Entomology 3rd edition - C.Gillott - Chapter 17 ppsx

17 The Circulator y S y ste m 1 . Intr oduc t ion T he circulator y s y stem of insects, like that of all arthropods, is of the “open” t y pe; that is, th e fl uid that circulates is not restricted to a network of conductin g vessels as, for example, i n v e rtebrates, but flows freel y amon g the bod y or g ans. An open s y stem results from the devel - o p ment, i nevo l ut i on, o f a h emocoe l rat h er t h an a true coe l om. A conse q uence o f t h eo p en system i st h at i nsects h ave on l y one extrace ll u l ar fl u id , h emo l ymp h , i n contrast to verte- b rates, w hi c hh ave two suc hfl u id s, bl oo d an dly mp h .T h e occurrence o f an open s y stem d oe s not mean that hemol y mph simpl y bathes the or g ans it surrounds because usuall y thin g ran - u lar membranes separate the tissues from the hemol y mph itself. Insects g enerall y posses s pump i ng structures an d var i ous di ap h ragms to ensure t h at h emo l ymp hfl ows t h roug h out t he b o d y, reac hi ng t h e extrem i t i es o f even t h e most d e li cate appen d ages. As t h eon l y extrace l- l u l ar fl u id , i t i s per h aps not surpr i s i n g t h at t h e h emo ly mp h , i n g enera l , serves t h e f unct i ons of both blood and l y mph of vertebrates. Thus, the fluid fraction (plasma) is important in providin g the correct milieu for bod y cells, is the transport s y stem for nutrients, hormones , an d meta b o li c wastes, an d conta i ns e l ements o f t h e i mmune system, w hil et h ece ll u l ar com- ponents ( h emocytes) prov id et h e d e f ense mec h an i sm aga i nst f ore i gn organ i sms t h at ente r th e b o dy an d are i mportant i n woun d repa i ran d t h e meta b o li sm o f spec ifi c compoun d s. 2. S tructur e The primary pump for moving hemolymph around the body is a middorsal vessel that runs more or l ess t h e ent i re l engt h o f t h e b o d y(F i gure 17.1). T h e poster i or port i on o f t h e vesse lh as ost i a(va l ves) an di s somet i mes k nown as t h e h eart, w h ereas t h e cep h a l ot h orac i c portion, which is often a simple tube, ma y be termed the aorta (Fi g ure 17.1A). In som e insects the heart is the onl y part that contracts, but in man y others the entire vessel is contractile. The vessel is held in position by connective tissue strands attached to the dorsal i ntegument, trac h eae, gut, an d ot h er organs an db y a ser i es o f pa i re d , usua ll y f an-s h ape d, a l ary musc l es. Norma ll y, t h e vesse li s a stra i g h ttu b e, t h oug hi n many spec i es t h e aorta may loop verticall y . Anteriorl y the aorta runs ventrall y to pass between the corpora cardiac a and under the brain. Generall y the dorsal vessel is closed posteriorl y ; however, in Diplura, 5 1 5 5 1 6 CHAPTER 17 F I GU RE 17.1 . ( A ) Ventra ldi ssect i on o f t h e fi e ld cr i c k et , Ac h eta assimi l is ,t o sh ow d orsa l vesse l an d assoc i ate d s tructures; and (B) circulatory system of C ampodea au g en s ( Diplura) showing anterior and posterior arteries running off the dorsal vessel. [A, after W. L. Nutting, 1951, A comparative and anatomical study of the heart an d accessor y structures o f t h e ort h optero id i nsects, J . Morp h o l . 89 :5 01– 5 97. B y permission of Wistar Press. B, fro m a figure kindly supplied by Dr. G¨unther Pass.] A rc h aeognat h a, Zygentoma, an d some Ep h emeroptera t h e d orsa l vesse l connects at i ts rear wi t h arter i es t h at run a l ong t h e cerc i an d me di an cau d a lfil ament (Gere b en-Krenn an d Pass, 2 000). In D i p l ura an arter y a l so supp li es eac h antenna (F ig ure 17.1B), an di nD i ct y optera and some Orthoptera there are pairs of se g mental arteries in the abdomen (Hertel and Pass, 2 002). However, except as noted, in pter yg otes circulation to appenda g es is achieved b y m eans o f accessory pu l sat il e organs an d septa (see b e l ow) . In most i nsects t h e d orsa l vesse li swe ll trac h eate d .T h e h eart may not b e i nnervate d or m a y rece i ve pa i re dl atera l nerves f rom t h e b ra i nan d /or se g menta l ventra lg an gli a. Ost i ama y be sim p le, slitlike valves or dee p , funnel-sha p ed structures in the wall of the heart, or internal 517 THE C IR CU L A T O R Y S Y S TE M F I GU RE 17.2 . Incurrent ost i ao f B om b yx s hown during diastole and systole. Ar - r ows i n di cate di rect i on o fh emo l ymp hfl ow . [ After R. F. Cha p man, 1971, The In s ect s : S tructure and Function . By permission o f E l sev i er/Nort h -Ho ll an d , Inc., an d t h e author. ] fl aps (Fi