Abstract The larvae of Canthon cyanellus LeConte show well-defined sexual dimorphism. This dimorphism is seen in the presence of a terminal ampulla in the ventral region of the ninth abdominal segment in males. The terminal ampulla is a cuticular structure apparently present only in the male larvae beginning at the first instar and shows morphological changes during subsequent larval instars. A detailed study of terminal ampulla morphogenesis is needed to understand exactly how the ampulla develops into the structures of the ejaculatory bulb and aedeagus in the imago. Terminal ampullae have also observed in third-instar larvae of some species of Scarabaeidae, Trogidae, Passalidae and Lucanidae.