The sperm ultrastructure of Caurinus dectes Russell (Mecoptera: Boreidae) and its phylogenetic implications

The sperm structure of the enigmatic mecopteran species Caurinus dectes (Boreidae) is described for the first time. Diagnostic features are the bi-layered acrosome, a cylindric nucleus provided with two longitudinal opposite grooves, and a simple 9 + 2 axoneme which degenerates in the posterior tail end. The results are conform with the monophyly of Mecoptera including Boreidae. A possible autapomorphy of the order is the presence of the two longitudinal opposite grooves along the nucleus, and the presence of two electron-dense fibres beneath the axoneme. Some apparently plesiomorphic features are preserved in the sperm of Caurinus. Features characterizing the distal part of the flagellum, including the presence of an axial cylindrical structure and the distinctive type of axoneme degeneration, are potential synapomorphies of Caurinus and Boreus, i.e. autapomorphic traits of Boreidae.