Entomology 3rd edition - C.Gillott - Chapter 14 ppt

14 M usc l es an d Locomotion 1 . Introductio n The ability to move is a characteristic of living animals and facilitates distribution, foo d p rocurement, location of a mate or e gg -la y in g site, and avoidance of unsuitable conditions. I nsects, l ar g e ly t h rou gh t h e i ra bili t y to fly w h en a d u l t, are amon g t h e most mo bil ean d wid e ly di str ib ute d o f an i ma l s. Deve l o p ment o f t hi sa bili t y ear ly i nt h eevo l ut i on o f t h ec l as s h as made the Insecta the most diverse and successful animal group (Chapter 2, Section 3.1). However, flight is only one method of locomotion employed by insects. Terrestrial specie s m a y walk, j um p , or crawl over the substrate, or burrow within it. A q uatic forms can swim i navar i et y o f wa y s or run on t h e water sur f ace. I nt h e i r l ocomotor y movements, i nsects con f orm to norma ldy nam i can d mec h an i ca l pr i nc i p l es. However, t h e i r genera ll y sma ll s i ze an dli g h twe i g h t h ave l e d to t h e d eve l opment of some unique structural, physiological, and biochemical features in their locomotor y s y s t ems . 2 . M us c le s Essent i a lly ,t h e structure an d contract il e mec h an i sm o fi nsect musc l es are com p ara ble t ot h ose o f verte b rate s k e l eta l (cross-str i ate d ) musc l e; t h at i s, t h ere are no musc l es i n i nsects o f the smooth (non-striated) type. Within muscle cells, the contractile elements actin and m yosin have been identified, and Huxley’s sliding filament theory of muscle contractio n a pp lies. Thou g h insect muscles are alwa y s cross-striated, there is considerable variation i n th e i r structure, bi oc h em i str y ,an d neura l contro l , i n accor d w i t h s p ec i fic f unct i ons . Because o f t h e i r sma ll s i ze an d t h evar i a bl e compos i t i on o f t h e h emo l ymp h o fi nsects , t he neuromuscular system has some unique features (Hoyle, 1974). Being small, an insect h as a limited space for muscles which are, accordingly, reduced in size. Though this is achieved to some extent b y a decrease in the size of individual cells (fibers), the p rinci p a l c h an g e h as b een a d ec li ne i nt h e num b er o f fi b ers p er musc l e suc h t h at some i nsect musc l es compr i se on l y one or two ce ll s. T h us, to ac hi eve a gra d e d musc l e contract i on, eac h fi b er must b e capable of a variable response, in contrast to the vertebrate situation where graded muscle r esponses result in part from stimulation of a varied number of fibers. Similarly, the volume o f nervous tissue is limited , so that there are few motor neurons for the control of muscle 43 7 4 3 8 C HAPTER 14 c ontraction. The hemolymph surrounding muscles may contain high concentrations of ions (es p eciall y divalent ions such as M g 2 + ) (Cha p ter 17, Section 4.1.1) that could interfere with i m p u l se transm i ss i on at s y na p ses an d neuromuscu l ar j unct i ons. T h at t hi s d oes not occur i s t h e resu l to f t h eevo l ut i on o f am y e li ns h eat h t h at covers g an gli a, nerves, an d neuromuscu l a r junctions . 2.1. Stru c tur e Insect muscles can be arranged in two categories: (1) skeletal muscles whose functio n is to move one p art of the skeleton in relation to another, the two p arts bein g se p arated b y a j o i nt o f some ki n d ,an d (2) v i scera l musc l es, w hi c hf orm l a y ers o f t i ssue enve l o pi n gi nterna l o r g ans suc h as t h e h eart, g ut, an d re p ro d uct i ve tract . A ttachment of a muscle to the integument must take into account the fact that period- ically the remains of the old cuticle are shed; therefore, an insertion must be able to brea k a nd re-form easil y .AsFi g ure 14.1 indicates, a muscle terminates at the basal lamina l y in g b eneat h t h ee pid erm i s. T h e musc l ece ll san d e pid erma l ce ll s i nter digi tate, i ncreas i n g t h e s ur f ace area f or attac h ment by a b out 10 t i mes, an dd esmosomes occur at i nterva l s, re pl ac- i ng t h e b asa ll am i na. Attac h ment o f a musc l ece ll to t h er i g id cut i c l e i sac hi eve d t h roug h l arge numbers of parallel microtubules (called “tonofibrillae” by earlier authors). Distally, the epidermal cell membrane is invaginated, forming