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A new bridge between studies of sexual selection and the massive literature on Caenorhabditis elegans behaviourand nervous system properties promise to provide important new insights into both fields. This paper shows that mate choice likely occurs in hermaphrodite C. elegans on the basis of stimulation from the male genital spicules, making it possible to apply the toolkit of extensive background knowledge of C. elegans and powerful modern techniques to test in unprecedented detail the leading hypotheses regarding one of the most sweeping trends in all of animal evolution, the especially rapid divergence of genital morphology. The recognition that sexual selection by mate choice may also occur in other contexts in C. elegans suggests additional payoffs from exploring previously unrecognized possibilities that female-active hermaphrodite reproductive behaviours are triggered by male stimulation. These facultative behaviours include attracting males, fleeing from or otherwise resisting males, opening the vulva to allow intromission, guiding sperm migration, avoiding rapid oviposition following copulation that results in sperm loss, expelling recently received sperm, and increasing feeding rates following copulation. [ABSTRACT FROM AUTHOR]

Simple Summary: The morphological designs and the behavior (rhythmic brushing, vibrating, scraping, and tapping) of the male genitalia in two species of crane flies indicate that male genital structures in both species function to stimulate the female during copulation. These observations are used to test current theoretical explanations of the rapid divergent evolution of male genitalia of animals with internal insemination. Male genitalia have been hypothesized to function as courtship devices during copulation, but it is difficult to use behavioral observations to test this hypothesis because male genitalia are usually hidden inside the female during copulation. In tipuloid flies, however, nearly all of the male's complex genital structures remain outside the female. Copulation behavior and genital morphology in Tipula (Triplicitipula) colei and T. (Lunatipula) translucida suggest that some male genital structures function to stimulate the female: male structures that contact the female bear tufts or dense arrays of modified setae on precisely the surfaces that contact the female; contact involves repeated, stereotyped rhythmic movements that include brushing, vibrating, scraping, and tapping; the movements are appropriately designed to utilize the morphology of the modified setae to stimulate the female; and the movements have little or no other perceptible mechanical effects on the female. The female structures contacted by these male genital movements fail to show the defensive designs predicted by the theories of genital evolution that are based on morphological species isolation or male–female morphological conflicts of interest; also unexplained by the conflict of interest hypothesis are female movements that seem designed to increase rather than avoid stimulation by the male. [ABSTRACT FROM AUTHOR]

Psychological mechanisms involved in perceiving and processing sensory information likely influence how sexual selection acts on mate choice signals. Two such mechanisms are habituation and neural adaptation. Both are nearly universal, and doubtless affect female perception and analysis of male mate choice signals, thus potentially affecting male reproductive success. Habituation may be especially important because it affects the female's attention. Both likely favor greater variation in mate choice signals, contrary to many typological models of mate choice based on "the" male display behavior of a species. They may favor variation in the sites on the female's body that are stimulated by male somatosensory stimuli, as well as signal intensity and rates and patterns of repetition. Both habituation and neural adaptation are advantageous under natural selection and could provide original, naturally selected biases in female responses that lead to Fisherian runaway evolution under sexual selection by female choice. The multiplicity of mechanisms affecting habituation and neural adaption may help explain the general evolutionary trend for male mate choice signals to diverge rapidly. Avoidance of female habituation and sensory adaptation may explain the previously unremarked but widespread trend in vertebrates and arthropods for male genitalia to make rhythmic, repetitive movements during copulation. [ABSTRACT FROM AUTHOR]