviolet light (Langer, 1975; Stavenga, 1989). Ommochromes participate in the antioxidative system in invertebrate photoreceptors, like melanin in the eyes of vertebrates (Dontsov et al., 1984; Dontsov, 1999; Ostrovsky et al., 1987; Sakina et al., 1987). The The Journal of Experimental Biology 211, 780-789 Published by The Company of Biologists 2008 doi:10.1242/jeb.014043 The functional morphology of color changing in a spider: development of ommochrome pigment granules Teresita C. Insausti* and Jérôme Casas Université de Tours, Institut de Recherche sur la Biologie de lʼInsecte, UMR CNRS 6035, Av. Monge, Parc Grandmont, 37200 Tours, France *Author for correspondence (e-mail: tere.insausti@univ-tours.fr) Accepted 13 December 2007 SUMMARY Studies on the formation of ommochrome pigment granules are very few, despite their generalized occurrence as screening pigments in insect eyes. This is particularly true for ommochrome granules responsible for epidermal coloration. The aims of this study were to characterize the localization of major body pigments in a color changing mimetic spider, Misumena vatia (Thomisidae), and to describe the formation and location of ommochrome pigment granules responsible for the spiderʼs color change from white to yellow. The unpigmented cuticula of this spider is transparent. Both the guanine localized in guanine cells in the opisthosoma and the uric acid localized in epidermis cells in the prosoma are responsible for the white coloration. The bright yellow color is due to the combination of ommochrome pigment granules and the white reflectance from coincident guanine and/or uric acid. The formation of ommochrome pigment granules in epidermis cells proceeds via three distinctive steps. Translucent, UV fluorescent, progranules (type I) are produced by a dense network of endoplasmic reticulum associated with numerous mitochondria and glycogen rosettes. These progranules are present in white spiders only, and regularly distributed in the cytoplasm. The merging of several progranules of type I into a transient state (progranule type II) leads to the formation of granules (type III) characterized by their lack of fluorescence, their spherical sections and their osmophilic-electron-dense contents. They are found in yellow spiders and in the red stripes on the body sides. Their color varies from yellow to red. Thus, white spiders contain only type I granules, yellow tinted spiders contain type II and III granules and bright yellow spiders contain only type III granules. We present a synthetic view of the ontogeny of ommochrome granules. We discuss the physiology of color changing and the nature of the chemical compounds in the different types of granules. Extended studies on the ultrastructural modification and physiological processes associated with color change are required before any statement about the adaptiveness of the color change can be made. Key words: animal color, epidermis, zoochromes, kynurenine, 3-OH-kynurenine, mimetism, crab-spider, Misumena vatia. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY