alien press Functional Significance of the Capitate Supra-Fronto-Orbital Bristles of Male Medflies

Experimental removal of the sexually dimorphic supra-fronto-orbital bristles from male Ceratitis capitata lowered the chances that courting males would succeed in mounting females. Bristle removal did not affect male courtship behavior, and females often did not contact male bristles, so the male bristles probably function as visual display devices, despite the surprising fact that neither placement of the bristles nor courtship behavior are appropriate to display either the bristles' exaggerated length or the expansions at their tips to full effect. Indirect evidence suggests that male eye colors are also important in courtship, but that bristle asymmetry is not. Despite the fact that the medfly Ceratitis capitata Wied. is a serious agricultural pest that causes millions of dollars of damage annually, many aspects of its biology are as yet unstudied. One such poorly understood characteristic is the pair of sexually dimorphic bristles on the supra fronto orbital area of the head (hereafter the male "bristles"). In male medflies these bristles have a relatively long, light-colored stem that is bent dorsally near the base, and a dark, flattened, more or less diamond-shaped expansion at the tip (Figs. 1, 2). Arita and Kaneshiro (1983) mention that the modified bristles may contact the female during courtship. A second hypothesis, derived from the possibility that "Every aspect of the courtship ritual is directed at dispersing the pheromone toward the female" (Kaneshiro, 1993:26), is that they affect pheromone concentrations near the female. A third possibility is that the bristles form part of a visual courtship display: the colors of male eyes differ from those of females (below), and male courtship behavior performed immediately preceding male attempts to mount females includes rapid head rotations that are combined with smaller turning and nodding movements (Fig. 3) (Feron, 1962; Briceno et al., 1996). Hasson and Rossler (in prep.) note that the degree of asymmetry in size of male bristles is negatively associated with their size, and suggest that the asymmetry may reveal male quality. This note presents data on the morphology and positioning of the bristles on the male's head, and on the effects of bristles on female responses to male courtship. These data suggest that the bristles function as visual display devices.