Optics of the ultraviolet reflecting scales of a jumping spider.

The jumping spider Cosmophasis umbratica from Singapore is strongly sexually dimorphic. The males, but not the females, reflect ultraviolet as well as green-orange light. The scales responsible for this are composed of a chitin-air-chitin sandwich in which the chitin layers are three-quarters of a wavelength thick and the air gap a quarter wavelength (where lambda=600 nm, the peak wavelength of the principal reflection maximum). It is shown that this configuration produces a second reflectance peak at approximately 385 nm, accounting for the observed reflection in the ultraviolet. Other scales have a similar thickness of chitin but lack the air gap and thus produce a dull purple reflection. This novel mechanism provides the spiders with two colour signals, both of which are important in mating displays.