Entomology 3rd edition - C.Gillott - Chapter 15 pptx

15 G as Exchan g e 1 . Intr oduc t ion I n all or g anisms g as exchan g e, the suppl y of ox yg en to and removal of carbon dioxide fro m cells, depends ultimatel y on the rate at which these g ases diffuse in the dissolved state. Th e d iffusion rate is proportional to (1) the surface area over which diffusion is occurrin g and (2 ) th e diff us i on gra di ent (concentrat i on diff erence o f t h e diff us i ng mater i a lb etween t h etw o po i nts un d er cons id erat i on di v id e db yt h e di stance b etween t h etwopo i nts). D iff us i on a l one, th ere f ore, as a means o f o b ta i n i n g ox yg en or excret i n g car b on di ox id e can b e emp l o y e d onl y b y small or g anisms whose surface area/volume ratio is hi g h (i.e., where all cells ar e relativel y close to the surface of the bod y ) and or g anisms whose metabolic rate is low . Organ i sms t h at are l arger an d /or h ave a hi g h meta b o li c rate must i ncrease t h e rate at w hi c h gases move b etween t h eenv i ronment an d t h e b o d yt i ssues b y i mprov i ng (1) an d /or (2 ) a b ove.Inot h er wor d s, spec i a li ze d resp i rator y structures w i t hl ar g e sur f ace areas an d /or t ransport s y stems that brin g lar g e quantities of the g as closer to the site of use or disposa l (thereb y improvin g the diffusion g radient) have been developed. For most terrestrial animals prevent i on o fd es i ccat i on i s anot h er i mportant pro bl em, an d t hi s h as h a d ama j or i n fl uenc e on t h e d eve l opment o f t h e i r resp i ratory sur f aces t h roug h w hi c h cons id era bl e l oss o f wate r m igh t occur. T y p i ca lly , resp i rator y sur f aces o f terrestr i a l an i ma l s are f orme d as i nva gi nate d structures within the bod y so that evaporative water loss is g reatl y reduced. I n insects the tracheal s y stem, a series of g as-filled tubes derived from the inte g ument , h as evo l ve d to cope w i t h gas exc h ange. Term i na ll yt h etu b es are muc hb ranc h e d , f orm i n g t rac h eo l es t h at prov id e an enormous sur f ace area over w hi c h diff us i on can occur. Furt h er- more, trac h eo l es are so numerous t h at g aseous ox yg en rea dily reac h es most parts o f t he b od y , and, equall y , carbon dioxide easil y diffuses out of the tissues. Thus, in most insects, in contrast to man y other animals, the circulator y s y stem is unimportant in g as transport . B ecause they are in the gaseous state within the tracheal system, oxygen and carbon dioxid e diff use rap idl y b etween t h et i ssues an d s i te o f upta k eorre l ease, respect i ve l y, on t h e b o d y sur f ace. Oxygen, f or examp l e, diff uses 3 m illi on t i mes f aster i na i rt h an i n water (M ill , 1972). A g ain, because the s y stem is g as-filled, much lar g er quantities of ox yg en can reac h t he tissues in a g iven time. (Air has about 25 times more ox yg en per unit volume than water.) The eminent suitability of the tracheal system for gas exchange is illustrated by the fac t th at, f or most sma ll i nsects an d many l arge i nsects at rest, s i mp l e diff us i on o f gases i n/ou t 4 6 9 4 7 0 CHAPTER 15 o f the tracheal s y stem entirel y satisfies their requirements (but see Section 3.3). In lar g e, active insects the g radient over which diffusion occurs is increased b y means of ventilation; that is, air is activel y pumped throu g h the tracheal s y stem. 2 . Organization and Structure of the Tracheal Syste m A tracheal s y stem is present in all Insecta and in other hexapods with the exception of t h e Protura an d many Co ll em b o l a. It ar i ses d ur i ng em b ryogenes i sasaser i es o f segmenta l i nvag i nat i ons o f t h e i ntegument. Up to 12 (3 t h orac i can d 9a bd om i na l )pa i rs o f sp i rac l es m a y be seen in embr y os, thou g h this number is alwa y s reduced prior to hatchin g , and furthe r reduction ma y occur in endopter yg otes durin g metamorphosis. Various terms are used to describe the number of p airs of functional s p iracles, for exam p le, holo p neustic (10 p airs , l ocate d on t h e mesot h orax