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Observations of oviposition behaviour by Chilo partellus revealed that antennae, ovipositor tip and possibly tarsi were all involved in the choice of oviposition site. Among a very large number of typical mechanoreceptor hairs on the tip of the ovipositor valves, two pairs of stouter, blunt-tipped hairs were discovered. These hairs alone took up silver nitrate stain and transmission electron microscope sections revealed that the hair shaft had a hollow lumen within which ran a group of four or five dendrites. It is concluded that these sensilla are contact chemoreceptors and they are positioned in such a way that, with the placing of each egg, they would contact the oviposition substrate. Similar hairs were found on the ovipositor of Spodoptera littoralis. It is suggested that the hairs may prevent oviposition on surfaces directly chemically harmful to the eggs.