Sperm morphology and ultrastructure in four species of ground beetles (Insects-Carabidae)

Ground beetles (Carabidae) exhibit diverse sperm morphologies, ranging from free cells to conjugated bundles embedded in spermatostyles. Detailed ultrastructural analyses of four species - Paussus favieri (Paussinae), Tachys scutellaris and Pogonus littoralis (Trechinae), and Laemostenus acutangulus (Harpalinae) - were conducted using transmission electron and fluorescence microscopy. Free sperm were observed in P. favieri, T. scutellaris, and P. littoralis, with notable features including a monostratified acrosome and polysaccharide-rich granular material in P. favieri, and unique accessory bodies in T. scutellaris. L. acutangulus displayed sperm conjugated within crystallized spermatostyles, suggesting the presence of enzymatic proteins. These findings reveal structural diversity linked to phylogenetic positions, suggesting that free sperm predominate in basal lineages, while conjugated sperm with spermatostyles are characteristic of higher Carabidae. The study enhances understanding of sperm evolution and reproductive adaptations in ground beetles, providing a foundation for future comparative and systematic investigations. As a final consideration, it was hypothesized that sperm conjugation could be the ancestral condition of insect reproduction.