They are solitary insects, and most are parasitoids—the larvae feeding on or in another insect which finally dies. Being in the same order, ichneumons are closely related to other hymenopterans such as ants and bees.
Despite looking formidable, the ovipositor does not deliver a sting like many wasps or bees. It can be used by the wasps to bore into and lay eggs inside rotten wood.
Ichneumon wasp species are highly diverse, ranging from 3 millimetres (0.12 in) to 130 mm (5.1 in) long. Most are slender, with the females of many species (particularly in the genus Megarhyssa) having an extremely long ovipositor for laying eggs. The female finds a host and lays an egg on, near, or inside the host's body