448 notches on the sternum, thereby "cocking the system." The jump is initiated when other laterally inserted muscles contract to pull the catches out of the notches, thus allowing the stored energy to be rapidly released (Rothschild et al., 1972).

Once airborne, an insect may or may not stabilize itself in preparation for landing or flight. For example, the flea beetle Chalcoides aurata sometimes jumps "out of control," its body rotating continuously till it hits the ground. However, it can, when necessary, control its jump by extending the wings so that a "feet-first" landing occurs (Brackenbury and Wang, 1995). Larger species such as Orthoptera use their hindlegs as rudders during the jump, facilitating an upright landing or takeoff for flight (Burrows and Morris, 2003).

3.1.3. Crawling and Burrowing

Many endopterygote larvae employ the thoracic legs for locomotion in the manner typical of adult insects; that is, they step with the legs in a specified sequence, the legs of a given segment alternating with each other. Usually, however, changes in body shape, achieved by synchronized contraction/relaxation of specific body muscles, are used for locomotion in soft-bodied larvae. In this method the legs, together with various accessory locomotory appendages, for example, the abdominal prolegs of caterpillars, are used solely as friction points between the body and substrate. Apodous larvae depend solely on peristalsis of the body wall for locomotion.

Where changes in body shape are used for locomotion the body fluids act as a hydrostatic skeleton. In other words, the insect employs the principle of incompressibility of liquids, so that contraction of muscles in one part of the body, leading to a decrease in volume, will require a relaxation of muscles and a concomitant increase in volume in another region of the body. Special muscles keep the body turgid, enabling the locomotor muscles to effect these volume changes.

Crawling in lepidopteran caterpillars is probably the best studied method of locomotion in endopterygote larvae and comprises anteriorly directed waves of contraction of the longitudinal muscles, each wave causing the body to be pushed upward and forward (Hughes and Mill, 1974; Brackenbury, 1999). Three main phases can be recognized in each wave of contraction (Figure 14.7). First, contraction of the dorsal longitudinal muscles and transverse muscles causes a segment to shorten dorsally and its posterior end to be raised so that the segment behind is lifted from the substrate. The dorsoventral muscles and leg retractor muscles then contract, lifting both feet of the segment from the substrate. Finally, contraction of the ventral longitudinal muscles, combined with the relaxation of the dorsoventral and leg retractor muscles, moves the segment forward and down to the substrate. Compared to walking, crawling and burrowing are relatively slow means of forward progression, with speeds of about 1 cm/sec for typical caterpillars. To take evasive action, for example, from a predator, caterpillars may walk backward by simply reversing the direction of peristalsis. Under extreme provocation, the caterpillar may coil up into a wheel and simply roll backward, achieving speeds up to 40 times greater than normal walking (Brackenbury, 1999). Little work has been done on the neural coordination of crawling, though it seems probable that endogenous activity within the central nervous system is responsible. However, pro-prioceptive stimuli undoubtedly influence the process.

Crawling or burrowing in apodous larvae is comparable to peristaltic locomotory movements found in other invertebrates, for example, mollusks and annelids. Larvae that crawl over the surface of the substrate grip the substrate with, for example, protrusible prolegs or creeping welts (transversely arranged thickenings equipped with stiff hairs) situated at

FIGURE 14.7. Phases in the passage of a peristaltic wave along the body of a caterpillar. [After G. M. Hughes, 1965, Locomotion: Terrestrial, in: The Physiology ofInsecta, 1st ed., Vol. II (M. Rockstein, ed.). By permission of Academic Press, Inc., and the author.]

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