an d metat h orax an d 8a bd om i na l segments), amp hi pneust i c ( 2pa i rs, on t h e mesot h orax an d at t h et i po f t h ea bd omen), an d apneust i c (no f unct i ona l sp i rac l es). T h e l ast con di t i on i s common i n aquat i c l arvae, w hi c h are sa id ,t h ere f ore, to h ave a closed tracheal s y stem (Section 4.1) . T he proportion of the bod y filled b y the tracheal s y stem varies widel y , both amon g spec i es an d w i t hi nt h e same i n di v id ua l t h roug h out a sta di um. In act i ve i nsects w h ose trac h ea l system i nc l u d es a i r sacs (see b e l ow) t h e trac h ea l system occup i es a greater f ract i on o f t h e b o dy t h an i n l ess act i ve spec i es. Furt h er, i nt h e f ormer, t h evo l ume o f t h e trac h ea l s y stem m a y decrease dramaticall y durin g a stadium (e. g ., i n L ocusta from 48% to 3% ) as th e air sacs become occluded b y the increased hemol y mph pressure that results from tissu e growt h .A f ter ec d ys i s, w h en b o d yvo l ume h as i ncrease d (C h apter 11, Sect i on 3.2), t he trac h ea l system expan d s b ecause o f t h e l owere dh emo l ymp h pressure. 2 . 1 .Tr ac h eae a n d Tr ac h eo l es In apterygotes otherthanlepismatid Zygentoma, thetracheae that run fromeach spiracle d o not anastomose e i t h er w i t h t h ose f rom a dj acent segments or w i t h t h ose d er i ve df ro m t h esp i rac l eont h e oppos i te s id e. In t h e Lep i smat id ae an d Pterygota b ot hl ong i tu di na l an d transverse anastomoses occur, and, thou g h minor variations can be seen, the resultant patter n o f the tracheal s y stem is often characteristic for a particular order or famil y . Generall y , a pai r o f large-diameter, longitudinal tracheae (the lateral trunks) run along the length of an insect j ust i nterna l to t h esp i rac l es. Ot h er l ong i tu di na l trun k s are assoc i ate d w i t h t h e h eart, gut, an d v entra l nerve cor d . Interconnect i ng t h e l ong i tu di na l trac h eae are transverse comm i ssures, usuall y one dorsal and another ventral, in each se g ment (Fi g ure 1 5 .1). Parts of the tracheal s y stem, for example, that of the pterothorax, ma y be effectivel y isolated from the rest of the s y stem b y reduction of the diameter or occlusion of certain lon g itudinal trunks. This arrangement i s assoc i ate d w i t h t h e use o f autovent il at i on as a means o fi mprov i ng t h e supp l y of oxygen to w i ng musc l es d ur i ng fli g h t (Sect i on 3.3). A l so, trac h eae are o f ten dil ate d t o f orm l ar g et hi n-wa ll e d a i r sacs t h at h ave an i mportant ro l e i n vent il at i on (Sect i on 3.3) an d o ther functions . N umerous smaller tracheae branch off the main tracts and under g o pro g ressive subdi - v ision until at a diameter of about 2– 5 µ m they form a number of fine branches each 1 µµ µ m µ µ o r l ess across k nown as trac h eo l es. Trac h eo l es are i ntrace ll u l ar, b e i ng enc l ose d w i t hi n a v er y thin la y er of c y toplasm from the tracheoblast (tracheal end cell) (Fi g ure 1 5 .2), and ramif y throu g hout most tissues of the bod y . The y are especiall y abundant in metabolicall y active tissues. Thus, in fli g ht muscles, fat bod y , and testes, for example, tracheoles indent 4 7 1 GAS EX C H A N G E F I GU RE 15.1. ( A) Dorsal tracheal system of abdomen of locust; and (B) diagrammatic transverse sectio n t hrough abdomen of a hypothetical insect to illustrate main tracheal branches. [A, from F. O. Albrecht, 1953, T h e Anatomy o f t h e Migratory Locust . By perm i ss i on o f At hl one Press. B, f rom R. E. Sno dg rass, P rincip l es o f Insec t Morpholo gy. