Eggshell fine structure of three lepidopteran pests: Cydia pomonella (L.) (Tortricidae), Heliothis virescens (Fabr.), and Spodoptera littoralis (Boisd.) (Noctuidae)

Abstract

The eggshells of 3 moths, Cydia pomonella (Tortricidae), Heliothis virescens, and Spodoptera littoralis (Noctuidae) were investigated by scanning (SEM) and transmission (TEM) electron microscopy. The surface of the noctuid eggs shows structural elements (micropylar rosette, ribs, cross-ribs, and aeropyles) and regional differentiation, all typical of Lepidoptera. The egg of C. pomonella shows a different regional morphology due to its watch-glass shape and its position, lying on the flank. The micropylar structures are on the lower egg face in contact with the substrate. For S. littoralis, the surface structure (sculpturing) of the egg is not species-specific, being indistinguishable from that of S. frugiperda (Salkeld, 1984). In all 3 moths, the eggshell fine structure is basically identical, as revealed by TEM. Both the vitelline envelope and the chorion consist of several distinct layers. The vitelline envelope, bi-layered and several μm thick, undergoes a marked structural change when embryogenesis begins. At the same time, Golgi vesicles bearing dense particles, appear in the periplasm of the egg cell in fertilized eggs of H. virescens and S. littoralis. The chorion of all 3 species consists of a basal layer (C-1), a cavity layer (C-2) supported by trabecles and opening to the exterior via aeropylar canals, and a lamellar layer (C-3), which probably consists of helicoidally arranged stacks of fibrils. In H. virescens and S. littoralis, an additional epicuticle-like layer (C-4) is present. Available data from the literature are summarized and a basic scheme of the radial eggshell fine structure of ditrysian Lepidoptera is proposed.