Butterflies of both sexes have two pairs of extraocular photoreceptors on the genitalia. Here we demonstrate in female Papilio xuthus that a pair of the genital photoreceptors, P1s, is crucial for oviposition. Mated females of Papilio lay eggs on citrus leaves. When a female finds a food plant of the larvae, the female lands on its leaf and curls the abdomen often pushing out the ovipositor. As soon as the ovipositor touches the leaf surface, the female deposits an egg and glues it on to the leaf. We observed the oviposition behavior of individuals whose P1s or the mechanoreceptors on the ovipositor were ablated. Females treated in either way could no longer lay eggs, although they actively curled the abdomen and pushed the leaf, often quite strongly, with exposed ovipositor. This indicates that the females first confirm whether the ovipositor is sufficiently pushed out by using the P1 response as the measure, and then they deposit an egg in response to the mechanical stimulation of the ovipositor.