Sexual selection and sexual dimorphism in a harem-polygynous insect, the alpine weta (Hemideina maori, Orthoptera Stenopelmatidae)

New Zealand tree weta (Orthoptera Stenopelmatidae; Hemideina spp.) have lif e-history traits that are typical of animals with female-defence mating sys tems. Individuals live for more than 1 year and single adult males fight to defend ha-rems of females from rival males in cavities in wood or under ro ck. The alpine weta, H. maori, inhabits schist outcrops ('tors') in the alp ine zone of New Zealand's South Island. In this species (and other Hemidein a) there is a large sexual dimorphism in cephalic weaponry; the mandibles u sed by males in fights are almost twice the size of those of females. In th is paper we (i) document the sexual difference in head and mandible allomet ry for H. maori and (ii) provide evidence for the hypothesis that sexual di morphism is a result of sexual selection for male defence of a cavity that contains from one to seven adult females. Within two populations of this sp ecies inhabiting single tors there were significant positive correlations b etween harem size and both the head and mandible size of males. However, a third population failed to show a significant relationship, possibly becaus e some of the large males were recently-moulted, young adults that had not yet acquired females.