Abstract
1. Experiments are described in which various concentrations of pyrethrum in oil are tested for speed of stimulation of Ornithodorus larvae. There is a logarithmic relation between concentration and speed of stimulation, except with concentrated solutions (stronger than 0·015% by weight of pyrethrin I) in which there is a relative retardation of response.2. As larvae grow older they respond more slowly, and this is probably due to an increase in thickness of the cuticle in natural growth.3. Mineral oils induce much swifter penetration of the pyrethrum than any of the vegetable oils tested.My thanks are due to Prof. Buxton in whose department the work was done, and also, to Dr J. T. Martin of Rothamsted Experimental Station, who kindly analysed samples of Pyremist ‘L’ for pyrethrin I.
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