Spider vibration receptors: Threshold curves of individual slits in the metatarsal lyriform organ

Abstract

The compound slit sense organ (lyriform organ) on the distal end of the spider metatarsus is highly vibration sensitive. Both its morphology and its topography are well adapted to perceive substrate vibrations picked up by the tarsus (Figs. 2, 3). We present threshold curves measured with carefully controlled sine-wave stimulation at frequencies from 0.1 Hz to 1 kHz (Fig. 1). They were determined individually for about half of the 21 slits which make up this organ. 1. In all cases and in all slits studied the receptors behave like high-pass filters with relatively low sensitivity of 10−3 cm to 10−2 cm up to ca. 40 Hz, and steeply decreasing threshold displacement down to 10−7 cm at 1 kHz. At low frequencies up to about 40 Hz (in some slits only 1 Hz) the threshold curves follow roughly constant displacement whereas at higher frequencies they follow roughly constant acceleration (Fig. 4). Variation in both the direction of tarsal displacement (either dorsoventral or lateral) and in the kind of coupling between the tarsus and the vibrator only moderately affects the tuning curve shapes. 2. No marked tuning of the slits to limited frequency ranges was found in any of our experiments, including cases where the tarsus was not firmly coupled to the vibrating substratum as it would under natural conditions. With loose coupling there is only a slight shift towards lower sensitivity below 10 Hz (Fig. 6). 3. All slits tested responded to lateral as well as to dorsoventral displacement of the tarsus. Threshold curves are essentially similar in shape and absolute values except for small dips between 600 Hz and 1 kHz found with two of the four slits tested and indicating slight resonances (Fig. 7). 4. Despite the overall similarity there are differences in the tuning curves of the various slits of the same organ. These differences are as large as 1.5 orders of magnitude at a certain frequency, and may form the basis for at least a crude frequency discrimination which depends on the ability of the central nervous system to recognize the changing activity pattern of several or all slits in this compound organ (Fig. 5). 5. Except for a few strictly phasic slits, the majority of the slits studied shows a slow decline of responses to sustained deflection of the tarsus. Vibration sensitivity is therefore not regarded as the full function of the metatarsal lyriform organ. An additional proprioceptive function monitoring tarsal movement during locomotion is suggested.