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.] i n di v id ua l ce ll s, so t h at gaseous oxygen i s b roug h t i nto extreme l yc l ose prox i m i ty w i t h t he ener gy -pro d uc i n g m i toc h on d r i a. I n Cal p odes ethlius cater p illars not all tracheae end in tracheoles within or on tissues. I n particular, some tracheae ori g inatin g from the spiracles of the ei g hth abdominal se g men t 4 7 2 CHAPTER 15 F I GU RE 15.2. Structure of (A) large; and (B) small tracheae. (C) Origin of tracheole. [After V. B. Wigglesworth, 1 965, T h e Princip l es of Insect P h ysio l og y ,6 th ed., Methuen and Co. By permission of the author.] f orm large, greatly branched tufts that are suspended within the hemolymph (Figure 15.3). T h e trac h eo l ar t i ps o f t h ese tu f ts are connecte d to h eart an d ot h er musc l es so t h at t h etu f ts are c onstant l y move d w i t hi nt h e h emo l ymp h (Loc k e, 1998). T h eo b servat i on t h at h emocytes w ere abundant within the tufts led Locke to speculate that the tufts are sites of g as exchan ge f or the blood cells, analo g ous to the lun g s of vertebrates. Thou g h similar tracheal tufts are found in cater p illars from other le p ido p teran families, their existence and function(s ) among ot h er i nsect groups h as not b een exam i ne d . As d er i vat i ves o f t h e i ntegument, trac h eae compr i se cut i cu l ar components, ep id erm i s, and basal lamina (Fi g ure 1 5 .2). Ad j acent to the spiracle, the tracheal cuticle includes, the c uticulin envelope, epicuticle and procuticle; in smaller tracheae and most tracheoles onl y the cuticulin envelope and epicuticle are present. Providin g the s y stem with stren g th y e t fl ex ibili ty, trac h ea l cut i c l e h as i nterna l r id ges t h at may b ee i t h er separate (annu li )or f or m a cont i nuous h e li ca lf o ld (taen idi um). In l arge trac h eae t h er id ges i nc l u d e some procut i c l e , b u t t hi s i sa b sent f rom t h ose o f trac h eo l es. Taen idi a are a b sent f rom, or poor ly d eve l ope d i n, air sacs. The epicuticle of tracheae comprises the same la y ers as that of the inte g ument. 4 7 3 GAS EX C H A N G E F I GU RE 15.3 . Tu f ts o f trac h eae i nt h ee igh t h a bd om i na l se g ment o f Ca l po d es et hl ius th at p er h a p s serve to aerat e h emocytes. Arrows indicate direction of hemolymph flow. [From M. Locke, 1998, Caterpillars have evolved lungs f or h emocyte gas exc h ange , J. Insect P h ysio l. 44 :1–20. W i t h perm i ss i on f rom E l sev i er.] I n the smallest tracheoles, however, onl y the cuticulin envelope is present and, furthermore , t his contains fine pores. These two features ma y be associated with movement of liquid into and out of tracheoles in connection with gas exchange (Section 3.1) (Locke, 1966) . 2 .2. Spiracles Onl y in some apter yg otes do tracheae ori g inate at the bod y surface. Normall y , the y arise sli g htl y below the bod y surface from which the y are separated b y a small cavit y , t he atrium (Fi g ure 15.4A). In this arran g ment, the term “spiracle” g enerall y includes bot h th e atr i um an d t h es pi rac l e s en s u s tr i ct o , t h at i s, t h e trac h ea lp ore. Exce p t f or t h ose o fa f ew i nsects t h at li ve i n h um id m i croc li mates, sp i rac l es may b e covere d , f or examp l e, by th ee ly tra or w i n g s i n Hem i ptera an d Co l eoptera, or are equ i ppe d w i t h var i ous va l ves f o r prevention of water loss. The valves ma y take the form of one or more cuticular plates that can be pulled over a spiracle b y means of a closer muscle (Fi g ure l5.4B–D). Openin g of th e va l ve(s) i se ff ecte d e i t h er b yt h e natura l e l ast i c i ty o f t h e surroun di ng cut i c l eor b y an opene r musc l e. A l ternat i ve l y, t h eva l ve may b e a cut i cu l ar l ever w hi c hb y musc l e act i on constr i cts t he trachea ad j acent to the atrium (Fi g ure l 5 .4E,F). In lieu of, or in addition to, the valves , t here ma y be hairs linin g the atrium or a sieve plate (a cuticular pad penetrated b y man y fi ne pores) coverin g the atrial pore. It is commonl y assumed that an important function o f th ese h a i rs an d s i eve p l ates i s to prevent d ust entry. However, as M ill er (1974) note d ,s i eve p l ates are not b etter d eve l ope d on i nsp i ratory t h an on exp i ratory sp i rac l es an d severa l ot h e r f unct i ons can b e suggeste d : (1) t h ey may prevent water l ogg i ng o f t h e trac h ea l system i n t errestrial species durin g rain, in aquatic insects, and in species that live in moist soil, rottin g 4 7 4 CHAPTER 15 F IGURE 15.4. S p