Essentially, the structure and contractile mechanism of insect muscles are comparable to those of vertebrate skeletal (cross-striated) muscle; that is, there are no muscles in insects of the smooth (non-striated) type. Within muscle cells, the contractile elements actin and myosin have been identified, and Huxley's sliding filament theory of muscle contraction applies. Though insect muscles are always cross-striated, there is considerable variation in their structure, biochemistry, and neural control, in accord with specific functions.

Because of their small size and the variable composition of the hemolymph of insects, the neuromuscular system has some unique features (Hoyle, 1974). Being small, an insect has a limited space for muscles which are, accordingly, reduced in size. Though this is achieved to some extent by a decrease in the size of individual cells (fibers), the principal change has been a decline in the number of fibers per muscle such that some insect muscles comprise only one or two cells. Thus, to achieve a graded muscle contraction, each fiber must be capable of a variable response, in contrast to the vertebrate situation where graded muscle responses result in part from stimulation of a varied number of fibers. Similarly, the volume of nervous tissue is limited, so that there are few motor neurons for the control of muscle

438 contraction. The hemolymph surrounding muscles may contain high concentrations of ions chapter 14 (especially divalent ions such as Mg2+) (Chapter 17, Section 4.1.1) that could interfere with impulse transmission at synapses and neuromuscular junctions. That this does not occur is the result of the evolution of a myelin sheath that covers ganglia, nerves, and neuromuscular junctions.

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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