ABSTRACT.Effects of three alkaloids, sparteine, nicotine and quinine on galeal sensilla of larvae and adults of Entomoscelis americana Brown (Coleoptera: Chrysomelidae) are described. Each of these deterrent compounds stimulates a cell which is stimulated also by glucosinolates (Mitchell&Gregory, 1979; Sutcliffe&Mitchell, 1980) which are feeding stimulants. They inhibit also the response of the sugar‐sensitive cell. It is suggested that the latter effect may be mainly responsible for the observed feeding deterrence caused by sparteine and implicated in the other two compounds. Possible mechanisms for the actions of these compounds are discussed with reference to work on other invertebrates. The results also lead us to suggest modification of recent interpretations of the role of secondary plant compounds in the evolution of insect chemosensitive system