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The 'ovipositor' is an organ used by some of the arthropods for oviposition, i.e. the laying of eggs. It consists of a maximum of three pairs of appendages formed to transmit the egg, to prepare a place for it, and to place it properly. In some of the insects the organ is used merely to attach the egg to some surface, but in many parasitic species (primarily in wasps and hymenoptera) it is a piercing organ as well. It is used by the grasshoppers to force a burrow in the earth to receive the eggs and by cicadas to pierce the wood of twigs for a similar purpose. Both long-horned grasshoppers and sawflies cut the tissues of plants by means of the ovipositor. None of these examples is quite as remarkable as the Megarhyssa species of Ichneumon wasp (parasitic Hymenoptera), the females of which have a slender ovipositor several inches long, used to drill into the wood of tree trunks. These species are parasitic in the larval stage on the larvae of wood-boring insects, hence the egg must be deposited in the burrow of the host (or, often, directly into the host's body as it is feeding.)
The sting of wasps, hornets, bees and some ants is also an ovipositor, in this case highly modified and associated with poison glands (to paralyze the prey so that the eggs can be laid without the host fighting back, and probably also to suppress the host's immune system so that it can't destroy the eggs or shake off the paralysis.) [1]
Some Roach-like fish, such as bitterlings, have an ovipositor as a tubular extension of the genital orifice in the breeding season for depositing eggs in the mantle cavity of the pond mussel.
The BBC documentary ''Walking with Dinosaurs'' portrayed a diplodocus mother using an ovipositor to lay her eggs, but it was pure speculation on the documentary's part.