SENSORY ECOLOGY OF A SEMI-AQUATIC SPIDER (DOLOMEDES-TRITON) .2. THE RELEASE OF PREDATORY BEHAVIOR BY WATER-SURFACE WAVES

The motor response of the fishing spiders D. triton and D. fimbriatus to water surface waves was studied by applying mono- and polyfrequency stimuli. At short distances (< 10 cm) from the stimulus source prey capture of D. triton can be elicited both by surface waves of the water (responsiveness of intact spiders to rod stimuli 68%) and by air-borne vibrations (buzzing fly) (responsiveness 48%). At distances larger than 10 cm, D. triton reacts exclusively to surface wave stimuli. Visual stimulation elicited prey capture behavior in only 14% of the trials. The spiders turn towards the wave source with a mean error angle of 13.6 .+-. 10.9.degree. SD (D. fimbriatus) and 9.3 .+-. 13.3.degree. (D. triton). After ablation of all trichobothria or after unilateral ablation of the metatarsal organs this angle increases significantly. For intact D. triton the ratio Q (running distance/source distance) is 1.3 .+-. 0.4. It decreases (improves) significantly if the spiders are blinded (1.1 .+-. 0.33) or if in addition the metatarsal organs are unilaterally ablated (1.05 .+-. 0.35). A further reduction of Q follows the ablation of all trichobothria (0.97 .+-. 0.35). For intact D. fimbriatus Q is significantly smaller than for D. triton (Q = 1.07 .+-. 0.34). Intact D. triton run towards the wave source at a speed of 36.6 .+-. 16.2 cm/s. The spiders become increasingly slow with various sensory deficits: blinded animals, 31.8 .+-. 17.2 cm/s; blinded and metatarsal lyriform organs unilaterally ablated, 27 .+-. 14.9 cm/s; all trichobothria removed, 21.2 .+-. 12.2 cm/s. The running speed of intact D. fimbriatus (27.1 .+-. 12.9 cm/s) is significantly smaller than that of intact D. triton. Using single frequency (sf) signals (10-140 HZ), the responsiveness of D. triton has a maximum (37%) at 40-50 Hz. If band-limited noise is offered, the responsiveness increases with increasing bandwidth of the stimulus (D. triton; -60 dB bandwidth 3-40 Hz: 26%; -60 dB bandwidth 3-130 Hz:68%). The behavioral threshold curve of D. triton to sf-signals is U-shaped, with the lowest values between 40 Hz (2.1 .mu.m peak-to-peak amplitude) and 70 Hz (1.0 .mu.m). If band-limited noise is offered the average threshold for 20% positive reactions are 3.3 .mu.m (3-130 Hz) vs. 9.0 .mu.m (3-40 Hz). Electrophysiologically determined threshold curves of individual slits of the vibration sensitive metatarsal lyriform organ show a highpass characteristic with low sensitivity (.apprx. 75 .mu.m) up to 10-30 Hz and steadily decreasing threshold amplitudes at higher frequencies (.apprx. 0.05 .mu.m at 1 kHz). Irregular signals with frequency components higher than those of waves cuased by abiotic origin are more attractive than sf-signals or signals containing only low freqeuncy components.