g ure 17.2). Their position and number are equall y varied. The y ma y be lateral, d orsa l , or ventra l an d may b e as numerous as 12 pa i rs ( i n coc k roac h es) or as f ew as 1 pa i r ( i n some d ragon fli es). Ost i a are usua ll y i ncurrent, t h at i s, t h ey open to a ll ow h emo l ymp h t o enter t h e h eart b ut c l ose to prevent b ac kfl ow. In some ort h optero id i nsects, h owever, some ostia are excurrent. Histolo g icall y , the dorsal vessel in its simplest form comprises a sin g le la y er of circular muscle fibers, thou g h more often lon g itudinal and oblique muscle l ayers a l so occur. U l trastructura l exam i nat i on o f t h e h eart musc l ece ll srevea l s, h owever, t h at th ey conta i n, i na ddi t i on to contract il ee l ements, prom i nent Go l g i comp l exes an d ves i c l es , su gg est i n g t h at t h e i nsect h eart i s secretor y an d , lik et h at o f verte b rates, ma yh ave a more si g nificant role in homeostasis than j ust pumpin g hemol y mph (Locke, 1989). Assistin g in directin g the flow of hemol y mph, especiall y in postlarval sta g es, are var - ious diaphragms (septa) (Figure 17.3) that include both connective tissue and muscular e l ements. T h e spaces d e li m i te db yt h e di ap h ragms are k nown as s i nuses. T h e per i car di a l septum ( d orsa ldi ap h ragm) li es i mme di ate l y b eneat h t h e d orsa l vesse l an d sprea d s b etween t he alar y muscles. Laterall y , it is attached at intervals to the ter g a and in most species ha s F I G URE 17.3 . D i a g rammat i c transverse sect i on t h rou gh a bd omen to s h ow arran g ement o f septa. [Fro m R . E. Snod g rass, Principles o f Insect Morphology . C op y ri g ht 193 5 b y McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company. ] 518 CHAPTER 17 o penin g s so that the pericardial sinus is in effect continuous with the perivisceral sinus. V entrall y , a perineural septum (ventral diaphra g m) ma y occur, which cuts off the perineu- ral sinus from the perivisceral sinus. Generall y , the ventral diaphra g m is restricted to the a bd omen an d occurs on l y i n spec i es w h ose ventra l nerve cor d exten d s i nto t hi sreg i on o f the body (Miller, 198 5 ). It is capable of performing posteriorly directed undulations and m a yh ave open i n g s. It ma y rece i ve motor nerves f rom se g menta lg an gli a, w hi c h re g u l at e the rate at which it undulates, thou g h the undulations ori g inate m y o g enicall y . In some in- sects, for example, caddisflies and cockroaches, the ventral diaphra g m is reduced to a fe w transverse or l ong i tu di na l musc l es, respect i ve l y. Frequent l y, t h ere i sac l ose p h ys i ca l asso - ci at i on b etween t h e di ap h ragm (or i ts vest i ges) an d t h e nerve cor d .T h us, i n coc k roac h es an d P seu d a l etia uni p uncta th e act i ons o f t h e l ong i tu di na l musc l e remnants cause t h e nerv e c ord to oscillate laterall y , brin g in g it into g reater mix with the hemol y mph and possibl y i mprovin g hemol y mph flow (Koladich et al. , 2002). Hemol y mph circulation throu g h the l egs and palps of some insects is assisted by the presence of a longitudinal septum tha t part i t i ons t h e appen d age i nto a ff erent an d e ff erent s i nuses. To f urt h er f ac ili tate h emo l ymp hfl ow, espec i a ll yt h roug h appen d ages, accessory pu l- s atile or g ans (auxiliar y hearts) commonl y occur (Pass, 1998, 2000). These have been iden - tified in the head, antennae, thorax, le g s, win g s, and ovipositor. In man y species the y are s aclike structures that have a posterior incurrent ostium and an anteriorly extended vessel. In antenna l pu l sat il e organs t h e vesse l may run t h e l engt h o f t h e appen d age b ut i s per f orate d at i nterva l s to perm i tex i to fh emo l ymp h .T h ewa ll o f t h e sac may b e muscu l ar,sot h at c onstriction of the sac is the active phase, and dilation results from elasticit y of the wall , o r the sac ma y have attached to it a discrete dilator muscle, and constriction is due to th e s ac’s elasticit y . In some situations, for example, the le g s of Orthoptera and Hemiptera, the accessory pu l sat il e organ i ss i mp l y one or two sma ll musc l es t h at attac h to t h e l ong i tu di na l s eptum. In d ee d , i n Hem i ptera, t h e organ i sc l ear l y d er i ve df romas k e l eta l musc l e, t h e pre - tarsal depressor (Fi g ure 14. 