New Zealand tree weta (Orthoptera Stenopelmatidae;Hemideinaspp.) have life-history traits that are typical of animals with female-defence mating systems. 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 rock. The alpine weta,H. maori, inhabits schist outcrops (‘tors’) in the alpine zone of New Zealand's South Island. In this species (and otherHemideina) there is a large sexual dimorphism in cephalic weaponry; the mandibles used by males in fights are almost twice the size of those of females. In this paper we (i) document the sexual difference in head and mandible allometry forH. maoriand (ii) provide evidence for the hypothesis that sexual dimorphism 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 species inhabiting single tors there were significant positive correlations between harem size and both the head and mandible size of males. However, a third population failed to show a significant relationship, possibly because some of the large males were recently-moulted, young adults that had not yet acquired females.