Muscles And Locomotion

between the myofibrils. The body musculature of apterygotes and some larval pterygotes, the 441

leg muscles of some adult pterygotes, and the flight muscles of Orthoptera and Lepidoptera are of the close-packed type. Here the myofibrils and mitochondria are concentrated in the center of the fiber and the nuclei are arranged peripherally. In close-packed flight muscles, the fibers are considerably larger than those of tubular flight muscles. In addition, tracheoles deeply indent the fiber, whereas in tubular muscles tracheoles simply lie alongside each fiber. It should be appreciated that the tracheoles do not actually penetrate the muscle cell membrane, that is, they are extracellular. In most insects the indirect muscles, which provide the power for flight, are nearly always fibrillar, so-called because individual fibrils are characteristically very large and, together with the massive mitochondria, occupy almost all of the volume of the fiber. Very little sarcoplasm is present, and the nuclei are squeezed randomly between the fibrils. Because of their size, there are often only a few fibrils per cell, and these are frequently quite isolated from each other by the massively indented and intertwining system of tracheoles. The presence of large quantities of cytochromes in the mitochondria gives these muscles a characteristic pink or yellow color. It should be apparent even from this brief description that fibrillar muscles are designed to facilitate a high rate of aerobic respiration in connection with the energetics of flight.

Visceral muscles differ from skeletal muscles in several ways. The cells comprising them are uninucleate, may branch, and are joined to adjacent cells by septate desmosomes. Their contractile elements are not arranged in fibrils and contain a larger proportion of actin to myosin. Nevertheless, the visceral muscles are striated (sometimes only weakly), and their method of contraction is apparently identical to that of skeletal muscles.

All skeletal muscles and many visceral muscles are innervated. The skeletal muscles always receive nerves from the central nervous system, whereas the visceral muscles are innervated from either the stomatogastric or the central nervous system. Within a particular muscle unit, each fiber may be innervated by one, two, or three functionally distinct axons. One of these is always excitatory; where two occur (the commonest arrangement), they are usually both excitatory ("fast" and "slow" axons), but may be a "slow" excitatory axon plus an inhibitory axon; in some cases all three types of axon occur. This arrangement, known as polyneuronal innervation, facilitates a variable response on the part of a muscle (Section 2.2). Each axon, regardless of its function, is much branched and, in contrast to the situation in vertebrate muscle, there are several motor neuron endings from each axon on each muscle fiber (multiterminal innervation) (Figure 14.4).

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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