D

FIGURE 20.5. Form and position of embryonic primordium in exopterygotes. (A) Periplaneta; (B) Platycnemis; (C) Zootermopsis; and (D) Notonecta. [After D. T. Anderson, 1972a, The development ofhemimetabolous insects, in: Developmental Systems: Insects, Vol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic Press Ltd., and the author.]

As a result of the differing amounts of yolk that exopterygote and endopterygote eggs contain, important differences occur in the formation of the embryonic primordium. In exopterygote eggs where there is initially little cytoplasm, the embryonic primordium is normally relatively small, and its formation depends on the aggregation and, to some extent, proliferation of cells. In these eggs it usually occupies a posterior midventral position (Figure 20.5A-D). In contrast, in endopterygote eggs with their greater quantity of cytoplasm, the primordium forms as a broad monolayer of columnar cells that occupies much of the ventral surface of the yolk (Figure 20.6A,B). In other words, the primordium in endopterygote eggs does not require to undergo much increase in size, as is necessary in eggs of exopterygotes, so that tissue differentiation can occur directly and embryonic growth more rapidly. At its extreme, seen in eggs of some Diptera and Hymenoptera, the primordium occupies both ventral and lateral areas of the egg, with the extra-embryonic ectoderm covering only the dorsal surface (Figure 20.6C).

The shape of the primordium is varied, though in most insects the anterior region is expanded laterally as a pair of head lobes (= protocephalon), behind which is a region of varied length, the protocorm (postantennal region) (Figure 20.5). In eggs of Paleoptera, hemipteroid insects, and some orthopteroid species, the protocorm is semilong and at its formation includes the mouthpart-bearing segments, the thoracic segments, and a posterior growth region from which the abdominal segments arise. In eggs of other orthopteroid insects the postantennal region consists initially of only the growth zone. Though the protocorm in most endopterygote embryos is long, it also includes a posterior growth zone from which rudimentary abdominal segments proliferate. As the embryonic primordium elongates and begins to differentiate, it becomes known as the germ band. During elongation and differentiation, the abdomen grows around the posterior end and forward over the dorsal surface of the egg (Figure 20.7). In eggs of some higher endopterygotes (Hymenoptera-Apocrita and Diptera-Muscomorpha), there is no posterior growth zone and the abdominal segments arise directly from the primordium.

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.

Get My Free Ebook


Post a comment