numbers of conical hemidesmosomes o nw hi c h t h em i crotu b u l es term i nate. Runn i n gdi sta df rom eac hh em id esmosome i s one , rare ly two, musc l e attac h ment fi b ers ( = tonofi b r il s). Eac h fi b er p asses a l on g a p ore cana l FI G URE 14.1. Musc l e i nsert i on. [A f ter A. C. Nev ill e, 1 975, B iology of the Arthropod Cuticle . By p ermissio n o f Spr i nger-Ver l ag, New Yor k . ] 4 3 9 M U S CLE S AND LO C OMOTIO N t o the cuticulin envelope of the epicuticle to which they are attached by a special cement. As the cuticulin la y er is the first one formed durin gp roduction of a new cuticle (Cha p te r 11, Sect i on 3.1), attac h ment o f new ly f orme d fi b ers can rea dily occur. Unt il t h e actua l mo l t, h owever , t h ese are cont i nuous w i t h t h eo ld fi b ers an d, t h ere f ore , norma l musc l e contract i on i s possible (Neville, 197 5 ). M uscles comprise a varied number of elongate, multinucleated cells (fibers) (not t o b e confused with the muscle attachment fibers mentioned above) that ma y extend alon g th e l en g t h o f a musc l e. A musc l e i s arran g e d usua lly i nun i ts o f 10–20 fi b ers, eac h un i t b e i n g se p arate df rom t h eot h ers by a trac h eo l ate d mem b rane. Eac h un i t h asase p arate nerve supp l y. T h e cytop l asm (sarcop l asm) o f eac h fi b er conta i nsavar i e d num b er o f m i toc h on d r i a ( sarcosomes). Even at the light microscope level, the transversely striated nature of muscles i s visible. Higher magnification reveals that each fiber contains a large number of myofibril s ( = fi b r illae = sarcost yl es) lyi n gp ara ll e li nt h e sarco pl asm an d exten di n g t h e l en g t h o f t h e ce ll . Eac h m y ofi b r il com p r i ses t h e contract il efi l aments, ma d eu pp r i mar ily o f two p rote i ns, act i nan d myos i n. T h et hi c k er myos i nfi l aments are surroun d e db yt h et hi nner b ut mor e n umerous actin filaments. Filaments of each myofibril within a cell tend to be aligned, and i t is this that creates the striated appearance (alternating light and dark bands) of the cell. Th e d ark bands (A bands) corres p ond to re g ions where the actin and m y osin overla p , whereas th e ligh ter b an d s i n di cate re gi ons w h ere t h ere i son ly act i n(I b an d s) or m y os i n(H b an d s) ( F i gure 14.2). E l ectron m i croscopy h as revea l e di na ddi t i on to t h ese b an d s a num b er o f t hin t ransverse structures in the muscle fiber. Each of these Z lines ( discs ) runs across the fibe r i n the center of the I bands, separating individual contractile segments called sarcomeres. Attached to each side of the Z line are the actin filaments, which in contracted muscle ar e F IGURE 14 . 2 . D etails of a muscle fiber. [After R. F. Chapman, 1971, T he Insects: S tructure and Function. By p erm i ss i on o f E l sev i er / Nort h -Ho ll an d , Inc., an d t h e aut h or. ] 44 0 C HAPTER 14 c onnected to the myosin filaments by a means of cross bridges present at each end of the m y osin. Periodicall y , the p lasma membrane (sarcolemma) of the muscle fiber is dee p l y i nva gi nate d an df orms t h e so-ca ll e d Ts y stem (transverse s y stem). In most i nsect musc l e s t h eTs y stem occurs m id wa yb etween t h eZ li ne an d H b an d ; i nfi b r ill ar musc l es, h owever , t h ere i snoregu l ar pattern f or t h e pos i t i on o f t h e i nvag i nat i ons . T hough the above description is applicable to all insect muscles, different types o f muscles can be distin g uished, p rimaril y on the basis of the arran g ement of m y ofibrils , m i toc h on d r i a, an d nuc l e i ;t h e d e g ree o f se p arat i on o f t h em y ofi b r il s; t h e d e g ree o fd eve l- op ment o f t h e sarco pl asm i c ret i cu l um; an d t h e num b er o f act i ns surroun di n g eac h m y os i n (F i gure 14.3). T h ese i nc l u d etu b u l ar ( l ame ll ar), c l ose-pac k e d ,an d fi b r ill ar musc l es, a ll o f w hich are skeletal, and visceral muscles. L eg and segmental muscles of many adult insects and the flight muscles of primitive fli ers, suc h as O d onata an d D i ct y o p tera, are o f t h etu b u l ar t yp e, i nw hi c h t h e fl attene d ( l ame ll ate) m y ofi b r il s are arran g e d ra di a lly aroun d t h e centra l sarco pl asm. T h e nuc l e i are di str ib ute d w i t hi nt h e core o f sarcop l asm an d t h es l a blik em i toc h on d r i a are i ntersperse d F IGURE 14.3. T ransverse sections of insect skeletal muscles. (A) Tubular leg muscle of V es pa (Hymenoptera) ; (B) tu b u l ar fli g h t musc l eo f E na ll a g ma ( O d onata); (C) c l ose-pac k e dfli g h t musc l eo f a b utter fl y; an d (D) fi b r ill a r fligh t musc l eo f T ene b ri o (Co l eo p tera). (Not to same sca l e.) [A, a f ter H. E. Jor d an, 1920, Stu di es on str ip e d musc l e structure. VI, A m. J . A nat . 