Postembryonal development of the testes in cotton leaf worm, Spodoptera littoralis (Boisd.) (Noctuidae, Lepidoptera)

During postembryonal development of males of Spodoptera littoralis the paired four-follicular larval testes undergo fusion and torsion, forming in the prepupal stage one gonad composed of eight testicular follicles. From the 6th larval till early pupal stage, the interior of the testicular follicles is divided into the following zones: 1) germarium with apical complex (an apical cell and two kinds of spermatogonia); 2) a zone, in which the single spermatogonia become surrounded by somatic cells, thus forming spermatogonial cysts; 3) a zone in which the spermatogonia inside the cysts undergo six incomplete mitotic divisions to form a syncytium of 64 spermatocytes (eupyrene spermatocytes with spherical nuclei or apyrene ones with polymorphic nuclei); 4) a zone, in which the spermatocytes transform into eupyrene or apyrene spermatids (256 per one cyst). In the mid-period of pupal stage two events occur: the apical cell in germarium degenerates and the eupyrene spermatogenesis ends. The apyrene spermatogenesis starts in the 6th larval instar and ends in the late pupa. In the late pupal and young imago testis, apyrene spermatozoa cysts form a compact layer under the gonadal wall, whereas the eupyrene cysts are loosely scattered in the central region of testicular follicles. The flagellum of the eupyrene spermatozoon is characterized by one mitochondrial derivative, by axonemal microtubules containing electron-dense material and by two kinds of appendage structures on the surface: lacinate appendages and satellite bodies. The flagellum of apyrene spermatozoon possesses two mitochondrial derivatives. It has neither extracellular appendages nor electron-dense material in microtubules. In seminal follicles the apyrene spermatozoa acquire a thick coating exhibiting periodic structure.