TEM micrographs of sperm head region of K. caelatata. A Longitudinal section of head region, showing the head region (including acrosome (a) and nucleus (n)) inserted into a homogenous matrix (ma) B Cross-section through the acrosome (a), showing the subacrosomal space (ss) located at an eccentric position of acrosome C Cross-section through the acrosome, showing the nucleus (n) located at an eccentric position of acrosome D–F Cross-sections through the posterior region of the acrosome (a), showing two acrosomal processes and the nucleus (n) G Lower magnification of cross-section through spermatodesmata, showing different transverse sections of spermatozoa. Scale bars: 2 μm (A, G), 100 nm (B–F).