Taste discriminating capability to different bitter compounds by the larval styloconic sensilla in the insect herbivore Papilio hospiton (Géné)

Herbivorous animals may benefit from the capability to discriminate the taste of bitter compounds since plants produce noxious compounds, some of which toxic, while others are only unpalatable. Our goal was to investigate the contribution of the peripheral taste system in the discrimination of different bitter compounds by an herbivorous insect using the larvae of Papilio hospiton Géné as the experimental model, showing a narrow choice range of host plants. The spike activity from the lateral and medial styloconic sensilla, housing two and one bitter-sensitive gustatory receptor neurons (GRNs), respectively, was recorded following stimulation with nicotine, caffeine, salicin and quercitrin and the time course of the discharges was analyzed. Nicotine and caffeine activated all three bitter-sensitive GRNs, while salicin and quercitrin affected only two of them. In feeding behavior bioassays, intact larvae ate glass-fiber disks moistened with salicin and quercitrin, but rejected those with nicotine and caffeine, while lateral sensillum-ablated insects also ate the disks with the two latter compounds. The capability to discriminate bitter taste stimuli and the neural codes involved are discussed.