1. The reactions, under field conditions, of Lucilia sericata (Mg.) to various substances placed on sheep are described.2. Attempts to produce either attraction or oviposition by placing cystine or cysteine hydrochloride in the living fleece gave negative results.3. Several organic sulphur compounds (see Table 1) of a type which might arise from the breakdown of cystine have been tested. All showed some power of attraction for L. sericata. Ethyl mercaptan and dimethyl disulphide were the most powerful as attractants, but none of these substances was able to induce oviposition.4. Tests with ammonium compounds and carbon dioxide have shown that, at the concentrations used, ammonia acted as an attractant. For oviposition to occur carbon dioxide had to be present.5. Hydrogen sulphide was detected as a constituent of the fleece atmosphere of certain sheep. This substance, whilst having no attractive powers when used alone on sheep, increased the attractiveness of organic sulphur compounds.6. Comparative tests with.ammonium carbonate-indole and ammonium carbonate-ethyl mer-captan mixtures have shown marked variations in the relative powers of these preparations to induce oviposition.7. A distinction is drawn between stimuli which attract L. sericata to sheep and those which induce oviposition. Ammonia and various organic sulphur compounds enhanced attraction, but both ammonia and carbon dioxide were necessary for oviposition. It is suggested that some of these materials may be produced under natural conditions from the break-down of fleece keratin or by the bacterial decomposition of sweat and similar products in the fleece.
