Effects on running speed of changes in sexual size dimorphism at maturity on in the cursorial huntsman spider, Delena cancerides (Sparassidae).

Running speed is a measure of whole-organism performance reflecting relative fitness. For spiders, increased speed translates into enhanced prey capture, mating success and reduced predation risk. In male spiders, leg length increases dramatically with the molt to sexual maturity. To determine how changes in leg length and body mass with sexual maturity influence running performance, we compared allometric and kinematic changes in a species without extreme size sexual dimorphism (SSD): male and female Delena cancerides (Sparassidae) during their penultimate and adult instars. Spiders in each age-sex class were filmed running in the lab, and body morphometrics, maximum velocity, body lengths per second, acceleration, stride length and stride frequency were compared. At maturity, females increase in overall size, whereas male's leg length increases over 30% with little associated increase in body mass or overall size. Adult male legs are similar in length to those of the adult females and maximum velocity did not differ between age-sex classes. However, both male age-classes have higher velocity scaled as body lengths per second than females, due to their lighter mass. Thus, for sparassids spiders without large SSD, lower mass and longer legs translate into lower energetic costs of running distances for males.