1. Females of Nasonia vitripennis cannot distinguish newly parasitized hosts from unparasitized ones but after 60 hr parasitization the parasitized puparia are rejected as oviposition sites.

2. The sheath and stylets of the ovipositor have nerves passing down their canals leading to several types of receptor at the tip of the ovipositor shaft.

3. Differences are detectable in the chemical composition of host haemolymph after 3 days parasitization as compared with the unparasitized condition. These may be the factors bringing about discrimination.

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