The central course of the hair afferents and the pattern of contralateral activation in the central nervous system of the scorpion, Heterometrus fulvipes.

There is a considerable amount of information on the central representation of the sensory input from the hairs in insects (Pumphrey & Rawdon-Smith, 1937; Fielden, 1960A; Rowell, 1961; Fielden & Hughes, 1962; Mill, 1963) and crustaceans (Prosser, 1935a, b, 1937; Wiersma, Ripley & Christensen, 1955; Fielden, 1960a; Hughes & Wiersma, i960; Wiersma & Hughes, 1961; Wiersma & Bush, 1963). To date, no such studies have been reported on the arachnid central nervous system. This dearth of knowledge precludes fruitful comparison between the major classes of arthropods. Furthermore, in most of the studies mentioned above the emphasis has been more upon the reaction patterns and mapping of the peripheral fields of the interneurones. Attempts have never been made to label the hairs in the periphery and systematically map the central spread of the sensory fibres associated with them. The ipsiand contralateral central discharges following the stimulation of a single hair have never been simultaneously monitored. Hence the available evidence is inadequate to explain whether the contralateral central representation of the tactile input in insects (Roeder, 1948) and crustaceans (Wiersma & Bush, 1963) is due to the terminal branches of the sensory neurones only or whether collaterals of the sensory axons passing ' through' a ganglion also participate. This point needs clarification since it is known that some of the hair afferents pass ' through' one or more central ganglia before terminating in the central nervous system (Horridge, 1968). With this in mind the hairs on the metasoma of the scorpion were ' labelled' and the central course of the hair afferents as well as the mechanisms of contralateral activation were studied in great detail (Sanjeeva-Reddy, 1969). The present paper represents a preliminary account of the results obtained and shows that the collaterals also take part in the contralateral activation. It also illustrates the suitability of the scorpion preparation for studies on the central representation of the tactile input.