Nervous System

Like that of other animals, the nervous system ofinsects consists of nerve cells (neurons) and glial cells. Each neuron comprises a cell body (perikaryon) where a nucleus, many mitochondria, and other organelles are located, and a cytoplasmic extension, the axon, which is usually much branched, the branches being known as neurites. Axons may be long, as in sensory neurons, motor neurons, and principal interneurons, or very short, as in local interneurons. Often, insect neurons are monopolar, lacking the dendritic tree characteristic of vertebrate nerve cells, though bipolar and multipolar neurons do occur (Figure 13.1). Motor (efferent) neurons, which carry impulses from the central nervous system, are monopolar, and their perikarya are located within a ganglion. Sensory (afferent) neurons are usually bipolar but may be multipolar, and their cell bodies are adjacent to

FIGURE 13.1. Neurons found in the insect nervous system. Arrows indicate direction of impulse conduction. (A) Monopolar; (B) bipolar; and (C) multipolar. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

the sense organ. Interneurons (also called internuncial or association neurons) transmit information from sensory to motor neurons or other interneurons; they may be mono- or bipolar and their cell bodies occur in a ganglion. Interneurons may be intersegmental and branched, so that the variety of pathways along which information can travel and, therefore, the variety of responses are increased.

Neurons are not directly connected to each other or to the effector organ but are separated by a minute space, the synapse or neuromuscular junction, respectively. Impulses may be transferred across the synapse either electrically or chemically (Section 2.3). The normal diameter of axons is 5 |am or less; however, some interneurons within the ventral nerve cord, the so-called "giant fibers," have diameters up to 60 |^m. These giant fibers may run the length of the nerve cord without synapsing and are unbranched except at their termini. They are well suited, therefore, for very rapid transmission of information from sense organ to effector organ; that is, they facilitate a very rapid but stereotyped response to a stimulus and for some insects are important in escape reactions (Hoyle, 1974; Ritzmann, 1984).

Neurons are aggregated into nerves and ganglia. Nerves include only the axonal component of neurons, whereas ganglia include axons, perikarya, and dendrites. The typical structures of a ganglion and interganglionic connective are shown in Figure 13.2. In a ganglion there is a central neuropile that comprises a mass of efferent, afferent, and association axons. Frequently visible within the neuropile are groups of axons running parallel, known as fiber tracts. The perikarya of motor and association neurons are normally found in clusters adjacent to the neuropile.

Surrounding the neurons are glial cells, which are differentiated according to their position and function. The peripheral glial (perineural) cells, which form the perineurium,

FIGURE 13.2. Cross-sections through (A) abdominal ganglion and (B) interganglionic connective to show general structure. [A, after K. D. Roeder, 1963, Nerve Cells and Insect Behaviour. By permission of Harvard University Press. B, after J.E. Treherne and Y. Pichon, 1972, The insect blood-brain barrier, Adv. Insect Physiol. 9:257-313. By permission of Academic Press Ltd., London, and the authors.]

FIGURE 13.2. Cross-sections through (A) abdominal ganglion and (B) interganglionic connective to show general structure. [A, after K. D. Roeder, 1963, Nerve Cells and Insect Behaviour. By permission of Harvard University Press. B, after J.E. Treherne and Y. Pichon, 1972, The insect blood-brain barrier, Adv. Insect Physiol. 9:257-313. By permission of Academic Press Ltd., London, and the authors.]

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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