2 7 : 1–66. By permission of Wistar Press. B, C, redrawn from electron micrograph s i n D. S. Smith, 1965, The flight muscles of insects, Scienti fi c America n , June 1965, W. H. Freeman and Co. B y p erm i ss i on o f t h e aut h or.D,re d rawn f rom an e l ectron m i cro g ra ph i nD.S.Sm i t h ,19 6 1, T h e structure o fi nsec t fi brillar muscles. A study made with special reference to the membrane systems of the fiber, J . Biophys. Biochem . Cy to l . 1 0 : 123–158. By permission of the Rockefeller Institute Press and the author.] 4 4 1 M U S CLE S AND LOCOMOTIO N b etween the myofibrils.The body musculature of apterygotes and some larval pterygotes,th e le g muscles of some adult p ter yg otes, and the fli g ht muscles of Ortho p tera and Le p ido p tera are o f t h ec l ose- p ac k e d t yp e. Here t h em y ofi b r il san d m i toc h on d r i a are concentrate di nt h e center o f t h efi b er an d t h e nuc l e i are arran g e dp er iph era lly .Inc l ose- p ac k e d fligh t musc l es, th efi b ers are cons id era bl y l arger t h an t h ose o f tu b u l ar fli g h t musc l es. In a ddi t i on, trac h eo l e s d eeply indent the fiber, whereas in tubular muscles tracheoles simply lie alongside each fiber. It should be a pp reciated that the tracheoles do not actuall yp enetrate the muscl e ce ll mem b rane, t h at i s, t h e y are extrace ll u l ar. In most i nsects t h e i n di rect musc l es, w hi c h p rov id et h e p ower f or fligh t, are near ly a l wa y sfi b r ill ar, so-ca ll e db ecause i n di v id ua l fi b r il s are c h aracter i st i ca ll y very l arge an d , toget h er w i t h t h e mass i ve m i toc h on d r i a, occupy a l most all of the volume of the fiber. Very little sarcoplasm is present, and the nuclei are squeezed r andomly between the fibrils. Because of their size, there are often only a few fibrils pe r ce ll ,an d t h ese are f re q uent ly q u i te i so l ate df rom eac h ot h er by t h e mass i ve ly i n d ente d an d i ntertw i n i n g s y stem o f trac h eo l es. T h e p resence o fl ar g e q uant i t i es o f c y toc h romes i nt he mi toc h on d r i ag i ves t h ese musc l esac h aracter i st i cp i n k or ye ll ow co l or. It s h ou ld b e apparen t ev e n from this brief description that fibrillar muscles are designed to facilitate a high rat e of aerobic respiration in connection with the energetics of flight . Visceral muscles differ from skeletal muscles in several wa y s. The cells com p risin g th em are un i nuc l eate, ma yb ranc h ,an d are j o i ne d to a dj acent ce ll s by se p tate d esmosomes . T h e i r contract il ee l ements are not arrange di nfi b r il san d conta i na l arger proport i on o f act i n t o myosin. Nevertheless, the visceral muscles are striated (sometimes only weakly), and t heir method of contraction is apparently identical to that of skeletal muscles. All skeletal muscles and man y visceral muscles are innervated. The skeletal muscles a l wa y s rece i ve nerves f rom t h e centra l nervous s y stem, w h ereas t h ev i scera l musc l es are i nnervate df rom e i t h er t h e stomato g astr i cort h e centra l nervous s y stem. W i t hi na p art i cu l a r m usc l eun i t, eac h fi b er may b e i nnervate db y one, two, or t h ree f unct i ona ll y di st i nct axons. One of these is always excitatory; where two occur (the commonest arrangement), the y are usuall y both excitator y (“fast” and “slow” axons), but ma y be a “slow” excitator y axon pl us an i n hibi tor y axon; i n some cases a ll t h ree t yp es o f axon occur. T hi s arran g ement, k nown as p o ly neurona li nnervat i on, f ac ili tates a var i a bl e res p onse on t h e p art o f a musc l e ( Sect i on 2.2). Eac h axon, regar dl ess o fi ts f unct i on, i s muc hb ranc h e d an d , i n contrast t o t he situation in vertebrate muscle, there are several motor neuron endings from each axo n on each muscle fiber (multiterminal innervation) (Figure 14.4). 