The Effect of the Web on Vibration Sensitivity in the Spider, Achaearanea

The common house spider, Achaearanea tepidariorum, has a sensitive vibration receptor in each of its eight legs. The receptor has been shown to respond to air-borne sound as well as to direct vibration (Walcott & Van der Kloot, 1959). But for a spider on its web a more usual source of information about the outside world comes from vibrations of the web strands themselves. What is the response of the receptor to the vibrations of the web? Liesenfeld (1956, 1961) has described some of the properties of spider webs and recorded the electrical response of the vibration receptors of a variety of species of spiders to vibrations of the substrate. Nowhere, however, can one find information on how vibrations of the web are translated by the spider's vibration receptor into electrical impulses in the sensory nerve. One of the striking characteristics of the spider's vibration receptor is its dramatic variation in sensitivity to closely adjacent frequencies of air-borne sound. It appears that the ten receptor units of which the sense organ is composed are tuned. The sharpness of this tuning is such that the threshold of the preparation may vary by as much as 1 db. as the frequency is changed 1 cyc./sec. from 250 to 251 cyc./sec. When the tip of the leg was cemented to a crystal phonograph pick-up cartridge instead of being left free to vibrate in response to air-borne sound, the variation in threshold sensitivity to different frequencies disappeared. But a spider responding to vibrations of the strands of its web must be undergoing a direct vibration of its legs. Are all the characteristics of the sense organ's response to air-borne sound, the peaks in the threshold and the frequency discrimination, just an artifact of the experimental arrangement or do they play a significant role in the life of the spider as an integral property of its vibration receptor? This paper will examine the role of the web in the spider's sensitivity to vibration and to air-borne sound. Included in this report are data on the transmission of vibration by single strands of spider silk and on the effect of these web strands upon the sensitivity, tuning and frequency discrimination of the receptor. In addition, the sense organ's response to web-borne vibrations is compared with its response to air-borne sound. These electrophysiological investigations have raised several questions about the spider's use of the information its receptor provides. For this reason, data on the frequencies of vibration and sound generated by insects snared in spider webs and on the responses of spiders to these vibrations are also included.