High sensitivity to sodium in the sugar chemoreceptor of the cherry fruit fly after emergence

Abstract Recordings from the tarsal contact chemoreceptor D-sensilla of the cherry fruit fly (Rhagoletis cerasi, Dipt., Tephritidae) revealed the presence of a cell which had a variable sensitivity spectrum. In about 60% of the sensilla of freshly emerged flies this cell was found to be very sensitive to sodium and to a lesser degree to lithium cations. Potassium and other alkali cations were non-stimulatory. The anions tested, Cl^-, F^-, Br, NO₃^-, and CO₃^-, had no effect on the response to sodium. The same Na⁺-sensitive receptor cells fired in response to stimulation with sucrose plus NaCl or sucrose plus KCI mixtures and were therefore considered to be sugar cells. This was confirmed by cross-adaptation experiments using NaCl, and sucrose dissolved in dilute NaCl or KCI. However, the two adaptive stimuli were not acting symmetrically: NaCl did inhibit the following stimulation with sucrose, whereas sucrose had no effect on the subsequent NaCl stimulation. The response to sucrose and NaCl were not additive, sucrose being apparently, in some sensilla, inhibitory to the stimulation by NaCl. This observation, the lack of symmetry in adaptation, as well as the fact that only a proportion of the sensilla were sensitive to NaCl, seems to indicate that sodium had a different stimulating mechanism than sucrose. In most sensilla of flies older than 24 h, the Na⁺ sensitivity of the sugar cell was either reduced or completely lost. Behavioural observations of cherry fruit flies during the first 3 ½ days of adult life revealed that the flies fed little or not at all in the first 12 h. Thus the pronounced sodium sensitivity of the sugar cell early in adult life seems not to be correlated with a specific need for sodium intake but may have some role in the functioning of the sugar cell.