2 .2. Ph y s i olog y Like those of vertebrates, insect muscles contract according to the sliding filament t heory. The arrival of an excitatory nerve impulse at a neuromuscular junction causes depo- larization of the ad j acent sarcolemma. A wave of de p olarization s p reads over the fiber and i nto t h e i nter i or o f t h ece ll v i at h eTs y stem. De p o l ar i zat i on o f t h eTs y stem mem b ranes i n d uces a momentar yi ncrease i nt h e p ermea bili t y o f t h ea dj acent sarco pl asm i c ret i cu l um, so that calcium ions, stored in vesicles of the reticulum, are released into the sarcoplas m surrounding the myofibrils. The calcium ions activate cross-bridge formation between the actin and m y osin, enablin g the filaments to slide over each other so that the distance between a dj acent Z li nes i s d ecrease d .T h e net e ff ect i s f or t h e musc l e to contract. Ener gy d er i ve d f rom t h e hyd ro ly s i so f a d enos i ne tr iph os ph ate (ATP) i sre q u i re df or contract i on, t h ou gh i ts prec i se f unct i on i sun k nown. It may b e use di n b rea ki ng t h e cross- b r id ges, or f or t h e active transport of the calcium ions back into the vesicles, or for both of these processes. In 442 C HAPTER 14 F I G URE 14 . 4 . Pol y neuronal and multitermi- na li nnervat i on o f an i nsect musc l e. [ A f ter G . H o yl e, 1974, Neura l contro l o f s k e l eta l mus- c le, in: T he Physiology of Insecta, 2nd ed., Vol. I V (M. Roc k ste i n, e d .). By perm i ss i on o f Aca- d em i c Press, Inc., an d t h e aut h or. ] addition to sliding over each other, both the actin and the myosin filaments may shorten (by c o ili n g ), an di n some m y ofi b r il st h eZ li nes di s i nte g rate to a ll ow t h eA b an d so f a dj acent sarcomeres to over l a p eac h ot h er, t h us ena bli n g an even g reater d e g ree o f contract i on to occ u r. E xtension (relaxation) of a muscle may result simply from the opposing elasticity of the cuticle to which the muscle is attached. More commonly, muscles occur in pairs, each member of the p air workin g anta g onisticall y to the other; that is, as one muscle is stimulate d to contract, i ts p artner (unst i mu l ate d ) i s stretc h e d . Norma lly ,t h e p rev i ous ly unst i mu l ate d musc l e i sst i mu l ate d to b eg i n contract i on w hil e act i ve contract i on o f t h e partner i sst ill o ccurring (cocontraction). This is thought to bring about dampening of contraction, perhap s thereby preventing damage to a vigorously contracting muscle. Also, in slow movements , it p rovides an insect with a means of p recisel y controllin g such movements (Ho y le, 1974). M usc l e anta g on i sm i sac hi eve dby centra li n hibi t i on, t h at i s, at t h e l eve l o fi nterneuron s wi t hi nt h e centra l nervous s y stem (C h a p ter 13, Sect i on 2.3). T h us, f or a gi ven st i mu l us , t h e passage o fi mpu l ses a l ong an axon to one musc l eo f t h epa i rw ill b e perm i tte d ,an d hence that muscle will contract. However, passage of impulses to the partner is inhibited and the muscle will be p assivel y stretched. It should be em p hasized that in this arran g emen t t h e axon to eac h musc l e i sexc i tator y .Ins l ow wa lki n g movements, f or exam pl e, a l ternat i n g st i mu l at i on o f eac h musc l e i s q u i te di st i nct. At high er s p ee d st hi s rec ip roca li n hibi t i on b rea k s d own, an d one o f t h e musc l es rema i ns permanent l y i nam ildl y contracte d state , serving as an “elastic restoring element” (Hoyle, 1974). The other muscle continues to be alternately stimulated and thus provides the driving power for the activity. As note d ear li er, common ly musc l es rece i ve two exc i tator y axons, one “s l ow,” t h eot h er “ f ast.” T h ese terms are somew h at m i s l ea di n gf or t h e yd o not i n di cate t h es p ee d at w hi c h i mpu l ses trave l a l ong t h e axons, b ut rat h er t h e spee d at w hi c h as i gn i ficant contract i on can be observed in the muscle. Thus, an impulse traveling along a fast axon induces a strong 4 4 3 M U S CLE S AND LOCOMOTIO N