The corolla spider (Araneae: Segestriidae: Ariadna) of the Namib Desert gravel plains typically places seven or eight stones in a circle around its burrow entrance. It was examined whether the spider selects and places stones according to physical characteristics. Circle composition can be explained by the allometric scaling to spider size of the burrow entrance and the stones. The stones were usually placed with the narrowest side or point towards the burrow. Quartz crystals were preferred to four other stone types available. The spiders could detect prey brushing the outside of the circle stones that were about as wide as the spider's path of attack. I conclude that corolla spiders use the selected stones as tools to extend their foraging range.