Physiological differences between two sugar-sensitive neurons in the galea and the maxillary palp of the spruce budworm larva Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae)

The L1 contact-chemoreceptor sensillum on the maxillary palp of the spruce budworm larva Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae) was examined electrophysiologically for its responses to stimulation by various pyranose and furanose sugars. The results were compared to those from previous work on the sugar-sensitive neuron of the LST sensillum on the galea. We show that the L1 contact-chemoreceptor sensillum contains one sugar-sensitive neuron with furanose but no pyranose sites. It has response characteristics that differ from those of the sugar-sensitive neuron in the LST. Behavioural 2-choice feeding experiments show that, even with both known sugar-sensitive neurons disabled, larvae can still discriminate between disks treated with either distilled water or alpha-D-glucose. We conclude that the epipharyngeal sensilla must thus also contain a sugar-sensitive neuron.