Neural Organization of the Sensory Appendages of the Whip Scorpion, Thelyphonus Indicus Stoliczka (Arachnida, Uropygi)

Abstract

In whip scorpion, the multisegmented whiplike flagellum and the antenniform legs are the modified telson and the first pair of legs respectively. These two appendages bear numerous sensory hairs. Each segment of the flagellum also bears at its base an oval membranous region called ‘Fenestra Ovalis’. The neural organisation of these two sensory appendages was examined with light and electron microscope and compared with the sensory appendages of other arthropods.

Each sensory hair of the antenniform leg has polyneural innervation. The afferent axons form tiny nerves. In contrast to the conventional arthropod neural organisation, an unusual finding is the formation of synapses by some of the axons in the periphery. Various types of synapses based on pre- and postsynaptic elements, and the nature of synaptic vesicles have been studied. The aggregation of the axons results in the formation of two large nerves which contain predominantly fine fibres and a few exceptionally large “Giant” fibres.

In the antenniform legs just below the cuticle, cells with striking microvillar formations similar to those found in rhabdoms of photoreceptor cells were observed. Associated with these structures were cells containing abundant osmiophilic granules.

The innervation of sensory hairs of the flagellum is similar to that of the antenniform legs. Fenestra Ovalis shows a thin and membranous cuticle below which sensory cells with broad, expanded dendrites containing elongated mitochondria were found.

These observations are discussed from a standpoint of neural organization in relation to information processing at the periphery and in comparison with the sensory appendages of arthropods.