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

The hearing system of Requena verticalis is sexually dimorphic. Previous wo rk has shown size of the auditory spiracle determines absolute threshold an d as female spiracles are, on average, larger than mates, females are more sensitive to the main energy of the male call. In all measured traits in mo rphology and physiology, females showed lower coefficient of variation than males. This difference was significant for bulla volume and hearing thresh old. In addition, female ear size covaries with thorax dimensions but this is not so in males. Such a finding suggests stabilising selection on ear si ze 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 brac hypterous 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 un der strong selection, may result in allometry. Female hearing traits show p ositive allometry with absolute size and while the relationship between bul la volume and spiracle area was positively allometric in females this was n ot the case for males.