FIGURE 20.6. Form and position of embryonic primordium in endopterygotes. (A) Tenebrio; (B) Sialis; and (C) Pimpla. [After D. T. Anderson, 1972a,b, The development of hemimetabolous insects, and The development of holometabolous insects, in: Developmental Systems: Insects, Vol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic Press Ltd., and the author.]

It is during the differentiation and elongation of the germ band that the primordial germ cells first become noticeable in most endopterygote eggs, though in those of some Coleoptera they are distinguishable even as the syncytial blastoderm is forming. They are largish, rounded cells in a distinct group at the posterior pole of the yolk, and accordingly are referred to as pole cells (Figure 20.3). In eggs of Dermaptera, Psocoptera, Thysanoptera, and homopterans also, the germ cells differentiate early at the posterior end of the primordium. In those of most exopterygotes, however, they are not apparent until gastrulation or somite formation has occurred.

As the germ band elongates and becomes broader, segmentation and limb-bud formation appear externally and are accompanied internally by mesoderm and somite formation. Growth of the germ band may occur either on the surface of the yolk (superficial growth) as seen in eggs of Dictyoptera, Dermaptera, Isoptera, some other orthopteroid insects, and all endopterygotes (Figure 20.7), or by immersion into the yolk (immersed growth) as occurs in eggs of Paleoptera, most Orthoptera, and hemipteroid insects (Figure 20.8). Immersion of the germ band (anatrepsis) forms the first of a series of embryonic movements, collectively known as blastokinesis. The reverse movement (katatrepsis), which brings the embryo back to the surface of the yolk, occurs later (see Section 6). Anatrepsis has developed secondarily (i.e., superficial growth is the more primitive method) and convergently among those exopterygotes in which it occurs. Its functional significance is, however, not clear (Anderson, 1972a; Heming, 2003).

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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