Comparative spermatology in Plecoptera (Insecta). II. An ultrastructural investigation on four species of Systellognatha.

Sperm structure of four Systellognatha species, Dinocras cephalotes (Curtis, 1827) and Perla grandis (Rambur, 1841), family Perlidae, Siphonoperla torrentium (Pictet, 1841), family Chloroperlidae, and Isoperla grammatica (Poda, 1761), family Perlodidae, was investigated by light microscopy, scanning and transmission electron microscopy, and immunofluorescence microscopy. The spermatozoa of all the species are filiform and flagellate; they have an elongated compact nucleus, topped by either a bi-layered or three-layered acrosomal complex, and a '9+9+2' axoneme flanked by two mitochondrial derivatives. However, some variations have been observed, particularly regarding the morphology of the acrosomal complex, the shape of the nucleus, two more or less evident accessory bodies flanking the axoneme and the degree of crystallisation of the mitochondrial derivatives. Our results support the suggestion that Systellognatha is a monophyletic group, showing a single sperm pattern. Chloroperlidae and Perlodidae retain the plesiomorphic condition of some sperm characters while Perlidae show some autoapomorphies. A phylogenetic affinity between Plecoptera and the other orders of Polyneoptera has been confirmed.