Muscles And Locomotion

rhythmic activities, the walking rhythm is centrally generated; that is, the endogenous 447

activity of a network of neurons within each thoracic ganglion regulates the alternating contraction of the leg flexor and extensor muscles, hence the flexion and extension of the limb. Intersegmental coordination of leg movements is achieved principally by "central coupling," that is, by signals traveling within the central nervous system from one network to another. Starting and stopping, turning, and change of speed are controlled by the brain and subesophageal ganglion, via so-called "command neurons," though how these centers exert their control is unclear. Superimposed on this central control is the input from the insect's sense organs, especially proprioceptors on the legs themselves, which permits the insect to adjust its walking rhythm to compensate for changing environmental conditions. (See Chapter 13, Section 2.3).

3.1.2. Jumping

Jumping is especially well developed in grasshoppers, fleas, flea beetles, click beetles, and Collembola. In the first three mentioned groups, jumping involves the hindlegs, which, like those of other jumping animals, are elongate and capable of great extension. Their length ensures that the limbs are in contact with the substrate for a long time during takeoff. Extension is achieved as the initially acute angle between the femur and tibia is increased to more than 90° by the time the tarsi leave the substrate. The length and extension together enable sufficient thrust to be developed that the insect can jump heights and distances many times its body length. For example, a fifth-instar locust (length about 4 cm) may "high jump" 30 cm and, concurrently, "long jump" 70 cm.

In Orthoptera the power for jumping is developed by the large extensor tibiae muscle in the femur (Figure 14.5C). The muscle is arranged in two masses of tissue that arise on the femur wall and are inserted obliquely on a long flat apodeme attached to the upper end of the tibia. The resultant herringbone arrangement increases the effective cross-sectional area of muscle attached to the apodeme, thereby increasing the power that the muscle develops.

As the extensor tibiae contracts, all activity ceases in the motoneurons running to its antagonist, the flexor tibiae muscle, thus permitting all of the power developed to be used in extending the tibia. This inhibition results from the activity of a single, branched inhibitory interneuron. Because the apodeme is attached to the upper end of the tibia, slight contraction of the extensor muscle will cause a relatively enormous movement at the tarsus (ratio of movements 60:1 when the tibiofemoral joint is tightly flexed).

It has been calculated that for the locust to achieve the maximum thrust for takeoff, the body must be accelerated at about 1.5 x 104 cm/sec2over a time span of 20 msec. The force exerted by each extensor muscle is about 5 x 105 dynes (= 500 g wt) for an insect weighing 3 g (Alexander, 1968, cited from Hughes and Mill, 1974). To withstand this force, the apodeme must have a strength close to that of moderate steel. The extremely short time period over which this acceleration is developed makes it unlikely that jumping occurs as a direct result of muscle contraction. Indeed, Heitler (1974) showed that the initial energy of muscle contraction is stored as elastic energy using a cuticular locking device that holds the flexor tendon in opposition to the force developed within the extensor muscle. At a critical level, the tendon is released, allowing the tibia to rotate rapidly backward.

Likewise, in fleas, the energy of muscle contraction is first stored as elastic energy. Prior to jumping, the flea contracts various extrinsic muscles of the metathorax, which are inserted via a tendon on the fused trochantofemoral segment. This serves to draw the leg closer to the body, compressing a pad of resilin and causing the pleural and coxal walls to bend. At a certain point of contraction, the thoracic catches (pegs of cuticle) slip into

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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