contraction of the “all or nothing” type; that is, a further contraction cannot be initiated unti l t he ori g inal ionic conditions have been restored. In contrast, a sin g le im p ulse from a slow axon causes on ly a wea k contract i on i nt h e musc l e. However, a ddi t i ona li m p u l ses arr i v i n g i nqu i c k success i on are a ddi t i ve i nt h e i re ff ect (summat i on) so t h at, w i t h t h es l ow axo n arrangement a graded response is possible for a particular muscle, despite the relativel y few fibers it may contain. Muscles with dual innervation use only the slow axon for mos t r e q uirements; the fast axon functions onl y when immediate and/or massive contraction i s n ecessar y . For exam pl e, t h e extensor t ibi a musc l eo f t h e hi n dl e g o f a g rass h o pp er i sor di nar ily contro ll e d so l e ly v i at h es l ow axon. For j um pi n g , h owever, t h e f ast axon i s b rou gh t i nto play . The function of inhibitory axons remains questionable. Electrophysiological work ha s shown that in normal activit y the inhibitor y axon is electricall y silent, that is, shows no el ectr i ca l act i v i t y ,an di sc l ear ly b e i n gi n hibi te df rom w i t hi nt h e centra l nervous s y stem. Dur i n gp er i o d so fg reat act i v i t y , i m p u l ses can somet i mes b eo b serve dp ass i n g a l on g t h e axon , per h aps to acce l erate musc l ere l axat i on, t h oug h norma ll yt h e use o f antagon i st i c musc l es and central inhibition is adequate. Hoyle (1974) suggested that peripheral inhibition may b e necessar y at certain sta g es in the life c y cle, such as moltin g , when central inhibition ma y n ot b e p oss ibl e. 3 .Lo c omotio n 3 .1. Movement on or Through a S ubstrat e 3 . 1 . 1 . Walk i n g I nsects can walk at almost imperceptibly slow speed (watch a mantis stalking its prey) o r run at seemingly very high rates (try to catch a cockroach). The latter is, however, a w ron g im p ression created b y the smallness of the or g anism, the rate at which its le g s move , an d t h e rate at w hi c hi t can c h an g e di rect i on. Ants scurr yi n g a b out on a h ot summer d a y are traveling only about 1. 5 km/hr, and the elusive cockroach has a top speed of just unde r 5 km/hr (Hughes and Mill, 1974). Nevertheless, an insect leg is structurally well adapted for locomotion. Like the limbs of o ther activel y movin g animals, it ta p ers toward the distal end, which is li g ht and easil y lifted. I ts tarsa l se g ments are e q u ipp e d w i t h c l aws or p u l v illi t h at p rov id et h e necessar yf r i ct i on b etween t h e li m b an d t h esu b strate. A l e g com p r i ses f our ma i nse g ments (C h a p ter 3, Sect i on 4 .3.1), which articulate with each other and with the body. The coxa articulates proximall y w ith the thorax, usually by means of a dicondylic joint and distally, with the fused trochante r and femur, also via a dicond y lic j oint. Dicond y lic j oints p ermit movement in a sin g le p lane. However, t h etwo j o i nts are set at r igh tan gl es to eac h ot h er an d ,t h ere f ore, t h et ip o f a l e g can move i nt h ree di mens i ons . T h e musc l es t h at move a l eg are b ot h extr i ns i c( h av i ng one en di nserte d on t h ewa ll o f the thorax) and intrinsic (having both ends inserted within the leg) (Figure 14. 5 ). Th e m a j orit y of extrinsic muscles move the coxa, rarel y the fused trochantofemoral se g ment, wh ereas t h e p a i re di ntr i ns i c musc l es move l e g se g ments i nre l at i on to eac h ot h er. Som e of t h e extr i ns i c musc l es h ave a d ua lf unct i on, serv i n g to b r i n g a b out b ot hl e g an d w i n g m ovements. Typ i ca ll y, t h e l eg musc l es i nc l u d e (1) t h e coxa l promotor an di ts antagon i st, t h e coxal remotor, which run from the tergum to the anterior and posterior edges, respectively, 444 C HAPTER 14 F IGURE 14 . 