Dorsal Closure and Katatrepsis

When germ band elongation and segmentation are complete, limb buds develop, the embryonic ectoderm grows dorsolaterally over the yolk mass, and internally organogenesis begins. This phase of growth is ended abruptly as the extra-embryonic membranes fuse and rupture and the germ band reverts to its original (pre-anatreptic) position (in most exopterygotes) or shortens (endopterygotes).

In embryos of most insects, the amnion and serosa fuse in the vicinity of the head, and the combined tissue then splits to expose the head and rolls back dorsally over the yolk (Figure 20.10A). As a result, the serosa is reduced to a small mass of cells, the secondary dorsal organ, and the amnion becomes stretched over the yolk, forming the provisional dorsal closure (Figure 20.10B). In some endopterygote embryos, variations of this process can be seen. In those of Nematocera (Diptera) and Symphyta (Hymenoptera), for example, it is the amnion that ruptures and is reduced, leaving the serosa intact. As noted above, in eggs of Apocrita only a serosa is formed, and this persists until definitive dorsal closure occurs, and in those of Muscomorpha no extra-embryonic membranes develop, and the yolk remains covered by the extra-embryonic ectoderm until definitive dorsal closure.

Except in dictyopteran embryos where the germ band remains superficial and ventral during elongation, extensive movement of the germ band now occurs in exopterygote eggs which serves (1) to bring an immersed germ band back to the surface of the yolk and (2) to restore the germ band to its pre-anatreptic orientation, that is, on the ventral surface of the yolk with the head end facing the anterior pole of the egg. This movement, the reverse of anatrepsis, is known as katatrepsis (Figure 20.8).

At the beginning of provisional dorsal closure, the germ band of most endopterygotes is quite long so that, although its anterior end is ventral, its posterior component passes round the posterior tip of the yolk and forward along the dorsal side (Figure 20.7F). During closure, the germ band shortens and broadens rapidly so that its posterior end now comes to lie near the posterior end of the egg (Figure 20.11A).

Definitive dorsal closure, that is, the enclosing of the yolk within the embryo, then occurs. It is achieved in all insect embryos by a lateral growth of the embryonic ectoderm, which gradually replaces the amnion or, rarely, the serosa (Figures 20.10C and 20.11B).

FIGURE 20.10. Diagrammatic representation of dorsal closure. (A) Initial fusion of amnion and serosa and beginning of rolling back; (B) provisional dorsal closure with amnion covering yolk; and (C) definitive dorsal closure with yolk enclosed within embryonic ectoderm.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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