A Study of the Function of the Neural Fat-body Sheath in the Stick Insect, Carausius Morosus

Two conflicting hypotheses have been advanced to explain the remarkable ability of the neurones of some phytophagous insect species to function in nerve cords bathed with haemolymph containing relatively low concentrations of sodium ions. The first of these attributes the maintenance of a high extraneuronal sodium level to the activity of the glial and perineurial elements (Treherne & Maddrell, 1967). According to this interpretation of the available experimental evidence, largely derived from research on Carausius morosus, the necessary concentration gradient maintained between the fluid immediately surrounding the central neurones and the haemolymph results from the active extrusion of sodium ions by the glial membranes into the restricted extraneuronal channels. An alternative hypothesis which has been advanced more recently (Weidler & Diecke, 1969, 1970) is that the extraneuronal sodium regulation in C. morosus is achieved by the active transport of sodium ions from the haemolymph by the extraneural fat-body sheath which envelopes the central nervous system of this insect (Maddrell & Treherne, 1966). This communication represents an attempt to test the validity of the latter hypothesis by the comparison of the effects of various concentrations of sodium ions on axonal function in intact abdominal connectives and in those subjected to varying degrees of experimental surgery.