The mating system of two hybridizing species of water striders (Gerridae)

Abstract

In Alberta and British Columbia, breeding males of pond-dwelling Limnoporus dissortis and L. notabilis defend territories around floating leaves and produce ripple signals (repel signals) at intruders. Before and during mating and oviposition, males also produce courtship signals, and repel signals if intruders appear. Females do not maintain territories or produce repel signals. In the field, repel signals were recorded and analyzed by computer, then played back by oscillating a leaf tip with a magnet. Dead females (models) in natural position were guided with thread into territories, to entice males to signal. Models were guided so the forelegs were within 3 mm of a magnetbearing leaftip. Repel-signaling males approached models closely, then switched to courtship signals, wherepon playbacks of repel signals caused the males immediately to reply with repel signals and attack the model. Males thus distinguished sex by presence or absence of the repel signal. With a dead male L. notabilis pinned near a magnet on a territory leaf, playbacks were made to patrolling Limnoporus and Gerris buenoi males that skated by and showed interest in the model. After playbacks, Limnoporus notabilis (large) males skated away at the usual slow patrolling pace, but hybrid (medium) and L. dissortis (small) males skated away rapidly, indicating that repel signals repel at least smaller intruder males. Gerris buenoi males skated away slowly after playbacks, their relatively small size indicating that small size was not by itself a necessary reason for a strider to skate rapidly away. Coexistent Limnoporus dissortis and L. notabilis hybridize extensively in Alberta and British Columbia. Waveform and frequency comparisons of repel signals from pure and hybrid individuals showed no significant difference in these characteristics. Repel signals from both species resulted in similar playback responses from both species, indicating that in this area ripple signals are not involved effectively in species discrimination.