The Spermatozoa of the Thysanuran Insects Petrobius brevistylis Carp. and Lepisma saccharina L.

The spermatozoa of Petrobius and Lepisma share a few general insect features (filamentous shape, two mitochondria, compact acrosome vesicle, bilateral symmetry) but differ fundamentally with regard to specializations. In Petrobius, a long coiled acrosome, a coiled nucleus, and a “body” with axonema, two mitochondria, and a pair of lateral bodies follow each other in normal sequence. In Lepisma the acrosome is a small vestige in the spoon‐shaped anterior end, the centriole is dislocated anteriorly, and nucleus, two mitochondria and axonema run like parallel filaments through most of the spermatozoon. The centriole adjunct develops into a postnuclear body in Lepisma but forms a pair of complicated “lateral bodies” in Petrobius. It is concluded that ancestral forms must have had fairly primitive spermatozoa and that specialization has proceeded independently within each evolutionary line.