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Stridulation and its significance in the waterbug genus Cenocorixa Jansson, Antti Risto Ilmari

Abstract

Stridulation in the waterbug genus Cenocorixa was studied in the field and experimentally in the laboratory. It was shown that both males and females stridulate. The stridulatory signals, analysed by use of a sound spectrograph, were shown to be species and sex specific, differing in temporal pattern of pulses, pulse rate, pulse structure, and signal length. It was shown that the annual rhythm of stridulation in both male and female is correlated with sexual maturity. Males will spontaneously stridulate when there is mature sperm in the testes, and this occurs in spring, early summer, and late fall. Females do not stridulate spontaneously, but can be induced to stridulate when they have chorionated eggs in the lateral oviducts, but no sperm in the receptaculum seminis; they are sexually mature only in the spring and early summer. Stridulation was shown to be important in behavior leading to successful copulation. Male stridulation functions as a calling signal facilitating pair-formation by attracting conspecific females, and as an agonistic signal serving to space out individuals. Males will answer almost any stridulatory signal, but only calls from a conspecific female initiate searching behavior. Receptive females respond to stridulatory stimuli from conspecific males by stridulating, and successful copulations were observed only when preceded by such signal recognition; female stridulation functions as an agreement signal. Stridulation serves as a premating isolating mechanism in Cenocorixa. However, it is not the only isolating mechanism, but is reinforced by geographic and ecological isolation in a number of cases. The Corixidae, since they mostly have only a single stridulatory signal that can function in at least two contexts, are considered to represent a primitive stage in evolution of stridulatory signals: a stage in which functional diversification of signals is just evolving.

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