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Whip spiders belong to a small arachnid order (Amblypygi) that is not well known. Their most conspicuous feature are the elongated, extremely thin front legs (“whips”, or antenniform legs). These are no longer used for walking but are modified extremities carrying various sense organs – very much like the antennae of insects. Whereas hundreds of olfactory hairs are concentrated near the tip of each antenniform leg, large bristles (contact chemo-receptors) are evenly distributed over the entire antenniform leg. The sensory hairs of each antenniform leg contribute about 30,000 small sensory axons which proceed toward the Central Nervous System (CNS). The sensory fibers originating from the mechano-receptive bristles make chemical synapses with a few giant interneurons in the periphery. The giant axons (10-20 μm in diameter) of these large interneurons transmit nerve impulses with a high velocity (6 m/s) to the CNS. The purpose of this fast pathway still needs to be determined. Originally it was thought that the fast giant axons would trigger quick escape reactions, but this was not confirmed in physiological experiments. However, other possible behaviors that may be aided by the giant interneurons are prey capture, fighting, and orientation.
What makes whip spiders unique is that both synapses and giant neurons are located far out in the peripheral nervous system. In all other arthropods – except for some arachnids – synapses and giant fiber systems are always found inside the CNS, never the in the periphery.