Across the animal kingdom, exaggerated weaponry is frequently used by one sex to contest access for potential mates. Within species, if disproportionate investment in weaponry confers an advantage to larger individuals, this may result in positive static allometry. It is predicted that the same selective pressures may also lead to positive evolutionary allometry, where larger species bear disproportionately large weapons on average, compared with smaller species. However it is unclear whether the slopes of species-specific static allometries are steeper among larger species, or remain consistent. All adult males across the New Zealand sheet-web spider genus Cambridgea bear exaggerated chelicerae which are used to compete for control of females’ webs. Here, we characterise the distribution of chelicera lengths within each sex of 12 Cambridgea species to show that chelicerae almost always exhibit positive static allometry in males while female chelicera lengths are consistently isometric. We use comparative phylogenetic methods to demonstrate that the slopes of static allometries steepen in males of larger species but that the ratio of average chelicera length to cephalothorax width is tightly conserved across taxa, leading to an isometric evolutionary allometry. While sexual selection may drive weapon exaggeration within species, resulting in steeper or shallower static allometries, this conservation of relative trait size suggests that chelicera length is subject to other stabilising selective pressures. Changes to species body plans might be constrained, while allowing for disproportionate investment in weapon traits at the extremes of body sizes within species.

Dataset includes measurements of cephalothorax width (CW) and chelicera length (Chl) in millimetres (mm) for adult males (m) and female (f) for 12 species of Cambridgea (Desidae), a genus of sheet-web spider endemic to Aotearoa New Zealand. Species included in dataset: C. ambigua, C. annulata, C. antipodiana, C. arboricola, C. fasciata, C. foliata, C. obscura, C. pallidula, C. peelensis, C. plagiata, C. quadromaculata, and C. reinga. Data was collected to examine intra- and inter-specific variations in chelicera length. Measured using digital hand callipers on specimens collected in the field and accessed from museums including the Entomology Research museum, Lincoln University (LUNZ); the Museum of New Zealand Te Papa Tongarewa, Wellington (MONZ); the New Zealand Arthropod Collection, Landcare research (NZAC); Auckland War Memorial Museum, Auckland (AMNZ); and Otago Museum (OMNZ). Field localities and collection details included in separate .csv files. 

The conservation status for the species included in this dataset has been designated as "data deficient" by the New Zealand Department of Conservation, indicating that these taxa cannot be assessed due to a lack of current information about their distribution and abundance. Specimens were collected under Department of Conservation permits (Authorizations: 45478-FAU, 45668-FAU).

Cambridgea chelicera length: Length of male and female Cambridgea of 12 species (cambridgea_chelicera_length.csv).