One of the means whereby an insect recognises a plant is "by detecting secondary plant compounds. Some such compounds are phagostimulatory but many act as feeding deterrents. The extent to which these deterrents are toxic was investigated in this thesis using Locusta migratoria, an oligophagous species which feeds mainly on grasses and thereby avoids many secondary plant compounds and Schistocerca gregaria, a polyphagous species which feeds on many plants deterrent to Locusta. Toxicity tests revealed that single injections of azadirachtin, allyl isothiocyanate, tomatine and nicotine into the haemolymph were significantly more toxic to Locusta than to Schistocerca. Sinigrin, umbelliferone and salicin were non-toxic in the short term to both species. Overall there was a significant positive correlation between the deterrency of the compounds to the insects and their toxicity. Tolerance was greater following cannulation of the compounds into the gut where only allyl isothiocyanate and nicotine were toxic to Locusta, and Schistocerca was unaffected by all the test compounds. Sinigrin is more deterrent to Locusta than to Schistocerca yet in the short term was non-toxic to both species. Long term feeding studies, in which the formulation of the test compound was shown to affect its toxicity, demonstrated however that Locusta was more susceptible to sinigrin than Schistocerca. Nicotine, which followed the overall pattern of being more deterrent and toxic to Locusta than to Schistocerca, was shown to be excreted from the haemolymph and voided from the gat more efficiently in Schistocerca. The target organs of Schistocerca were also less permeable to nicotine than those of Locusta. These factors, together with the ability of Schistocerca to induce its mixed-function oxidase system are suggested as the reasons for its greater tolerance to nicotine compared to Locusta . Azadirachtin is 1000 times more deterrent to Schistocerca than to Locusta, yet was more toxic to Locusta. Azadirachtin was shown to decrease body growth and increase the instar length in both species of insect. This effect is associated with a lack of feeding which could be caused by a direct toxic action to the gat as suggested by histological evidence. Gut contractions in Locusta were reduced 'in vitro' after application of azadirachtin. The inductive effect of azadirachtin on the mixed-function oxidase levels was greater in Schistocerca than Locusta. It is concluded that Locusta is physiologically less able to deal with secondary plant compounds than Schistocerca as a result of having evolved from a polyphagous to a graminivorous feeding strategy.