Revisiting the formation of midgut epithelium in Zygentoma (Insecta) from a developmental study of the firebrat Thermobia domestica (Packard, 1873) (Lepismatidae)

Zygentoma is an order of wingless insects, representing the sister group of Pterygota and constituting Dicondylia together with Pterygota. Contrasting views exist regarding midgut epithelium formation in Zygentoma. According to some reports, in Zygentoma, the midgut epithelium is entirely derived from yolk cells as in other wingless orders; however, according to other reports, the midgut epithelium is of dual origin in Zygentoma, similar to that in Palaeoptera of Pterygota, i.e., the anterior and posterior midgut epithelia are stomodaeal and proctodaeal respectively, whereas the middle part of the midgut originates from yolk cells. Aiming to provide a sound basis to evaluate the true image of midgut epithelium formation in Zygentoma, we examined the formation of the midgut epithelium in detail in Thermobia domestica, and concluded that the midgut epithelium is exclusively derived from yolk cells in Zygentoma, without the stomodaeal and proctodaeal elements involved in its formation. The participation of the anlagen differentiated at or around the stomodaeal and proctodaeal extremities in the formation of the midgut epithelium (bipolar formation) may be regarded as having first appeared not in Dicondylia but in Pterygota, of which the major part is represented by Neoptera with the midgut epithelium formed through bipolar formation.

Egg structure of five antarctoperlarian stoneflies (Insecta: Plecoptera, Antarctoperlaria)

The egg structures of five antarctoperlarian species - Stenoperla prasina of Eustheniidae; Austroperla cyrene of Austroperlidae; and Zelandobius truncus, Megaleptoperla grandis, and Acroperla trivacuata of Gripopterygidae, were examined in detail, and the groundplan of the egg structure was considered within the representative lineages of Antarctoperlaria and Plecoptera. The flattened egg shape and the circular arrangement of micropyles along the equator are regarded as potential autapomorphies for not only Eustheniidae but also for Eusthenioidea. Austroperlidae has eggs with thin, less-sclerotized chorion, a gelatinous layer on the surface, and micropyles roughly and randomly arranged along the equator. A significant ultrastructural difference between the attachment disc in Gripopterygidae and the anchor plate of arctoperlarian Systellognatha suggests that these structures were independently derived. The thin less-sclerotized chorion represents a groundplan feature in Plecoptera, along with micropyles arranged in a circle, including those circularly arranged along the equator of the egg. On the other hand, in contrast to previous understanding, the sclerotized hard chorion is regarded as a derived feature, having been independently acquired in each of Eustheniidae and Gripopterygidae of Antarctoperlaria and Systellognatha of Arctoperlaria.

Development and ultrastructure of the thickened serosa and serosal cuticle formed beneath the embryo in the stonefly Scopura montana Maruyama, 1987 (Insecta, Plecoptera, Scopuridae)

We aimed to describe the development and ultrastructure of the thickened serosa and serosal cuticle formed beneath the embryo of Plecoptera, using Scopura montana of Scopuridae as a euholognathan representative. Using transmission electron microscopy, we found that the egg membranes were composed of a thick exochorion, a thicker endochorion consisting of two sublayers, and an extremely thin vitelline membrane. The egg membrane construction represents a groundplan feature of the euholognathan egg membranes. The serosa converges beneath the embryo to form a thickened serosa, comprising cells in a radial arrangement, in association with the formation of the amnioserosal fold. The thickened serosa then deposits the thickened serosal cuticle, consisting of four layers differing in fine structure and electron density. After achieving its secretory function, the thickened serosa then disintegrates, and the liberated serosal cells float for a short period in the peripheral region of the egg inside. Collectively, our findings should provide the basis for further characterization of the serosal structures concerned, but we were unable to corroborate previous studies assigning the thickened serosa and serosal cuticle in Plecoptera to the water absorption function.