Comparison of the ovariole structure and oogenesis between Bittacus cirratus Tjeder and Bittacus planus Cheng (Mecoptera: Bittacidae).

The eggs are subspherical in some species of Bittacidae, but are cuboidal in others. However, the formation mechanism of cuboidal eggs remains unclear so far. Here, we compared the ovariole structure and oogenesis between the hangingflies Bittacus cirratus Tjeder, 1956 with cuboidal eggs and Bittacus planus Cheng, 1949 with subspherical eggs using light and transmission electron microscopy. The results show that the ovariole is of the meroistic-polytrophic type, consisting of a terminal filament, a germarium, a vitellarium, and an ovarian pedicel. During the oogenesis, each follicle has one oocyte and three nurse cells surrounded by follicle cells. The difference occurs between B. planus and B. cirratus during the choriogenesis. In B. cirratus, the follicle is in an elliptical shape along the anterior-posterior axis during the late previtellogenesis, but transforms into a cuboidal shape during the choriogenesis. The formation mechanism of the cuboidal eggs is briefly discussed for hangingflies.