Fine Structural Characteristics of the Chorionic Microspheres on the Egg Surface of the Orb Web Spider Trichonephila clavata

Abstract

The eggs laid by the orb web spider Trichonephila clavata must overwinter in very harsh weather conditions before hatching, but there does not seem to be any protection like a compact silk case covering the entire eggmass. Instead, the surface of the eggmass is completely coated with a milky coating called chorionic microspheres (CM). Therefore, we investigated the fine structural characteristics of CM to demonstrate the role of their ecological importance. Although the CM exhibits an uneven size distribution in outer eggmass, the chorionic surface is evenly covered with a single layer with a diameter of 2.3 µm approximately. The surface structure of aggregated CM shows short papillary projections demonstrating segmental adhesion of mucous components. CM is insoluble in water but partially soluble in absolute ethanol, and its spherical structure is completely decomposed by HFIP, a strong organic solvent. Since our fine structural observations clearly show that CM is not derived from vitellogenic or choriogenetic processes, the CM adhesive coatings during ovipositional process appears to be equivalent to cocoon silk for various protective functions in silken eggcase.