In this paper we define and describe morphological features that have colloquially been termed spiders and map their distribution in the south polar region of Mars. We show that these features go through a distinct seasonal evolution, exhibiting dark plumes and associated fan-shaped deposits during the local defrosting of the seasonal cap. We have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain. These observations are consistent with a geyser-like model for spider formation. Association with the transparent (cryptic) portion of the seasonal cap is consistent with basal sublimation and the resulting venting of CO2 gas. Also consistent with such venting is the observation of dark fan-shaped deposits apparently emanating from spider centers. Spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface.