Muscles and Locomotion

Abstract

All insect muscles are striated,though their structuraldetails vary according to their function. Skeletal muscles can be categorized as tubular (leg and body segment muscles, and wing muscles of Odonata and Dictyoptera), close-packed (wing muscles of Orthoptera and Lepidoptera), and fibrillar (flight muscles of most insects). All skeletal and many visceral muscles are innervated. Each musclefiber may receive one to three functionally distinct axons, one of whichisalways excitatory. Excitatory axons may besubdividedinto “fast” axons, whose impulses each initiate strong, rapid contractions, and “slow” axons, whose impulses individually cause a weak contraction but are additive in effect, facilitating a graded response in the muscle. Some fibers also receive an inhibitory axon. Most muscles operate in pairs, with one antagonistictotheother so that as one isstimulated to contract theother is passively stretched.Where there is no antagonistic partner, a muscle may be stretched because of the elasticity of the cuticle to which it is attached. Most insects use six legs during walking (hexapodal gait). Others use only two pairs oflegs(quadrupedal gait) but may use the tip of the abdomen as a point of support. On all occasions, however, there are at least three points of contact with thesubstrate, forming atriangleof support. Stepping movements are coordinated endogenously, though external stimuli clearly can affect the basic rhythm. Jumping, in most insects that use it, is a function of the hindlegs, which are elongate, muscular, and capable of great extension because of the wide angle that can be developed between femur and tibia. In order to obtain sufficient power 465 for takeoff, the energy of muscle contraction must be stored temporarily as elastic energy. This energy, at a critical point, is suddenly and rapidly released to produce the acceleration necessary to overcome gravity. Most soft-bodiedlarvae that crawl over, or burrow through, thesubstratedependonsynchronizedcontractionandrelaxationofmuscles to effect changes in body shape, the legs and accessory locomotory appendages serving simply as points of friction between the body and substrate. In these larvae the body fluids serve as a hydrostatic skeleton. Insects thatmove slowly over the surface of,orthrough,water typically use a hexapodal gait. More rapidly moving species, which maybe streamlined, usually employ arowing motion of the midlegs, occasionally the hindlegs. The legs are often modified to increase the surface area presented during the active stroke and their point of insertion on the body is designed so as to obtain maximum power from the stroke. Some aquatic insects swim by other means, for example, body curling, jet propulsion, or flapping thewings. Primitively,the direct muscles (those that connect directly with thewing articulations) are usedbothfor supplying power for flightandfor controlling the nature of thewing beat. However, in most flying insects efficiency is increased by separating the supply of power (the role of large indirect muscles in the thorax) from the control of wing beat (which remains the function of the direct muscles). In fliers that have a low wing-beat frequency, control of muscle contraction issynchronous (neurogenic); that is, there is a 1:1 ratio between wing-beat frequency (= frequency of wing-muscle contraction) and the number of nerve impulses arriving at the muscle. High wing-beat frequencies are achieved by the use of fibrillar muscles and asynchronous (myogenic) control. Fibrillar muscles alwaysoperate in antagonistic pairs. Theirrhythmof contraction originates endogenously andis initiated when a muscle is stretched toacritical tension,Thus, the contractions of an antagonistic pair of fibrillar muscles are self-perpetuating, though their initiation and termination are under nervous control. The indirect muscles serve to change the shape of the pterothorax, which thus acts as an elastic box, these changes in shape causing the wings to be moved up and down. Energy released at the end of each wing stroke (when a wing’s momentum rapidly decreases) is stored as elastic energy, inthewall of the pterothorax, inthe resilinof the wing hinge, and (in myogenic fliers) inthefibrillar muscles. This energy isthen released to powerthe followingstroke. Inmostinsects lift is generated through a combination of delayed stall, rapid wing rotation, and wake capture. However, in some very small species a special form of rotation-generated lift, the “clap and fling” mechanism is used. In both systems the generation of vortices, creating regions oflow pressure above thewings, isacritical component. Insects use a variety of substrates as energy sources for flight. Diptera and Hymenoptera metabolize trehalose; locusts, many Lepidoptera, Hemiptera, and Odonata oxidize lipid, sometimes after an initial phase of trehalose breakdown. A few insects use proline to fuel flight. Regulation of flight metabolism is under hormonal control, with adipokinetic hormone playing amajor role. Enhancedlocomotor activity,whichfollows receiptof astimulus, but whose direction iswithout spatial reference to that stimulus, is known as a kinesis. When the direction of movement is with reference to the source of the stimulus, for example, attraction to an odor and the light-compass reaction, the movement is described as a taxis. 5.