Recent comparative analyses of sexual size dimorphism in web-building spiders have not included data from the Hypochilidae, an ancient group of spiders in which half of the species have geographic distributions that are restricted to the Appalachian Mountains. Females are slightly larger than males as measured by cephalothorax width, but male leg 1 is much longer than that of females. We document the development of this dimorphism in the field and discuss the possible adaptive significance of this trait.