i racu l ar structure. (A) Sect i on t h roug h sp i rac l etos h ow genera l arrangement; (B, C) oute r an di nner v i ews o f secon d t h orac i csp i rac l eo fg rass h opper; (D) di a g rammat i c sect i on t h rou gh sp i rac l etos h ow m echanism of closure. The valve is opened by movement of the mesepimeron, closed by contraction of the m usc l e; (E) c l os i ng mec h an i sm on fl ea trac h ea; an d (F) sect i on t h roug hfl ea trac h ea at l eve l o f c l os i ng mec h an i sm. [A–C, f rom R. E. Sno dg rass , Princip l es of Insect Morp h o l ogy . Cop y ri g ht 193 5 b y McGraw-Hill, Inc. Used wit h permission of McGraw-Hill Book Company. D, after P. L. Miller, 1960, Respiration in the desert locust. II. The c ontro l o f t h esp i rac l es , J. Exp. Bio l . 3 7:237–2 6 3. By permission of Cambridge University Press. E. F, after V. B. Wi gg lesworth, 196 5 , T h e Princip l es of Insect P h ysio l og y ,6 th ed., Methuen and Co. B y permission of the author. ] ve getat i on, etc.; (2) t h ey may prevent entry o f paras i tes, espec i a ll ym i tes, i nto t h e trac h ea l s y stem; and (3) the y ma y reduce bulk flow of g ases throu g h the s y stem caused b y bod y m ovements, thereb y reducin g evaporative water loss. This would be disadvanta g eous in i nsects that ventilate the tracheal system, and it is of interest, therefore, that those spiracle s i mportant i n vent il at i on common l y l ac k as i eve p l ate or h aveap l ate t h at i s di v id e dd own t h em iddl esot h at i t may b e opene dd ur i ng vent il at i on . 3. Movement of Gases within the Tracheal S ystem Gas exc h an g e b etween t i ssues an d t h e trac h ea l s y stem occurs a l most exc l us i ve ly across the walls of tracheoles, for it is onl y their walls that are sufficientl y thin as to permit a satis- f actor y rate of diffusion. It is necessar y , therefore, to ensure that a sufficient concentration of 4 7 5 GAS EX C H A N G E ox yg en is maintained in tracheoles to suppl y tissue requirements and, at the same time, tha t carbon dioxide produced in metabolism is removed quickl y , preventin g its buildup to toxic levels. In small insects and inactive sta g es of lar g er insects, diffusion of g ases between th e sp i rac l ean d trac h eo l es i ssu ffi c i ent l y rap id t h at t h ese requ i rements are met. However, if t h e sp i rac l es are k ept permanent l y open, t h e amount o f water l ost v i at h e trac h ea l system ma y b ecome i mportant. T h us, man yi nsects ut ili ze di scont i nuous g as exc h an g e as a means o f reducin g this loss. The needs of lar g e, active insects can be satisfied onl y b y shortenin g the d istance over which diffusion must occur. This is achieved b y active ventilation movements. 3.1. D iff us i o n The absence of obvious breathin g movements led man y 19th centur y scientists to assume that insects obtained ox yg en b y simple diffusion. It was not, however, until 1920 t hat Krogh (cited in Miller, 1974) calculated, on the basis of (1) measurements of the a v erage trac h ea ll engt h an ddi ameter, (2) measurements o f oxygen consumpt i on, an d (3) th e permea bili ty constant f or oxygen, t h at f or Tene b ri o an d C ossus (goat mot h ) l arvae a t rest with the spiracles open, the ox yg en concentration difference between the spiracles and t issues is onl y about 2% and easil y maintainable b y diffusion . Even in large active insects that ventilate (Section 3.3), diffusion is a significant process, b ecause t h e vent il at i on movements serve on l ytomovet h ea i r i nt h e l arger trac h eae. Fo r examp l e, i nt h e d ragon fl y A e sh na , oxygen reac h es t h e fli g h t musc l es b y diff us i on b etween th epr i mar y (vent il ate d )a i rtu b es an d trac h eo l es, a di stance o f up to 1 mm. Even i n fligh t , w hen the ox yg en consumption of the muscle reaches 1.8 m1/ g /min and the difference in ox yg en concentration between the primar y tube and tracheoles is 5–13%, diffusion is quit e adequate (Weis-Fogh, 1964). D iff us i on i sa l so i mportant i nmov i ng gases b etween t h e trac h eo l es an d m i toc h on d r ia o f t h et i ssue ce ll s. Because diff us i on o fdi sso l ve dg ases i sre l at i ve ly s l ow , t h e di stance ove r w hich it can function satisfactoril y (in structural terms, half the distance between ad j acen t t racheoles) is directl y related to the metabolic activit y of the tissue. In hi g hl y active fli g h t muscles of Diptera and Hymenoptera, for example, it has been calculated that the maximum t heoretical distance bet w een tracheoles is 6 – 8 µ m. In practice, tracheoles, which indent the µ µ musc l ece ll s , are w i t hi n 2–3 µ mof µ µ e ac h ot h er, a ll ow i ngas i gn ifi cant “sa f ety marg i n” (We i s- F o g h, 19 6 4) . I n man y insects, distal parts of tracheoles are not filled with air but liquid under norma l resting conditions. During activity, however, the tracheoles become completely air-filled; th at i s, fl u id i sw i t hd rawn f rom t h em on l y to return w h en act i v i ty ceases. W i gg l eswort h (19 5 3) suggested that the level of fluid in tracheoles depends on the relative strengths of the capillar y force drawin g fluid alon g the tube and the osmotic pressure of the hemol y mph. Durin g metabolic activit y , the osmotic pressure increases as or g anic respirator y substrate s are de g raded to smaller metabolites, causin g fluid to be withdrawn from the tracheole s (per h aps v i at h e pores ment i one d ear li er) an d ,t h ere f ore, b r i ng i ng gaseous oxygen c l oser t o t he tissue cells (Figure 1 5 . 5 ). As the metabolites are fully oxidized and removed, the osmoti c pressure w ill f a ll ,an d once a g a i nt h e cap ill ar yf orce w ill d raw fl u id a l on g t h e trac h eo l es . Thou g h carbon dioxide is more soluble, and has a g reater permeabilit y constant, tha n ox yg en in water and could conceivabl y move b y diffusion throu g h the hemol y mph to leav e th e b o d yv i at h e i ntegument, t hi s route d oes not norma ll ye li m i nate a s i gn ifi cant quant i ty o f t h e gas (e.g., 2–10% o f t h e tota li n some di pteran l arvae w i t h t hi n cut i c l es). T h e grea t ma j or i t y o f car b on di ox id e l eaves by g aseous diff us i on v i at h e trac h ea l s y stem. 47 6 CHAPTER 15 F I GU RE 15.5 . Ch an g es i n l eve l o f trac h eo l ar fl u id as a resu l to f muscu l ar act i v i t y . (A) Rest i n g musc l e; an d (B) active muscle. [After V. B. Wigglesworth, 1965 , T he Principles o f Insect Physiology , 6th ed., Methuen and Co . B y perm i ss i on o f t h e aut h or. ] 3 .2. D i scont i nuous G as Exchange O ri g inall y discovered in diapausin g pupae o f H yalophora cecropia and other le p - i dopterans, the discontinuous g as exchan g ec y cle (DGC) [formerl y known as passiv e ( suct i on) vent il at i on] i snow k nown to occur i na ll ants, many b ees, some wasps, sev - e ral different families of beetles, cockroaches, grasshoppers and locusts (Lighton 1996, 1 998). T h e DGC, w hi c h ma k es use o f t h e hi g h so l u bili ty o f car b on di ox id e i n water, com- prises three phases (Fi g ure 1 5 .6): constricted- or closed-spiracle phase, flutterin g -spiracl e p hase, and o p en-s p iracle p hase. In the constricted-s p iracle p hase the s p iracular valves are k ept almost closed. As oxygen is used in metabolism, the carbon dioxide so produced is store d , l arge l yas bi car b onate i nt h e h emo l ymp h an d t i ssues b ut part i a ll ya l so i nt h e gaseou s state i nt h e trac h ea l system. T h us,as li g h t vacuum i s create d w i t hi nt h e trac h ea l system t h at sucks in more air. Eventuall y , the tracheal ox yg en concentration falls to about 3. 5 % and the c arbon dioxide concentration rises to about 4%. At this point, the low ox yg en concentratio n i nduces “fluttering” (rapid opening and closing of the valves), the effect of which is to allo w some outwar d diff us i on o f n i trogen an d more a i rto fl ow i nto t h e trac h ea l system. However , c arbon dioxide cannot escape and its concentration increases to about 6. 