Fine structure of the egg envelopes in Silo mediterraneus saturniae (Trichoptera, Goeridae)

The temporal sequence of eggshell organization (vitelline envelope and chor ion) in Silo mediterraneus saturniae (Trichoptera, Goeridae) is highlighted by means of ultrastructural investigation on ovarioles of newly emerged fe males. Precursors of the vitelline envelope, initially accumulating in the space around egg microvilli, merge together to give rise to a striated bord er. Discrete striae fuse into the final electron-dense vitelline envelope. Choriogenesis takes place along the apical region of the follicle cells fac ing the egg, throughout the progressive release of precursor material synth esized by the follicular epithelium. The onset of chorionic layer depositio n is marked by irregular patches which gradually interconnect. As choriogen esis progresses, the follicular epithelium reduces in thickness. An epichor ion, with a fibrillar texture, becomes gradually distinct from the follicle cell cytoplasmic matrix and is superimposed on the chorion. At the end of choriogenesis (chorion and epichorion synthesis) the follicular epithelium progressively separates from the egg and folds markedly, giving the chorion a pleated appearance. The synthesis and morphology of the eggshell in S. m editerraneus saturniae are compared with those described for other insects laying eggs in water.