Stabilizing Sexual Selection and Density-Dependent Correlates of Copulatory Success in the Ambush Bug, Phymata wolffii (Hemiptera: Reduviidae)

Abstract

Laboratory competition experiments using males of the ambush bug Phymata wolffii Stal (Hemiptera: Reduviidae), with either the mid or rear leg tarsi amputated, demonstrated that the rear leg is more important in facilitating preand postcopulatory guarding of females. In a natural Indiana population, the rear leg of males was significantly longer than that of females relative to body size (protonal width), suggesting sexual selection on male rear leg length. However, analysis of the allometric relationship between rear leg length and pronotal width revealed that sexual differences could be accounted for by the average greater size (pronotal width) of females and a nonlinear relationship between rear leg length and pronotal width, with males and females lying at different ends of the same allometric curve. Nevertheless, stabilizing sexual selection on the allometric relationship was detected in the smaller variance of the ratio, rear leg length: pronotal width among copulating plus guarding males vs. singletons, and in the overall greater variance in the ratio among females than males. In general, males of greater dry weight had greater copulatory success. However, only at low density was male size, measured as pronotal width, associated with copulatory success. Males mated most often with heavier and presumably more fecund females. The greater energetic commitment males make before and after copulation and in preand postcopulatory guarding selects for male preference for larger females. INTRODUCTION In insects, differences among males in the ability to compete for females or attract females with nutritional resources result in variation in the copulatory success of males (Thornhill and Alcock, 1983). Variation in the number of females inseminated is a major component of male reproductive variance (Wade and Arnold, 1980) and gives rise to sexual selection for male characteristics which increase competitive ability or access to females (Darwin, 1871). Since sexual selection is frequently intense (West-Eberhard, 1983), it may represent an important force directing phenotypic evolution (Arnold and Wade, 1984). The present study on a natural population of the ambush bug Phymata wolffii Stal (Heteroptera: Reduviidae) examines male characteristics associated with copulatory success and characteristics which may have evolved in response to sexual selection. In addition, the study identifies characteristics differentially associated with copulatory success in microhabitat areas of varying population density. Ecological variables such as habitat heterogeneity (e.g, McLain, 1981, 1984a and b) and population density (e.g, McLain, 1982a, 1985a; Dodson and Marshall, 1984a) have received relatively little attention for their effects on pair bonding in insects. In fact, very little is known of intraspecific variation in insect mating systems (Alcock, 1979). Investigations of phymatid sexual behavior in Phymata fasciata revealed a tendency for males to choose larger females only under higher population densities (Dodson and 'Present address: Department of Biology, Emory University, Atlanta, Ga. 30322