In some species of insects there is only a single mating, but the egg-laying period may extend over a considerable length of time. Under such circumstances there must be some provision in the female for the storage and preservation of the supply of seminal fluid received during coitus, so that spermatozoa will be available as eggs mature. This is generally effected by an organ known as the spermatheca or receptaculum seminis, a flask-like structure which may open by means of a duct directly into the vagina, but more frequently empties into a bursa copulatrix, which in turn opens into the vagina.