Memoirs: Development of the Female Genital Ducts in Trichoptera

Abstract

The adult common oviduct of Brachycentrus is derived from two ectodermal rudiments, a hinder unpaired and a front paired one. The junction between the anterior and posterior portions occurs about midway in the eighth segment, the oviducal aperture is between the eighth and ninth sternites. The main length of the paired oviducts is mesodermal in origin, but a portion at their extreme posterior ends just before their junction is probably ectodermal. These mesodermal ducts are present as rudiments in the larva before any of the ectodermal structures arise. The various accessory structures are also ectodermal, the bursa and spermatheca arising as dorsal unpaired outgrowths of the oviduct in the seventh segment, the accessory glands as independent paired invaginations on the ninth sternite. The glands bursa and spermatheca are drawn from their segments of origin into the eighth segment by the dorsal growth of the ectodermal tube. The ectodermal genital structures in female Trichoptera correspond in general plan to the supposed primitive condition in Lepidoptera, i.e. in which there is a single opening for egg-laying and copulation and only the oviduct has a direct opening to the exterior with the accessory copulatory structures leading dorsally on to the oviduct. The relative position of the Trichopteran bursa and spermatheca is the reverse of that in the young stages of Lepidoptera. The names of the Trichopteran bursa and spermatheca should therefore be reversed in order to homologize with the Lepidoptera. There is also histological evidence in support of this view as stated. The copulatory opening in Lepidoptera represents the original common gonopore as still present in Trichoptera. The backward shifting of the gonopore has taken place in many groups of insects, but the Lepidoptera are peculiar in having retained the original opening for copulation.