5 %, at which point the valves are opened and remain open between 1 5 and 30 minutes. Durin g this period there is rapid diffusion of carbon dioxide out of the tracheal s y stem and massive release o f carbon dioxide from the hemol y mph. When the concentration of carbon dioxide in the trac h ea l system f a ll s to 3.0%, t h eva l ves rec l ose. Exper i ments i nw hi c h gases o f diff eren t c ompos i t i on are per f use d t h roug h t h e trac h ea l system or over t h e segmenta l gang li a h av e s h own t h at t h ere i s d ua l contro l over t h e open i n g o f t h eva l ves. H y percapn i a(a b ove norma l c arbon dioxide concentration) directl y stimulates relaxation of the valve closer muscle (the v a lve opens as a result of cuticular elasticit y ), whereas h y poxia (insufficient ox yg en) acts a t t h e l eve l o f t h e centra l nervous system (pro b a bl yt h e metat h orac i c gang li on). In di apaus i n g Hy a l op h ora pupae t h e per i o d s b etween b ursts o f car b on di ox id ere l ease may b eas l ong as 7h ours. F or d iapausin g pupae and certain other postembr y onic sta g es of insects, the sli g ht net i nflow of air durin g the constricted-spiracle phase of the DGC ma y serve as a means of c onserv i ng mo i sture t h at wou ld ot h erw i se b e l ost as a resu l to f gas exc h ange. However , t h ere are many i nsects t h at s h ow DGC b ut d o not norma ll y exper i ence water- l oss pro bl ems. Converse l y, t h ere are many d esert i nsects i nw hi c h DGC d oes not occur. T hi s h as l e d to t h e proposal that DGC evolved primaril y to facilitate g as exchan g einh y poxic and h y percapni c 4 7 7 GAS EX C H A N G E F I GU RE 15.6. D iscontinuous release of carbon dioxide in p u p ao f H y alophora cecropia in relation to s p iracular valve opening and closing. [After R. I. Levy and H. A. Schneiderman, 1966, Discontinuous respiration in insects. I V , J . I nsect P h ysio l . 12 : 46 5 –492. B y permission of Per g amon Press Ltd. ] environments (Lighton 199 6 , 1998). Certainly, DGC is common in some subterranea n g roups, nota bly ants an db eet l es t h at b urrow i nso il , d un g or woo d . However, t h ere are exceptions to this g eneralization, perhaps the ma j or one bein g termites (Shelton and Appel, 2000, 2001). Further, some species that normall y use DGC ma y abandon it under h y poxi c con di t i ons (C h own an d Ho l ter, 2000; C h own, 2002) . 3.3. Act i ve Vent i lat i o n B y alternatel y decreasin g and increasin g the volume of the tracheal s y stem throu gh compression and expansion of lar g er air tubes, a fraction of the air in the s y stem is pe- riodically renewed and the diffusion gradient between tracheae and tissues kept near the max i mum. T h ese b reat hi ng (vent il at i on) movements may b e cont i nuous as i n l ocusts an d d ragon fli es, i nterm i ttent as i n coc k roac h es, or occur on l ya f ter act i v i ty as i s seen i n wasps. T he volume chan g es are normall y brou g ht about b y contraction of abdominal dorsoventra l and/or lon g itudinal muscles, which increases the hemol y mph pressure, thus causin g th e 4 7 8 CHAPTER 15 tracheae to flatten or collapse. In some species both inspiration and expiration are brou g h t about b y muscles; in others, onl y expiration is under muscular control, and inspiration oc - c urs as a result of the natural elasticit y of the bod y wall. Durin g hi g h metabolic activit y, supp l ementary vent il at i on movements may occur. For examp l e, t h e d esert l ocust norma ll y v ent il ates b y means o fd orsoventra l movements o f t h ea bd omen b ut can supp l ement t h es e by “te l escop i n g ”t h ea bd omen an d protract i on/retract i on o f t h e h ea d an d prot h orax (M ill er, 1 9 6 0). Remarkabl y , it has recentl y been reported that man y insects showin g no obvious si g ns o f breathin g have rapid c y cles of tracheal compression in the head and thorax (Westneat et a l. , 2003). In ana l ogy w i t h t h es i tuat i on i nt h ea bd omen, trac h ea l compress i on i nt h ese reg i ons i s i n d uce di n di rect l y, b y contract i on o f man dibl ean dl eg musc l es. W h en t h ese mus- cl es re l ax, t h ee l ast i c i ty i nt h e taen idi a l r i ngs returns t h e trac h eae to t h e i ror i g i na l s h ape an d v olume. This discover y ma y require reconsideration of