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Abstract
1. Our electrophysiological investigations have primarily been focused on those primary chemoreceptor cells which were considered to be important in con- nection with the aspects of larval behaviour as described in the foregoing chapters. Response characteristics of these receptors were studied.
2. The epipharyngeal papilla-like organs were identified as contact chemo- sensitive sensilla. The three bipolar neurons which were found to be present in each sensillum were electrophysiologically characterized as a salt cell, a sugar cell, and a cell sensitive to alkaloids and steroids. The Ep do not contain chemoreceptor cells sensitive to mustard oil glucosides.
3. Analogies in receptor activities during stimulation with different qualities of stimuli and different strengths of one and the same stimulus quality were found between the salt sensitive cells in Ss I and Ep and also between the cells sensitive to alkaloids and steroids in Ss I and Ep. With the sugar receptors an- alogies were present to a certain extent between the Ss II and the Ep receptor cells.
4. The sugar receptor cell identified in the Ss I differed completely from the Ss II or Ep receptors, among others by possessing a much broader sensitivity spectrum which included l-fucose, sucrose, d-glucose and d-fructose, whereas the Ss II and Ep receptors were found to respond exclusively to stimulation with sucrose and glucose. None of the sensilla studied had a cell responsive to m-inositol.
5. The stereochemical basis of the stimuli determining the specificity of the sugar receptor cells is not readily identified as is exemplified by the sensitivity spectra of different species of lepidopterous larvae.
6. Feeding inhibitory compounds were found to act on a specialized receptor cell, present in the Ss I and the Ep. They did not influence the receptor activities of any other chemoreceptor cell. Effective stimuli for this type of receptor cell were provided by alkaloids and steroids which generally had configurations of complicated ringsystems of high molecular weight.
7. The responsiveness of the Ep and Ss II sugar receptors could be changed by sodium chloride and calcium chloride. The action of calcium chloride, but not that of sodium chloride, was exclusively inhibitory. Over a certain range sodium chloride acted antagonistically on the action of calcium chloride. Calcium chloride also had an inhibitory influence on the receptor cell sensitive to alka- loids and steroidal compounds.
8. In both Ss I and Ep sensilla an absolute cellular specificity could not be assumed under all conditions of stimulation.
9. The cell membrane of the sugar receptor in Ss II and Ep is likely to have one type of receptor site with possibly two subunits each site being occupied by one molecule of sucrose or two molecules of glucose.
