Crawling wriggling swimming and walking

Soft-bodied larvae with hydrostatic skeletons move by crawling. Muscular contraction in one part of the body gives equivalent extension in a relaxed part elsewhere on the body. In apodous (legless) larvae, such as dipteran "maggots", waves of contractions and relaxation run from head to tail. Bands of adhesive hooks or tubercles successively grip and detach from the substrate to provide a forward motion, aided in some maggots by use of their mouth hooks to grip the substrate. In water, lateral waves of contraction against the hydrostatic skeleton can give a sinuous, snake-like, swimming motion, with anterior-to-posterior waves giving an undulating motion.

Larvae with thoracic legs and abdominal prolegs, like caterpillars, develop posterior-to-anterior waves of turgor muscle contraction, with as many as three waves visible simultaneously. Locomotor muscles operate in cycles of successive detachment of the thoracic legs, reaching forwards and grasping the substrate. These cycles occur in concert with inflation, deflation, and forward movement of the posterior prolegs.

Insects with hard exoskeletons can contract and relax pairs of agonistic and antagonistic muscles that attach to the cuticle. Compared to crustaceans and myriapods, insects have fewer (six) legs that are located more ventrally and brought close together on the thorax, allowing concentration of locomotor muscles (both flying and walking) into the thorax, and providing more control and greater efficiency. Motion with six legs at low to moderate speed allows continuous contact with the ground by a tripod of fore and hind legs on one side and mid leg of the opposite side thrusting rearwards (retraction), whilst each opposite leg is moved forwards (protraction) (Fig. 3.3). The center of gravity of the slow-moving insect always lies within this tripod, giving great stability. Motion is imparted through thoracic muscles acting on the leg bases, with transmission via internal leg muscles through the leg to extend or flex the leg. Anchorage to the substrate,

Fig. 3.3 (right) A ground beetle (Coleoptera: Carabidae: Carabus) walking in the direction of the broken line. The three blackened legs are those in contact with the ground in the two positions illustrated - (a) is followed by (b). (After Wigglesworth 1972.)

needed to provide a lever to propel the body, is through pointed claws and adhesive pads (the arolium or, in flies and some beetles, pulvilli). Claws such as those illustrated in the vignette to Chapter 2 can obtain purchase on the slightest roughness in a surface, and the pads of some insects can adhere to perfectly smooth surfaces through the application of lubricants to the tips of numerous fine hairs and the action of close-range molecular forces between the hairs and the substrate.

When faster motion is required there are several alternatives - increasing the frequency of the leg movement by shortening the retraction period; increasing the stride length; altering the triangulation basis of support to adopt quadrupedy (use of four legs); or even hind-leg bipedality with the other legs held above the substrate. At high speeds even those insects that maintain triangulation are very unstable and may have no legs in contact with the substrate at intervals. This instability at speed seems to cause no difficulty for cockroaches, which when filmed with high-speed video cameras have been shown to maintain speeds of up to 1 m s-1 whilst twisting and turning up to 25 times per second. This motion was maintained by sensory information received from one antenna whose tip maintained contact with an experimentally provided wall, even when it had a zig-zagging surface.

Many insects jump, some prodigiously, usually using modified hind legs. In orthopterans, flea beetles (Alticinae), and a range of weevils, an enlarged hind (meta-) femur contains large muscles whose slow contraction produces energy stored by either distortion of the femoro-tibial joint or in some spring-like sclerotiza-tion, for example the meta-tibial extension tendon. In fleas, the energy is produced by the trochanter levator muscle raising the femur and is stored by compression of an elastic resilin pad in the coxa. In all these jumpers, release of tension is sudden, resulting in propulsion of the insect into the air - usually in an uncontrolled manner, but fleas can attain their hosts with some control over the leap. It has been suggested that the main benefit for flighted jumpers is to get into the air and allow the wings to be opened without damage from the surrounding substrate.

In swimming, contact with the water is maintained during protraction, so it is necessary for the insect to impart more thrust to the rowing motion than to the recovery stroke to progress. This is achieved by expanding the effective leg area during retraction by extending fringes of hairs and spines (Fig. 10.8). These collapse onto the folded leg during the recovery stroke. We have seen already how some insect larvae swim using contractions against their hydrostatic skeleton. Others, including many nymphs and the larvae of caddisflies, can walk underwater and, particularly in running waters, do not swim routinely.

The surface film of water can support some specialist insects, most of which have hydrofuge (water-repelling) cuticles or hair fringes and some, such as gerrid water-striders (Fig. 5.7), move by rowing with hair-fringed legs.

3.1.4 Flight

The development of flight allowed insects much greater mobility, which helped in food and mate location and gave much improved powers of dispersal. Importantly, flight opened up many new environments for exploitation. Plant microhabitats such as flowers and foliage are more accessible to winged insects than to those without flight.

Fully developed, functional, flying wings occur only in adult insects, although in nymphs the developing wings are visible as wing buds in all but the earliest instars. Usually two pairs of functional wings arise dorsolaterally, as fore wings on the second and hind wings on the third thoracic segment. Some of the many derived variations are described in section 2.4.2.

To fly, the forces of weight (gravity) and drag (air resistance to movement) must be overcome. In gliding flight, in which the wings are held rigidly outstretched, these forces are overcome through the use of passive air movements - known as the relative wind. The insect attains lift by adjusting the angle of the leading edge of the wing when orientated into the wind. As this angle (the attack angle) increases, so lift increases until stalling occurs, i.e. when lift is catastrophically lost. In contrast to aircraft, nearly all of which stall at around 20°, the attack angle of insects can be raised to more than 30°, even as high as 50°, giving great maneuverability. Aerodynamic effects such as enhanced lift and reduced drag can come from wing scales and hairs, which affect the boundary layer across the wing surface.