the proposal that diffusion alon e satisfies the g as-exchan g e requirements of small insects . T he diffusion gradient can be further improved by increasing the volume of air in t h e trac h ea l system t h at i s renewe dd ur i ng eac h stro k e(t h et id a l vo l ume). T hi s h as b ee n ac hi eve d t h roug h t h e d eve l opment o fl arge, compress ibl ea i r sacs. However, s i mp l et id a l flow (pumpin g of air in to and out of all spiracles) is still somewhat inefficient because a c onsiderable volume of air (the dead air space) remains within the s y stem at each stroke. The size of the dead air space is greatly reduced by using unidirectional ventilation in which a i r i sma d eto fl ow i n one di rect i on (usua ll y anteroposter i or l y) t h roug h t h e trac h ea l system. Un idi rect i ona l a i r fl ow i sac hi eve db y sync h ron i z i ng t h e open i ng an d c l os i ng o f sp i racu l a r v alves with ventilation movements. In the restin g desert locust, for example, the first, second, and fourth spiracles are inspirator y , while the tenth (most posterior) is expirator y . When the insect becomes more active, the first four spiracles become inspirator y , the remainder e xp i ratory. T h esp i racu l ar va l ves d o not f orm an a i rt i g h t sea l , h owever, so t h at a proport i o n of t h e i nsp i re d a i r cont i nues to move t id a ll y rat h er t h an un idi rect i ona ll y (20% i nt h e rest i n g d esert l ocust ). D urin g fli g ht, the ox yg en consumption of an insect increases enormousl y (up to 24 times in the desert locust), almost entirel y because of the metabolic activit y of the fli g ht m usc l es. To f ac ili tate t hi s act i v i ty, a mass i ve exc h ange o f a i r occurs i nt h e pterot h orax, ma d e poss ibl e b y certa i n structura lf eatures o f t h e pterot h orac i c trac h ea l system an db yc h anges i nt h e b o d y’s norma l (rest i ng) vent il at i on pattern. As note d ear li er, t h e trac h ea l system o f the pterothorax is effectivel y isolated from that of the rest of the bod y b y reduction in the diameter or occlusion of the main lon g itudinal tracheae. Autoventilation of fli g ht muscl e tracheae also occurs. This is ventilation that results from movements of the nota and p leura d ur i ng w i ng b eat i ng, an di t b r i ngs a b out a cons id era bl e fl ow o f a i r i nto an d out o f t he t h orac i c trac h eae. Dur i ng autovent il at i on, norma l un idi rect i ona lfl ow , w h ere suc h occurs, becomes masked b y the massive increase in tidal flow in the pterothorax. To achieve this tidal flow, in the desert locust, spiracles 2 and 3 remain permanentl y open. Spiracles 1 an d 4–10, however, continue to open and close in synchrony with abdominal ventilation so t h at some un idi rect i ona lfl o w occurs. T h e rate o f a bd om i na lv ent il at i on mo v ements a l s o i ncreases d ur i ng fli g h ttoa b out f our t i mes t h e rest i ng va l ue, b ut t h ese movements pro b a bl y serve pr i mar ily to i ncrease t h e rate o ffl ow o fh emo ly mp h aroun d t h e b o dy , b r i n gi n gf res h supplies of metabolites to the fli g ht musculature. However, keepin g the central nervous s y stem well supplied with ox yg en also appears to be important. Autovent il at i on i s use db y many O d onata, Ort h optera, D i ctyoptera, Isoptera , H em i ptera, Lep id optera, an d Co l eoptera, b ut i so fli tt l e i mportance i nD i ptera an d Hy- m enoptera. Its s ig n ifi cance can b e b roa dly corre l ate d w i t hb o dy s i ze, t h et y pe o f musc l es used in fli g ht (Chapter 14, Section 2.1), and the extent of movements of the thorax durin g [...]... The gas store may be subelytral or may occur as a thin film FIGURE 15. 7 Hydrofuge hairs surrounding a spiracle (A) Position when submerged; and (B) position when at water surface [After V B Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co By permission of the author.] 481 GAS EXCHANGE 482 CHAPTER 15 FIGURE 15. 