After constructing their vertical triangle-webs, Hyptiotes cavatus (Hentz) tense them by reeling in monitoring line thread and holding it between their second and third legs . When a prey strikes its web, a spider releases this slack silk, suddenly reducing web tension and causing the web to shake (Lubin 1986; Opell 1982) . This behavior may also change the properties of the web's cribellar capture threads that extend across its four diverging "radii ." Like the cribellar threads of other uloborids, those of H. cavatus are composed of torus shaped puffs of fine cribellar fibrils deposited around supporting axial fibers (Fig. 1 ; Opell 1989a) . The reduction of web tension that occurs when spiders respond to prey may increase the width of these cribellar puffs, thereby exposing more surface area per unit length of cribellar thread and increasing its ability to hold prey . To determine if this occurs, we measured the properties of taut and slack cribellar threads of H. cavatus .
