Morphology of the male reproductive system and sperm of Discodon minutum Pic, 1928 (Coleoptera: Cantharidae).

The morphology of the male reproductive system and sperm of adults of Discodon minutum Pic, 1928 (Elateriformia: Cantharidae) was described using light microscopy (LM) and transmission electron microscopy (TEM). The morphology of the reproductive system and sperm provides information of taxonomic value and reveals aspects of reproductive biology, although such morphological characteristics are poorly understood in Cantharidae. The reproductive system of D. minutum consists of two free testes, each with 28-30 follicles, two vasa deferentia, and three pairs of accessory glands, as well as an ejaculatory duct with its median portion functioning as an ejaculatory pump. It also has two atypical seminal vesicles formed by evaginations at the base of the vasa deferentia, which serve as sperm storage and secretion. The follicles contain developed cyst cells and mature sperm, grouped in a spermatodesma-like cystic bundle. However, sperm are found free and immersed in secretions within the seminal vesicles, whose wall folds indicate an intense exchange of substances, potentially involved in sperm maintenance, with the hemolymph. The sperm ultrastructure resembles that observed in other Elateriformia, characterized by a nucleus positioned laterally to the flagellar components and a similar arrangement of the mitochondrial derivatives and axoneme. However, they differ in the asymmetrical position of the dense bodies and the triangular cross-sectional shape of the dense bodies and the nucleus. Histological confirmation of an ejaculatory pump in D. minutum is the first report of this structure in Elateroidea. Previous descriptions in Elateridae may have misidentified the ejaculatory pumps as "accessory glands of the ejaculatory duct".