Vibrations in the orbwebs of Nuctenea sclopetaria (Araneidae)

Summary

Orbweaving spiders build an elaborate web to trap aerial prey. The web transmits vibration cues about the presence and location of entangled prey to the spider, which is usually waiting at the hub of the web. This paper examines the web as a medium for vibration transmission between the prey-catching region and the hub. Three types of vibration are propagated along a radius: longitudinal (motion directed along the strand's axis), transverse (motion perpendicular to the strand and to the web), and lateral (motion perpendicular to the strand and in the plant of the web). The web was stimulated in the middle of the preycatching region with one type of vibration, and the amount of that same type of vibration appearing at the hub was measured. In an empty web longitudinal vibration reaches the hub with almost no attenuation over the frequency range of 1 to 10,000 Hz (Fig. 3). Transverse vibration is attenuated by ∼15 dB at 1 Hz and transmission drops more or less linearly to ∼35 dB attenuation at 10 kHz. Lateral vibration is attenuated ∼23 dB between 1 and ∼200 Hz, and attenuation increases to ∼40 dB between 1 and 10 kHz. Individual webs (Fig. 4) show resonance effects (peaks and troughs in transmission) above ∼1 kHz. The distance over which the signal was transmitted (63±11 mm) was not correlated with the amount of attenuation. The three types of vibration vary in directionality (Fig. 5), that is, in how greatly the amplitude of the stimulated radius differs from that of its nonstimulated neighbors at the hub. Longitudinal vibration is the most directional and consequently it may play the dominant role in the spider's choice of radius to run out along to reach trapped prey. In a web containing the spider and/or prey, the web/object system tends to resonate at frequencies that depend on the weight of the objects, their position in the web, and the types of vibration (Fig. 6). A 200 mg spider at the hub of the web will oscillate at ∼10 Hz for longitudinal and lateral vibration, and at ∼4 Hz for transverse vibration. A 1 to 2 mg insect in the catching zone oscillates at ∼100 Hz for longitudinal vibration but a 100 mg insect oscillates at only ∼4 Hz for transverse vibration. Generally, resonance effects due to web loading will be confined to frequencies below a few hundred Hz. The presence of a spider at the hub modifies the transmission curves derived from the empty web. For longitudinal vibration, signals from the prey-catching zone to the spider's tarsus (Fig. 7) are attenuated by several dB below ∼300 Hz and by ∼20 dB/decade above this frequency. For transverse and lateral vibration, transmission to the spider could not be measured and must be estimated from that obtained in the empty web. Qualitative aspects of vibration transmission across web crossings are discussed using the physical properties of radial and sticky-spiral strands (Table 1).