Guanine as a colorant in spiders: development, genetics, phylogenetics and ecology

Coloration in spiders may serve a number of important functions, for example crypsis, mimicry, aposematism and thermoregulation (Oxford & Gillespie, 1998). However, the evolutionary elaboration of bright colours for these purposes is not straightforward. The opisthosomas of spiders have rather thin, elastic cuticles (Barth, 1973; Dalingwater, 1987) to allow for large changes in volume associated with sporadic feeding (Riechert & Harp, 1987) and, in females, the development of eggs. A consequence of this thinness is that the cuticle is virtually transparent. The peripheral volume of the opisthosoma is dominated by the digestive mass which, in most species, is a shade of brown. This, then, dictates the basic background colour upon which pigments have to be deployed. For species in which black or brown coloration is selectively advantageous, sufficient pigment can be deposited in the hypodermis for the optimum overall colour to be achieved. However, lighter pigments, for example yellows and reds, remain almost invisible against the brown background of the digestive mass. Spiders have evolved two devices to circumvent this problem when bright colours are advantageous. The first involves effectively increasing the thickness, but not the stiffness, of the cuticle by the use of hairs or flattened setae which may contain pigments or appear white or silvery as a result of structural coloration (Hill, 1979; Holl, 1987). This option is exploited in a number of families, for example the Salticidae. A second solution to the problem, adopted by many taxa, is to generate a white background between the brown digestive diverticula and the hypodermis, against which hypodermal pigments are viewed. This is achieved by depositing often massive amounts of guanine, the principal nitrogenous excretory product in spiders (Anderson, 1966), in peripheral cells of the digestive diverticula (Millot, 1926). Although normally confined to the opisthosoma, guanine is sometimes also found in the portion of the digestive mass that extends forwards into Guanine as a colorant in spiders: development, genetics, phylogenetics and ecology