The ultrastructure of the seminal vesicle's spermatozoa of the butterfly Euptoieta hegesia was analyzed. The apyrene spermatozoa measure about 300 mum in length and swim freely in a secretion. The anterior end consists in a cap with a cylindrical extension and a globular structure. The flagellum has a 9+9+2 axoneme, two mitochondrial derivatives with paracrystalline matrices and an external coat formed by concentric layers. The eupyrene spermatozoa measure about 550 mum in length and are grouped into bundles. The anterior end consists in an amorphous globule, Posterior to this globule, a coat with a dense material covers the spermatozoon where an acrosome and a nucleus appear. The flagellum has a 9+9+2 axoneme and two mitochondrial derivatives. External to the coat and attached to the dense material, there is a reticular appendage, which has a paracrystalline core and extends to the distal tip of the spermatozoon. (C) 2001 Harcourt Publishers Ltd.333301308