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The thermal isomerisation of the pyrethrins to isopyrethrins has been studied, the progress of the reaction being followed by the rise in optical density at 2700 A and the fall at 2300 A. The isomerisation is a first-order reaction with Io6k equal to 4.58 and 481 at 125' and 195' respectively ; these values give E* = 24,610 cal. mol.-l ; log A = 8.17 ; AS+ = -21.8 cal. deg.-l.In normal (unheated) pyrethrum extract the ratio of the optical densities a t 2700 A and 2300 A is 0.08 ; in pyrethrum extract which has been fully isomerised by heat this ratio is 0.62.The biological activities of normal and of partially and completely isomerised pyrethrum extract have been examined by three methods (two on flies and one on grain weevils). Extract in which the pyrethrins have been completely isomerised to isopyrethrins has approximately one-half of the lethal, and one-quarter of the knockdown, activity of normal extract on houseflies.Pyrethrum extract which has been submitted to heat treatment shows no loss in biological activity if the ratio of the optical density a t 2700 A to that at 2300 A has not risen above 0.095. IntroductionThe structural alteration of the pyrethrins effected by heat was first observed by Staudinger 8r Ruczicka,l and confirmed by Haller & LaForge2 who reported that the biological activity of pyrethrum extract decreased when the material was subjected to prolonged heating.

The carotenoid composition of flower heads of Chrysanthemum cinerariaefolium at different stages of maturity was investigated. More than 90% of the carotenoid fraction was shown to be related to cis and trans isomers of the xanthophyll lutein, which occur mainly as diesters but are also present as monoesters and the free diols.

The fatty acid fraction from an unrefined commercial pyrethrum extract containing 30% "pyrethrins" was isolated and the methyl esters were separated by column chromatography on silica gel into saturated or olefinic nonhydroxy acids and a hydroxy acid fraction. Combined fatty acids represented up to 40% of the pyrethrum extract, approximately 18% being nonhydroxy fatty acids. The latter were examined by gas chromatography and a series of normal saturated acids from C!4 to C30 was identified. The major acids were palmitic, stearic, oleic, linoleic, and linolenic, which together constituted up to 90% of the nonhydroxy fatty acids. Pyrethrum flowers of clonal origin containing different proportions of the six insecticidal constituents had similar fatty acid composition.

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