Hearing dimorphism, trait variation and conflicts over space in the thorax of the bushcricket Requena verticalis (Listroscelidinae: Tettigoniidae: Orthoptera)

The hearing system of Requena verticalis is sexually dimorphic. Previous work has shown size of the auditory spiracle determines absolute threshold and as female spiracles are, on average, larger than males, females are more sensitive to the main energy of the male call. In all measured traits in morphology and physiology, females showed lower coefficient of variation than males. This difference was significant for bulla volume and hearing threshold. In addition, female ear size covaries with thorax dimensions but this is not so in males. Such a finding suggests stabilising selection on ear size in females, perhaps explained by the requirement of females to recognise and locate the male. As the auditory bulla is larger in females than males, so occupying thoracic space, we suggest a possible trade-off in this brachypterous species between hearing sensitivity and sound production. Finally, we examine relative growth of body structures not associated with hearing and those that influence hearing sensitivity. Scaling, where traits are under strong selection, may result in allometry. Female hearing traits show positive allometry with absolute size and while the relationship between bulla volume and spiracle area was positively allometric in females this was not the case for males.