Prey catching behavior in most spiders is dominated by vibratory signals generated by prey animals. We have studied the release of the predatory behavior in two orb weavers,Zygiella x-notata andNephila clavipes (Araneidae) and measured threshold curves by applying sinusoidal stimuli to the webs (Fig. 1). 1. Resting position: WhileZygiella mostly stays in its retreat with one or both of its 1st legs on the end of the signal thread,Nephila sits in the hub of its orb web with its prosoma pointing downward (Figs. 2, 3). 2. Reaction: When stimulated by web vibrations the spiders show different behaviors ranging from weak movement of one leg to turning and then running toward the stimulus. The shortest latency time was 0.1 s for both spiders (video-analysis). The time for the turn toward the vibration source was 0.6 s forZygiella (upon arrival in hub) and 0.04 s forNephila. The resulting “error angles” measured 3.6 ° (±7.7 S.D.) forZygiella and 7.0 ° (±8.2 S.D.) forNephila (Fig. 5). 3. Airborne vibrations: A fly buzzing in the air above the spider rarely elicits elements of predatory behavior inNephila orZygiella. Instead, the spiders often lift their forelegs in this situation (Fig. 6b) displaying a defensive response never shown with web-borne vibrations. The threshold displacements of the web necessary to elicit predatory behavior did not change after ablation of all trichobothria (Zygiella). 4. Transverse sinusoidal vibrations: Stimuli were applied 6 cm away from the nearest leg of the spider to the signal thread (Zygiella) or radius (Nephila).Zygiella: The threshold displacement necessary to elicit predatory behavior is high at low frequencies (10 Hz, 61.3 dB) and steadily decreases toward high frequencies (1000 Hz, 12.9 dB) (0 dB = 1 μm). When keeping the displacement constant, the percentage of reactions increases considerably with stimulus frequency.Nephila: Unlike our finding inZygiella the threshold curve shows a minimum between 280 Hz and 420 Hz (30 Hz, 57.1 dB; 420 Hz, 24.2 dB). When using constant stimulus displacement the percentage of reactions is highest in the same frequency range (Figs. 7, 8). 5. Longitudinal vibrations: The threshold displacement needed to elicit predatory behavior ofZygiella is 3–4 dB lower for predominantly longitudinal vibrations than for transverse vibrations of the signal thread. The number of spiders reacting more than doubles both at low (160 Hz) and high frequencies (730 Hz) by using longitudinal instead of transverse stimulation at constant displacement (Fig. 8). 6. Active localization:Zygiella andNephila actively locate objects hanging motionless in their webs by vibrating the radii with their forelegs.Zygiella detected a 0.4 mg particle 6 cm away from the hub in 75 % of the cases and in one case even found a 0.05 mg particle. The information used by the spider is not derived from the thread tension itself but most likely from a change in the vibratory echo.