506 Comparative Animal Physiology 



dome which magnifies surface strains (Fig. 178); campaniform sensilla are 

 abundant on the basal joints of extremities, on wings, on the halteres of Dip- 

 tera, and elsewhere. Chordotonal sensilla have a sensory filament on an elas- 

 tic strand stretched between two points on the body wall; they often occur 

 at the base of wings. Johnston's organs resemble chordotonal sensilla and 

 occur on the second antennal segment (Fig. 171); they are stimulated by 

 movement of the antenna at its base. 



There is also wide variation in the location of the nuclei of the sensory 

 neurones. In the earthworm, for example, epidermal sense cells synapse in a 

 peripheral plexus with the afferent neurones, and there are many sense 

 cells per afferent fiber. In Crustacea a single fiber enters a sensory hair and 

 the nucleus lies at its base. The cell bodies of most tactile and proprioceptive 

 endings of vertebrates are located in dorsal spinal ganglia or in sensory cen- 

 ters in the lower brainstem. It is apparent from the differences in form and 

 arrangement that the various histological types of mechanoreceptors differ in 

 the kind of deformation which produces stimulation; they differ also in sen- 

 sitivitv and in the persistence of response to maintained stimulation. 



Outer lamella of Inner 1 amella 



cap membrane oF cap membrane 



Cuticular connection 

 of the sense cell 



Distal process 

 of the sense cell 



Fig. 178. Diagrammatic campaniform sensilla of an insect. From Pringle.™ 



Behavioral Responses. Distance Mechano- and Vibration-Sense. Mechano- 

 receptors usually signal either movement of a body region or direct contact 

 with some object; they may signal movement of a distant object if pressure 

 waves are transmitted bv the medium. The tactile endings on antennae and 

 vibrissae are, in a sense, distance receptors, and rats lacking both eyes and 

 vibrissae have difficulty in running a maze, whereas either eyes or vibrissae 

 can give adequate cues."'^ A distant source of vibration can be detected by 

 various kinds of mechanoreceptors. A spider locates its prey by sight and by 

 vibration of the web, and some spiders use a special signal thread. 



The proprioceptors, largely sensilla, of arthropods are exceedingly sensi- 

 tive to vibrations. The "feeling" of vibration by mammals is due to stretch 

 receptors, particularly those of muscles and tendons; this is shown by the 

 sensory impulses when a muscle or bone is touched b\' a tuning fork; the 

 threshold is lower for a contracted than for a relaxed muscle. A single 

 stretch receptor of a toe muscle of a frog discharged at a maximum frcquencv 

 of 330/sec. when stimulated by vibration, and 316/sec. when stimulated by 



