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aedeagus Intromittent organ or penis of males of most insect groups, which is often of great diagnostic value. Its inner wall is a continuation of the ejaculatory duct (see ENDOPHALLUS). It may be inflatable and everted during copulation. It has a gonopore at its base and opens at its apex through a phallotreme, the whole structure being surrounded by a sclerotized cuticle. By permitting sperm to be transferred without exposing it to the air, the aedeagus frees those species that possess it from the need to mate in water.