The responses of the African migratory locustLocusta migratoria migratorioides R. & F. to the chemical composition of the soil at oviposition

Abstract 1. Locusta was able to detect the presence of inorganic salts, hydrochloric acid and quinine hydrochloride in the medium provided for oviposition, and if the concentration was high enough would reject it for oviposition. 2. In choice experiments between NaCl and distilled water, increase in the concentration of NaCl above 0·05M resulted in an increase in the number of unsuccessful diggings and a decrease in the number of eggs deposited in the NaCl. Both of these parameters were logarithmically related to increase in the molar concentration of the NaCl. 3. The females had two thresholds of response towards the molar concentration of NaCl. There was an initial threshold, which was the concentration at which the salt was first detected, but at which laying still occurred, and a rejection threshold in which oviposition never occurred. 4. Sucrose elicited an acceptance response, and females laid in NaCl solutions which they would otherwise have rejected if a high enough concentration of sucrose was added to the NaCl first. 5. The stimulating efficiency of cations was found to be: Na + =K + =NH + 4 ++ ++ and this corresponds to the lyotropic series for cations. 6. The stimulating efficiency of anions was found to be: Ac − − =NO − 3 −− 4 which corresponds to their ionic mobilities. 7. Local stimulation experiments, cautery and electrophysiological studies all gave negative results in a search for possible chemoreceptor sites in the ovipositor region.