Ultrastructural features and formation of the micropylar apparatus in the cherry fly Rhagoletis cerasi.

The micropylar apparatus (MA) in Rhagoletis cerasi (Diptera, Tephritidae) is located at the anterior pole of the egg and consists of two parts: an outer chorion and an inner vitelline membrane. Sperm entry takes place through the micropylar canal, 2.0-2.5 μm in diameter, which penetrates the micropylar endochorion and terminates in the thick vitelline membrane, thus forming the "pocket." The pore of the micropylar canal, i.e., the micropyle, is covered by the exochorionic tuft. The formation of the MA is accomplished by 40 micropylar cells during oogenesis. These cells secrete the successive eggshell layers: the vitelline membrane, the wax layer, the innermost chorionic layer, the micropylar endochorion, and the exochorion. Two among 40 micropylar cells differentiate and form two tightly connected projections. The latter contain a bundle of parallel microtubules and participate in the formation of the micropylar canal and the pocket. At the tip of the projections there are two thin extensions full of microfilaments. In late developmental stages the two projections and the extensions degenerate and leave the canal and the pocket behind. We also discuss the structural features of the MA in relation to its physiology among Diptera.