Weapon allometry and phenotypic correlation in the New Zealand sheetweb spider Cambridgea plagiata

Sexual selection has repeatedly driven the evolution of exaggerated secondary sexual traits in male animals. When multiple traits are used in competitive contexts, producing or bearing one trait may be costly and come at the expense of other traits via trade-offs. Conversely, sexually selected traits may show positively correlated patterns of relative investment. Males of the sheetweb spider, Cambridgea plagiata, have exaggerated chelicerae and use their chelicerae and forelegs in male–male contest behaviour. In the present study, we describe the linear scaling relationship of the chelicerae and forelegs of both males and females. Chelicerae length was positively allometric for males, but not for females, whereas fore-tibiae and fore-femora showed a slightly negative allometric relationship for both sexes. We found no evidence of a trade-off between the length of tibiae and femora with chelicerae length when comparing simple phenotypic correlations. Rather, the length of the tibia and femur, relative to body size, both increased when compared with relative chelicerae length, indicating that these traits might be under correlational selection. This suggests that larger chelicerae and forelegs are advantageous to males and that when resources are available, males will invest in both these traits.