Inhibition of the spider heartbeat by gravity and vibration.

The rate and vigor of the spider heartbeat is controlled by an external pacemaker. A mechanical feature of the spider cardio-vascular system is the production of high serum pressure in the prosoma and the legs. This appears to be the source for leg extension. The lyriform organ on the patella of the leg is sensitive to vibratory and kinesthetic stimuli. This sensitivity depends upon the degree of leg extension. Thus the activity of the heart and the response characteristics of the sense receptor are related. The effect of a supra-threshold vibratory or gravitational stimulus is to produce an inhibition and a tachycardia of the spider heartbeat.