8 Examples of gas stores (A) Hair pile on abdominal sternum of... Plastron-bearing species known to inhabit water whose oxygen content may fluctuate daily, for example, that 483 GAS EXCHANGE 484 CHAPTER 15 of marshes which drops greatly at night, may employ behavioral means to overcome the danger of asphyxiation, such as moving closer to the water surface or even climbing out of the water Hinton (1968) also pointed out that the waters occupied by plastron-bearers... gills,” have evolved, though these often become important only under oxygen-deficient conditions 4.1 Closed Tracheal Systems For small aquatic insects or those with a low metabolic rate, diffusion of gases across the body wall provides an adequate means of obtaining oxygen and excreting carbon dioxide 479 GAS EXCHANGE 480 CHAPTER 15 In larger and/or more active forms, all or part of the body wall has a... (Hinton, 1968) In almost all instances they include a plastron In many dipteran pupae the gill is a long, hollowed-out structure (Figure 15. 8C) that carries a plastron over its surface The plastron is held in place by means of rigid cuticular struts and connects via fine tubes with the gas-filled center of the gill (and hence the spiracle) Because the volume of the plastron remains constant, the nitrogen... the insect is completely independent of atmospheric air Surface-breathing insects must solve two problems First, they must prevent waterlogging of the tracheal system when they are submerged, and second, they must be able to overcome the surface tension force at the air-water interface Both problems are solved by having hydrofuge (water-repellent) structures around the spiracles In many dipteran larvae... aquatic insects in the way that they obtain oxygen Most endoparasites satisfy a proportion of their requirements by cutaneous diffusion In some first-instar larvae of Hymenoptera and Diptera the tracheal system may be liquid-filled, but generally it is gas-filled with closed spiracles and includes a rich network of branches immediately beneath the integument Many endoparasitic forms, especially larval... surrounded by the respiratory funnel formed by ingrowth of the host’s integument [From A D Imms, 1937, Recent Advances in Entomology, 2nd ed By permission of Churchill-Livingstone, Publishers.) respiratory funnel produced by the host in an attempt to encapsulate the parasite (Figure 15. 9B) The funnel is produced by inward growth of the host’s integument or tracheal wall Within it, the parasite attaches... Miller (1966) Whitten (1972) and Mill (1997) provide structural details of the tracheal system 485 GAS EXCHANGE 486 CHAPTER 15 Burnside, C A., and Robinson, J V., 1995, The functional morphology of caudal lamellae in coenagrionid (Odonata: Zygoptera) damselfly larvae, Zool J Linn Soc 114 :155 –171 Bustami, H P., Harrison, J F., and Hustert, R., 2002, Evidence for oxygen and carbon dioxide receptors in insect... Microscopic Anatomy of Invertebrates, V 11a (F W Harrison and Vol M L Locke, eds.), Wiley-Liss, New York Miller, P L., 1960, Respiration in the desert locust I-III., J Exp Biol 37:224–236, 237–263, 264–278 Miller, P L., 1966, The regulation of breathing in insects, Adv Insect Physiol 3:279–344 Miller, P L., 1974, Respiration-aerial gas transport, in: The Physiology of Insecta, 2nd ed., Vol VI (M Rockstein,... (Kollar): Effects of caste, mass, and movement, J Insect Physiol 47:213–224 Weis-Fogh, T., 1964, Diffusion in insect wing muscle, the most active tissue known, J Exp Biol 41:229–246 Westneat, M W., Betz, O., Blob, R W., Fezzaa, K., Cooper, W J., and Lee, W.-K., 2003, Tracheal respiration in insects visualized with synchrotron X-ray imaging, Science 299:558–560 Whitten, M J., 1972, Comparative anatomy of . o f car b on di ox id e i n water, com- prises three phases (Fi g ure 1 5 .6): constricted- or closed-spiracle phase, flutterin g -spiracl e p hase, and o p en-s p iracle p hase. In the constricted-s p iracle p hase. trac h eae compr i se cut i cu l ar components, ep id erm i s, and basal lamina (Fi g ure 1 5 .2). Ad j acent to the spiracle, the tracheal cuticle includes, the c uticulin envelope, epicuticle. trac h ea l s y stem. 47 6 CHAPTER 15 F I GU RE 15. 5 . Ch an g es i n l eve l o f trac h eo l ar fl u id as a resu l to f muscu l ar act i v i t y . (A) Rest i n g musc l e; an d (B) active muscle.