You can see the ovipositor on this Myathropa florea .
The ovipositor is a tubular structure that is used for laying eggs. The ovipositor is attached to the abdomen of insects and the eggs pass down the tube. In parasitoids the ovipositor can be used to pierce the skin of another animal, such as a caterpillar, and the egg of the parasitoid can then be laid inside the host organism. Insects that possess ovipositors include many of the solitary wasps, crickets and many species of fly. The species of bees, ants and wasps that sting (remember that not all bees, ants and wasps sting) do so using a modified ovipositor.