5 . (A) Musculature of coxa; (B) segmental musculature of leg; and (C) musculature of hindleg of g rass h opper. [A, C, f rom R. E. Sno d grass, Princip l es o f Insect Morp h o l og y . Copyright 193 5 by McGraw-Hill, I nc. Used with p ermission of McGraw-Hill Book Com p an y .B,re p roduced b yp ermission of the Smithsonia n I n st i tut i o nPr ess fr o m S mithsonian Miscellaneous Collection s , V olume 80, Morphology and mechanism of the VV i nsect thorax, Number 1, June 2 5 , 1927, 108 p a g es, b y R. E. Snod g rass: Fi g ure 39, p a g e 89. Washin g ton, D.C. , 1 928, Smithsonian Institution.] of t h e coxa; contract i on o f t h e coxa l promotor causes t h e coxa to tw i st f orwar d ,t h ere b y effecting protraction (a forward swing) of the entire leg; (2) the coxal adductor and abductor (attached to the sternum and pleuron, respectively), which move the coxa toward or away f rom the bod y ; (3) anterior and p osterior coxal rotators, which arise on the sternum an d ass i st i nra i s i n g an d mov i n g t h e l e gf orwar d or b ac k war d ;an d (4) an extensor ( l evator ) an dfl exor ( d e p ressor) musc l e i n eac hl e g se g ment, w hi c h serve to i ncrease an dd ecrease , respectively, the angle between adjacent segments. It should be noted that the muscles that move a particular leg segment are actually located in the next more proximal segment. Fo r exam p le, the tibial extensor and flexor muscles, which alter the an g le between the femur an d t ibi a, are l ocate d w i t hi nt h e f emur an d are attac h e dby s h ort ten d ons i nserte d at t h e h ea d of t h et ibi a . It i st h e coor di nate d act i ons o f t h e extr i ns i can di ntr i ns i c musc l es t h at move a l eg and propel an insect forward. In considering how propulsion is achieved, it must also b e remembered that another im p ortant function of a le g is to su pp ort the bod y , that is, to kee p 44 5 M U S CLE S AND LOCOMOTIO N F I G URE 14.6. M a g n i tu d eo f t h e l on gi tu di na l an dl atera lf orces resu l t i n gf rom t h e strut e ff ect f or eac hl e gi n i ts extreme p osition. [After G. M. Hu g hes, 19 5 2, The coordination of insect movements. I. The walkin g movements of insects , J. Ex p . Biol. 2 9 : 267–284. By permission of Cambridge University Press, London. ] i to ff t h e groun d .Int h e l atter s i tuat i on, a l eg may b e cons id ere d asas i ng l e-segmente d structure—a rigid strut. If the strut is vertical, the force along its length (axial force) will be solely supporting and will have no propulsive component. If the strut is inclined, the axia l force can be resolved into two com p onents, a vertical su pp ortive force and a horizonta l p ro p u l s i ve f orce. Because t h e l e gp rotru d es l atera lly f rom t h e b o dy ,t h e h or i zonta lf orce can b e f urt h er reso l ve di nto a transverse f orce pus hi ng t h e i nsect s id eways an d a l ong i tu di na l force that causes backward or forward motion. The relative sizes of these horizontal force s d epend on (1) which leg is being considered and (2) the position of that leg. Figure 14.6 i ndicates the size of these forces for each le g at its two extreme p ositions. It will be a pp arent th at i na l most a ll o fi ts p os i t i ons t h e f ore l e g w ill i n hibi t f orwar d movement, w h ereas t h e mid -an dhi n dl egs a l ways promote f orwar d movement. In equ ilib r i um, t h at i s, w h en an i nsect is standing still, the forces will be equal and opposite. Movement of an insect’s body w ill occur only if the center of gravity of the body falls. This occurs when the forces become i mbalanced, for exam p le, b y chan g in g the p osition of a forele g so that its retardin g effec t i sno l on g er e q ua l to t h e p romot i n g e ff ect o f t h eot h er l e g s, w h ereu p on t h e i nsect to ppl e s f orwar d (Hu gh es an d M ill , 1974) . A l so i mportant f rom t h epo i nt o f v i ew o fl ocomot i on i st h e l eg’s a bili ty to f unct i on a s al e ver, that is, a solid bar that rotates about a fulcrum and on which work can be done. The fulcrum is the coxothoracic j oint and the work is done b y the lar g e, extrinsic muscles . 