The male reproductive organ or penis-like structure in insects and some other arthropods, used to transfer sperm during mating.
From Greek 'aedoeagus', likely from 'aed' (variant root) and 'agos' (leader/guide). The term entered scientific English in the 18th-19th centuries as entomologists systematically studied insect anatomy.
Some beetle aedeagi are so specialized that they only fit perfectly into the reproductive tract of their own species—like a biological lock-and-key system that prevents accidental crossbreeding even when insects look nearly identical!
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