Most insects glide a little, and dragonflies (Odonata) and some grasshoppers (Orthoptera), notably locusts, glide extensively. However, most winged insects fly by beating their wings. Examination of wing beat is difficult because the frequency of even a large slow-

flying butterfly may be five times a second (5 Hz), a bee may beat its wings at 180 Hz, and some midges emit an audible buzz with their wing-beat frequency of greater than 1000 Hz. However, through the use of slowed-down, high-speed cine film, the insect wing beat can be slowed from faster than the eye can see until a single beat can be analyzed. This reveals that a single beat comprises three interlinked movements. First is a cycle of downward, forward motion followed by an upward and backward motion. Second, during the cycle each wing is rotated around its base. The third component occurs as various parts of the wing flex in response to local variations in air pressure. Unlike gliding, in which the relative wind derives from passive air movement, in true flight the relative wind is produced by the moving wings. The flying insect makes constant adjustments, so that during a wing beat, the air ahead of the insect is thrown backwards and downwards, impelling the insect upwards (lift) and forwards (thrust). In climbing, the emergent air is directed more downwards, reducing thrust but increasing lift. In turning, the wing on the inside of the turn is reduced in power by decrease in the amplitude of the beat.

Despite the elegance and intricacy of detail of insect flight, the mechanisms responsible for beating the wings are not excessively complicated. The thorax of the wing-bearing segments can be envisaged as a box with the sides (pleura) and base (sternum) rigidly fused, and the wings connected where the rigid tergum is attached to the pleura by flexible membranes. This membranous attachment and the wing hinge are composed of resilin (section 2.1), which gives crucial elasticity to the thoracic box. Flying insects have one of two kinds of arrangements of muscles powering their flight:

1 direct flight muscles connected to the wings;

2 an indirect system in which there is no muscle-towing connection, but rather muscle action deforms the thoracic box to move the wing.

A few old groups such as Odonata and Blattodea appear to use direct flight muscles to varying degrees, although at least some recovery muscles may be indirect. More advanced insects use indirect muscles for flight, with direct muscles providing wing orientation rather than power production.

Direct flight muscles produce the upward stroke by contraction of muscles attached to the wing base inside the pivotal point (Fig. 3.4a). The downward wing stroke is produced through contraction of muscles that extend from the sternum to the wing base outside the pivot point (Fig. 3.4b). In contrast, indirect flight mus cles are attached to the tergum and sternum. Contraction causes the tergum, and with it the very base of the wing, to be pulled down. This movement levers the outer, main part of the wing in an upward stroke (Fig. 3.4c). The down beat is powered by contraction of the second set of muscles, which run from front to back of the thorax, thereby deforming the box and lifting the tergum (Fig. 3.4d). At each stage in the cycle, when the flight muscles relax, energy is conserved because the elasticity of the thorax restores its shape.

Primitively, the four wings may be controlled independently with small variation in timing and rate allowing alteration in direction of flight. However, excessive variation impedes controlled flight and the beat of all wings is usually harmonized, as in butterflies, bugs, and bees, for example, by locking the fore and hind wings together, and also by neural control. For insects with slower wing-beat frequencies (<100 Hz), such as dragonflies, one nerve impulse for each beat can be maintained by synchronous muscles. However, in faster-beating wings, which may attain a frequency of 100 to over 1000 Hz, one impulse per beat is impossible and asynchronous muscles are required. In these insects, the wing is constructed such that only two wing positions are stable - fully up and fully down. As the wing moves from one extreme to the alternate one, it passes through an intermediate unstable position. As it passes this unstable ("click") point, thoracic elasticity snaps the wing through to the alternate stable position. Insects with this asynchronous mechanism have peculiar fibrillar flight muscles with the property that, on sudden release of muscle tension, as at the click point, the next muscle contraction is induced. Thus muscles can oscillate, contracting at a much higher frequency than the nerve impulses, which need be only periodic to maintain the insect in flight. Harmonization of the wing beat on each side is maintained through the rigidity of the thorax - as the tergum is depressed or relaxed, what happens to one wing must happen identically to the other. However, insects with indirect flight muscles retain direct muscles that are used in making fine adjustments in wing orientation during flight.

Direction and any deviations from course, perhaps caused by air movements, are sensed by insects predominantly through their eyes and antennae. However, the true flies (Diptera) have extremely sophisticated sensory equipment, with their hind wings modified as balancing organs. These halteres, which each comprise a base, stem, and apical knob (Fig. 2.22f), beat in time

Fig. 3.4 Direct flight mechanisms: thorax during (a) upstroke and (b) downstroke of the wings. Indirect flight mechanisms: thorax during (c) upstroke and (d) downstroke of the wings. Stippled muscles are those contracting in each illustration. (After Blaney 1976.)

but out of phase with the fore wings. The knob, which is heavier than the rest of the organ, tends to keep the halteres beating in one plane. When the fly alters direction, whether voluntarily or otherwise, the haltere is twisted. The stem, which is richly endowed with sensilla, detects this movement, and the fly can respond accordingly.

Initiation of flight, for any reason, may involve the legs springing the insect into the air. Loss of tarsal contact with the ground causes neural firing of the direct flight muscles. In flies, flight activity originates in contraction of a mid-leg muscle, which both propels the leg downwards (and the fly upwards) and simultaneously pulls the tergum downwards to inaugurate flight. The legs are also important when landing because there is no gradual braking by running forwards - all the shock is taken on the outstretched legs, endowed with pads, spines, and claws for adhesion.

0 0

Post a comment