44 6 C HAPTER 14 B ecause of the large angle through which it can rotate and because of its angle to the body, the forele g is most im p ortant as a lever. In contrast, the mid- and hindle g s, which each rotate t h rou gh on ly a sma ll an gl e, exert on ly as ligh t l ever e ff ect an d serve p r i mar ily a s struts ( i nt h e f u lly exten d e d ,r igid p os i t i on). For t h e f ore l e gi n i ts f u lly p rotracte dp os i t i on , c ontract i on o f t h e retractor musc l e( i .e., t h e l ever e ff ect) w ill b esu f fic i ent to overcome t h e o pposing retarding (strut) effect and, provided that the frictional forces between the ground and tarsi are sufficient, the bod y will be moved forward. H owever, t h e l ar g est com p onent o f t h e p ro p u l s i ve f orce i s d er i ve d as a resu l to f t he l e g ’s a bili t y to fl ex an d exten dby v i rtue o f t h e i r j o i nte d nature. F l exure (a d ecrease i nt h e ang l e b etween a dj acent l eg segments) w ill ra i se t h e l eg o ff t h e groun d so t h at i t can be moved forward without the need to overcome frictional forces between it and the ground . In the case of the foreleg, flexure first will remove, by lifting the leg from the ground, the retar di n g e ff ect as a resu l to fi ts act i on as a strut an d , secon d ,w h en t h e l e gi sre pl ace d o n t h esu b strate, w ill cause t h e b o dy to b e p u ll e df orwar d .F l exure o f t h e f ore l e g cont i nues unt il t h e l eg i s perpen di cu l ar to t h e b o d y, at w hi c h po i nt extens i on b eg i ns so t h at now t he body is pushed forward. For the mid- and hindlegs, flexure serves to bring the legs into a new forward position. Extension, as in the case of the foreleg, will push the body forward. B ecause the hindle g is usuall y the lar g est of the three, it exerts the g reatest p ro p ulsive force. As note d a b ove, t h e h or i zonta l ax i a lf orce a l on g eac hl e gh as a transverse as we ll a s a l ong i tu di na l component. T h us, as an i nsect moves, i ts b o d yz i gzags s li g h t l y f rom s id eto side, the transverse forces exerted by the fore- and hindlegs of one side being balanced by an opposite force exerted by the middle leg of the opposite side in the normal rhythm of le g movemen t s . R hythms o f Leg Movements. Most i nsects use a ll s i x l egs d ur i ng norma l wa lki ng. O ther species habitually employ only the two anterior or the two posterior pairs of legs bu t may use all legs at higher speeds. In all instances, however, the legs are lifted in an orderly se q uence (thou g h this ma y var y with the s p eed of the insect), and there are alwa y s at least t h ree p o i nts o f contact w i t h t h esu b strate f orm i n g a “tr i an gl eo f su pp ort” f or t h e b o dy . (In some s p ec i es t h at em pl o y two p a i rs o fl e g s, t h et ip o f t h ea bd omen ma y serve as a p o i nt o f support.) Two other generalizations that may be made are (1) no leg is lifted until the le g behind has taken up a supporting position and (2) the legs of a segment alternate in their movemen t s . In t h et ypi ca lh exa p o d a lg a i tat l ow s p ee d ,on ly one l e g atat i me i sra i se d o ff t h e g roun d, so t h at t h e ste ppi n g se q uence i s R3, R2, R l , L3, L2, L l (w h ereRan d L are r igh tan dl e ft l egs, respect i ve l y, an d 1, 2, an d 3 i n di cate t h e f ore-, m id -, an dhi n dl egs, respect i ve l y). W i t h increase in speed, overlap occurs between both sides so that the sequence first becomes R3 Ll, R2, Rl L3, L2, etc., and, then, R3 Rl L2, R2 L3 Ll, etc., that is, a true alternatin g tri p odal g a i t. Th e ort h o p teran R hipipter yx h as a q ua d ru p e d a lg a i t, us i n g on ly t h e anter i or two p a i r s of l egs an d us i ng t h et i po f t h ea bd omen as a support. Its stepp i ng sequence i sR l L2, R2 LI., etc. Mantids are likewise quadrupedal at low speed, using the posterior two pairs of legs (sequence R3 L2, R2 L3, etc.). At high speed, the forelegs are brought into action though the insects remain effectivel yq uadru p edal (se q uence Ll R3, L3 R2, L2 R1, etc.). Av a r i et y o f met h o d s f or turn i n gh ave b een o b serve d ,o f ten i nt h e same s p ec i es. T h e y i nc l u d e i ncreas i ng t h e l engt h o f t h e str id eont h e outs id eo f t h e turn, i ncreas i ng t h e f requency o f stepping on the outside of the turn, fixing one or more of the “inside” legs as pivots, and moving the legs on the inside of the turn backward. Coordination of the movements both amon g se g ments of the same le g and amon g diff erent l e g sre q u i res a high l eve l o f neura l act i v i t y , b ot h sensor y an d motor. L ik eot h er [...]... Aerofoils on fixed-wing aircraft are designed with their upper surface curved, 453 MUSCLES AND LOCOMOTION 454 CHAPTER 14 FIGURE 14. 