Ovarian structures of the adult alderfly, Sialis mitsuhashii Okamoto (Megaloptera : Sialidae)

Abstract

The ovary of Sialis mitsuhashii consists of about 100 telotrophic ovarioles. No transverse septum was observed at the boundary between the terminal filament and the germanium. The germarial region is composed of the extensive syncytial nutritive tissue or core, the peripheral cells, and the interstitial cells. The cells of the nutritive tissue and the peripheral cell layer seem to originate from the same mother cells, because each peripheral cell is connected to the cytoplasm of the nutritive tissue by a cytoplasmic bridge (ca. 1.5 μm in diameter). Some of the peripheral cells situated in the posterior region of the germanium develop into young oocytes, surrounded by the several prefollicular cells. Clear nutritive cords are seen connecting the nutritive tissue to the developing oocytes. In the nutritive cord connecting with the previtellogenic oocytes, a large number of ribosomes and mitochondria are distributed evenly throughout the cytoplasm. This pattern seems to indicate that the flow of these organelles from the nutritive tissue to oocytes occurs mainly during the previtellogenic stage. The degree of development of the follicular cell structure and function does not seem to be as advanced as that of the telotrophic ovaries in hemipteran and polyphagous coleopteran insects.