Paraxial organ of a scorpion: structural and ultrastructural studies of Euscorpius tergestinus paraxial organ (Scorpiones, Euscorpiidae)

By combining ultrastructural and histochemical approaches, the paraxial organs of Euscorpius tergestinus (syn. E. carpathicus; Scorpiones, Euscorpiidae) have been studied for the first time. The paraxial organs are symmetrical structures in the male genital system. They are located on each side of the mesosoma and end at the genital aperture. The paraxial organs have a glandular function. They are directly involved in the secretion of the two hemispermatophores, which are deposited in the glandular lumen. When the hemispermatophores are extruded, they constitute the spermatophore, which is involved in sperm transfer. The epithelium of the paraxial organs is complex, being either unistratified or pseudostratified. Moreover, in the different parts of the gland, such as at the pars bulbosa, the epithelium can show folds. In addition, the appearance of the epithelial cells varies with secretory activity. The cells deposit the different components of the hemispermatophore in the lumen. We show that the cells have two successive methods of secretion: firstly merocrine and then apocrine. This has never been described before. The histochemical observations of the hemispermatophore show its complex structure with the presence of different constituents.