Central Nervous System

Soon after somite formation has commenced, specialized ectodermal cells on each side of the midventral line, the neuroblasts (Figure 20.9E), begin to proliferate, resulting in the formation of paired longitudinal neural ridges separated by a neural groove. As proliferation occurs, the cells move slightly inward so that they become separated from the ectoderm. They then begin to divide vertically, unequally and repeatedly, the small daughter cells eventually developing into ganglion cells from which both neurons and glial cells differentiate (Figure 20.12). Remarkably, the number and arrangement of neuroblasts is highly conserved across the Insecta: each half-ganglion has 30 or 31 neuroblasts from which all neurons are produced (Thomas et al., 1984). However, the number of neurons in

FIGURE 20.12. Transverse sections to show development of nervous tissue and mesodermal derivatives in prothoracic segment of Tachycines (Orthoptera). In A-D, which are prekatatreptic stages, the serosa is omitted. [After D. T. Anderson, 1972a, The development of hemimetabolous insects, in: Developmental Systems: Insects, Vol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic Press Ltd., and the author.]

FIGURE 20.12. Transverse sections to show development of nervous tissue and mesodermal derivatives in prothoracic segment of Tachycines (Orthoptera). In A-D, which are prekatatreptic stages, the serosa is omitted. [After D. T. Anderson, 1972a, The development of hemimetabolous insects, in: Developmental Systems: Insects, Vol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic Press Ltd., and the author.]

a ganglion can be widely varied; for example, in a thoracic ganglion of an aptergygote there are about 1500 neurons, whereas in the thoracic ganglia of grasshoppers and flies there are about 3000 and 4000 neurons, respectively, reflecting the increased demands on the nervous system associated with the evolution of flight (Truman and Ball, 1998).

With the onset of segmentation, neuroblasts in the intersegmental regions become less active, so that paired segmental swellings, the future ganglia, now become apparent. As the neurons form, their cell bodies become arranged peripherally around the central axis (neuropile). Subsequent growth of the axons leads to formation oflongitudinal connectives, transverse commissures, and to motor nerves innervating a variety of effector organs. As noted above, sensory nerves form as a result of the inward growth of axons from peripheral sense cells.

An important aspect of the development of the nervous system is how developing neurons find other neurons so that the correct connections are made within the insect's body. The first neurons to arise (from the central nervous system) are the central pioneer neurons. Their axons grow along predetermined paths when their tips recognize particular cues on

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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