8 (A) Indirect; and (B) accessory indirect muscles and direct muscles of right side of wing-bearing segment, seen from within [After J W S Pringle, 1957, Insect Flight By permission of Cambridge University Press, London.] their lower surface flat (Figure 14. 10A), so as to... topspin), the lift is negative Rotation-generated lift is probably of particular importance in the fine control of flight maneuvres The remaining contribution is derived from wake 458 CHAPTER 14 FIGURE 14. 12 Air flow around the wing and the resulting forces at points during a wing stroke Delayed stall (1) results from formation of a leading edge vortex on the wing Rotation-generated lift (2,3) occurs when... and wing-beat frequency) Wing-beat frequency varies widely among different insects as has been noted already in the introduction to this discussion of flight In fliers whose wing-beat frequency is low (e.g., the desert locust, about 15–20 beats/sec) neurogenic (synchronous) control occurs That is, there is a 1:1 ratio between wing-beat frequency and nervous input In such fibers, therefore, wing-beat frequency... each beat of the wings is initiated by a burst of impulses to the power-producing muscles, which are of the tubular or close-packed type (Figure 14. 3B, C) This applies to all users of direct muscles for powering flight, plus Lepidoptera in which the indirect muscles are used In contrast, in fliers that use indirect muscles and whose wing-beat frequency is high (up to 1000 beats/sec), muscle contraction is... depleted, more is formed from lipids stored in the fat body or from hemolymph alanine Wheeler (1989) suggested that its advantages 464 CHAPTER 14 over the use of lipid substrates are its water solubility and the non-necessity of maintaining the metabolically expensive lipoprotein-carrier system Hormones regulate the supply of fuels to, and their use by, the flight muscles Especially well studied is the control... (which remains the function of the direct muscles) In fliers that have a low wing-beat frequency, control of muscle contraction is synchronous (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)... A few, especially very small, insects such as thrips, whitefly, and parasitoid wasps f (whose wingspans are about 1 mm or less) use a particular form of rotation-generated lift known as the “clap and fling” mechanism (Weis-Fogh, 1973) (Figure 14. 13) In this system lift is generated by rotation of the wings as they separate after they have clapped together at the end of the upstroke and are flung apart... of the wing with the pleural wing process which serves as a fulcrum (Figure 14. 14A) In most insects indirect muscles are also used to lower the wing Shortening of the dorsal longitudinal muscles causes the tergum to bow upward, raising the anterior and posterior notal processes above the tip of the pleural wing process FIGURE 14. 13 Clap and fling mechanism for generating lift The wings clap together at... speed over the upper wing surface compared to the lower wing surface, thereby creating lift) [From T Weis-Fogh, 1975, Unusual mechanisms for the generation of lift, Sci Amer 233 (November):80–87 Original drawn by Tom Prentiss By permission of Nelson H Prentiss.] 459 MUSCLES AND LOCOMOTION FIGURE 14. 14 Diagrammatic transverse sections of thorax to show muscles used in upstroke and downstroke (A) Use of... muscles to lower wing; and (C) use of direct muscles to lower wing [After R F Chapman, 1971, The Insects: Structure and Function By permission of Elsevier/North-Holland, Inc., and the author.] and, therefore, the wing to be lowered (Figure 14. 14B) In Coleoptera and Orthoptera some power for a downstroke is also obtained by contraction of the basalar and subalar muscles and in Odonata and Blattodea, . o f contract i on to occ u r. E xtension (relaxation) of a muscle may result simply from the opposing elasticity of the cuticle to which the muscle is attached. More commonly, muscles occur in. However, ca l cu l at i ons s h owe d t h at conven- tional (steady-state) aerodynamic theory, which is based on rigid wings moving at con - stant velocit y , cannot account for the p roduction. muscle contractio n a pp lies. Thou g h insect muscles are alwa y s cross-striated, there is considerable variation i n th e i r structure, bi oc h em i str y ,an d neura l contro l , i n accor d w i t h s p ec i c f unct i ons . Because