Female-female competition in katydids: Sexual selection for increased sensitivity to a male signal?

In contrast to studies of sex-specific weaponry and other sexually selected traits, there has been no examination of Darwin's (1871, p. 418) suggestio n that elaborations or enlargements of "the organs of sense" function to en hance mating success. In certain katydids the size of thoracic spiracles, w hich are a main input into the hearing system, determines auditory sensitiv ity of females. sere we present evidence that sexual dimorphism in the spir acle size of a pollen katydid, Kawanaphila nartee, is a result of sexual se lection on females competing to locate nuptial-gift giving males. In field experiments in which female K. nartee were attracted to a calling male, we show a pairing advantage to females with larger auditory spiracles. The spi racle-size advantage was not a correlated result of a larger body size or m ass of winners. Finally, there was no spiracle-size advantage or body mass advantage for mating females in a later stage of competition when experimen tal females struggled for access to a silent male. We suggest that research on the detection of displays has lagged behind work on the displays themse lves; the focus has been on the species specificity of signal perception ra ther than on the fitness consequences of variation in the ability to detect cues from mates or predators.