The structure of the ovipositor sheaths is examined in a number of parasitic wasps as well as a few non-parasitic taxa. Parasitic wasps in the families Aulacidae, Braconidae, Ichneumonidae, Megalyridae, and Stephanidae with a very elongate, external ovipositor have the lateral wall of the sheath finely transversely subdivided by narrow furrows. This makes the sheath highly flexible, allowing it to support the ovipositor proper during ovipositing. The taxa having such a flexible sheath all drill into wood to lay their eggs. Support at the tip of the ovipositor is crucial in the initial stages of oviposition. The flexible ovipositor sheath is possibly a very ancient trait, arising prior to the radiation of extant Apocrita. This is corroborated by their presence in fossil Hymenoptera from the Upper Jurassic. The occurrence of short and/or concealed ovipositors with rigid sheaths in many extant apocritan taxa must therefore be assumed to be the result of multiple reversals.