Morphology and physiology of peripheral giant interneurons in the forelegs (whips) of the whip spider Heterophrynus elaphus Pocock (Arachnida: Amblypygi)

The tarsi of the modified front legs (whips) of the whip spider Heterophrynus elaphus contain two afferent giant fibers, GN1 and GN2, with diameters at the tibia-tarsus joint of ca. 21 micrometer and 14 micrometer, respectively. The somata of these two neurons lie in the periphery, about 25 cm away from the CNS. These two neurons are interneurons which receive mechanoreceptive inputs from approximately 750 and 1500 bristles, respectively. The receptive fields of GN1 and GN2 overlap; they extend for 40 mm (GN1) and 90 mm (GN2) along the length of the tarsus. About 90% of the synapses onto the giant fibers are axo-axonic. Mechanical stimulation of a single bristle is sufficient to elicit action potentials in one or both interneurons. The response of the interneurons adapts quickly. Average conduction time from the soma to the CNS is 45 ms for GN1 and 55 ms for GN2. Mean conduction velocities are 5.5 and 4.2 m/s, respectively. Activity in the giant fibers does not elicit a motor response; hence the giant fibers do not mediate an escape response. Possible functions of these giant fibers are discussed and compared to those of giant fiber systems in other arthropods.