Fine structure of the egg chorion in two anisopteran dragonflies from central India (Libellulidae)

The fine structure of the egg chorion in Brachydiplax sobrina (Ramb.) and Orthetrum s. sabina (Dru.), is described using the scanning electron microscope. The unwetted eggs of B. sobrina are bluish-green and spindle-shaped while those of O. s. sabina are oval and light brown in colour. The egg chorion is distinctly divided into an outer exochorion and an inner tough endochorion. The exochorion expands into a thick, sticky, jelly-like structure in water during oviposition, whereas the endochorion remains unchanged. The endochorion is thin and smooth in O. s. sabina, but in B. sobrina the undersurface of the endochorion is pitted and rough. The apical micropylar apparatus is composed of a sperm storage chamber (atrium) and a median projecting stalk, which possesses a pair of sub-terminal orifices. The atrium in B. sobrina is dome shaped with a tiny stalk whereas in O. s. sabina the micropylar apparatus is triangular with a longer stalk and a pair of almost apically placed orifices. Significant variations occur in the shape and size of the micropylar apparatus. The functional interrelationship of the micro morphological modifications in the chorionic structures is discussed.