5 C) (Hantschk, 1991). Contraction narrows the efferent sinus , w hile enlar g in g the afferent sinus. Valves ensure that hemol y mph is pushed toward the limb tip, then back toward the bod y cavit y . Normall y , accessor y hearts are quite separate from th e d orsa l vesse l ,t h oug hi n some O d onata t h ey are connecte d v i as h ort vesse l sw i t h t h e aorta i nto w hi c h t h ey pump h emo l ymp h . Most accessory pu l sat il e organs are not i nnervate d. Hemopo i et i c organs h ave b een d escr ib e df or a num b er o fi nsects. For examp l e, in G ryllu s there are pairs of such or g ans, in the second and third abdominal se g ments, directl y c onnected with the dorsal vessel. Like those of vertebrates, the hemopoietic or g ans serv e both as the site of production of at least some types of hemocytes and as centers for p h agocytos i s. T h e same ce ll sw i t hi nt h e h emopo i et i c organ can carry out b ot h o f t h es e f unct i ons, t h oug h not s i mu l taneous l y; t h us, d ur i ng per i o d so fi n f ect i on, di v i s i on o f t h ece ll s to form new prohemoc y tes is g reatl y retarded . At specific locations in the circulator y s y stem are sessile cells, usuall y conspicuousl y pigmented, called athrocytes (Locke and Russell, 1998). They occur singly, in small groups, o r f orm di st i nct l o b es, an d are a l ways surroun d e db ya b asa ll am i na, a f eature t h at di st i n- g u i s h es t h em f rom h emocytes. In most spec i es at h rocytes are s i tuate d on t h e sur f ace o f t h e h eart (occas i ona lly a l so a l on g t h e aorta), an d t h ese are re f erre d to as per i car di a l ce ll s . The y ma y also be found as scattered cells in the fat bod y (i n Le pi sma ) , in clusters at th e bases of le g s(i n Gryllus a n d P eriplaneta), or as a g arland of cells around the esopha g us (in some l arva l D i ptera). W h en mature t h ey may conta i n severa l nuc l e i ,aswe ll as m i toc h on - d r i a, Go l g i apparatus, an d p i gment granu l es or crysta l so f var i ous co l ors. T h ece ll s are a bl e to accumu l ate co ll o id a l part i c l es, f or examp l e, certa i n dy es, h emo gl o bi n, an d c hl orop hyll, 519 THE C IR CU L A T O R Y S YSTE M w hich led to an earl y su gg estion that the y se g re g ated and stored waste products (hence thei r alternate name of nephroc y tes). The usual view is that the cells accumulate and de g rad e lar g e molecules such as proteins, peptides, and pi g ments, and the products are then used o r excrete d . However, t h e i r structure i nc l u d es muc h roug h en d op l asm i c ret i cu l um an d we ll - d eve l ope d Go l g i comp l exes, c h aracter i st i cs o f ce ll s pro d uc i ng prote i n f or export. In d ee d , F if e et al. ( 1987) demonstrated that the pericardial cells s y nthesized and secreted several proteins into the hemol y mph. 3 .Phys i ology 3.1. Circulation C ontract i ons o f t h e d orsa l vesse l an d accessory pu l sat il e organs, a l ong w i t h movement s o f ot h er i nterna l or g ans an d a bd om i na l vent il ator y movements (coe l opu l ses), serve to mov e h emol y mph around the bod y (Miller, 1997). In P eri p lanet a l arvae, for exam p le, circulatio n t ime is 3–6 minutes; i n T e n eb r io the time for complete mixin g of in j ected radioisotope i s 8–10 m i nutes. Genera ll y h emo l ymp hi s pumpe d rap idl yt h roug h t h e d orsa l vesse lb ut move s s l ow l yan ddi scont i nuous l yt h roug h s i nuses an d appen d ages . T h e di rect i on o fh emo ly mp hfl ow i n most i nsects i s i n di cate di nF ig ure 17.4A–C . Hemol y mph is pumped anteriorl y throu g h the dorsal vessel from which it exits via eithe r excurrent ostia of the heart or mainl y the anterior openin g of the aorta in the head. The resu l tant pressure i nt h e h ea d reg i on f orces h emo l ymp h poster i or l yt h roug h t h e per i v i scera l F I G URE 17.4. D i a g rams s h ow i n gdi rect i on o fh emo ly mp hfl ow. (A) Lon gi tu di na l sect i on; (B) transverse sect i on t hrou g h thorax; and (C) transverse section throu g h abdomen. Arrows indicate direction of flow. [After V. B . W igglesworth, 1965, T he Principles o f Insect Physiology , 6th ed., Methuen and Co. By permission of the author.] 520 CHAPTER 17 and perineural sinuses. Undulations of the ventral diaphra g m aid the backward flow of hemol y mph. Relaxation of the heart muscle results in an increase in heart volume, and, b y n e g ative pressure, hemol y mph is sucked in via incurrent ostia. As noted earlier, circulatio n t h roug h appen d ages i sa id e db y accessory pu l sat il e organs. In most i nsects h emo l ymp h e nters t h ew i ngs v i at h e anter i or ve i ns an d returns to t h et h orax v i at h e ana l ve i ns. T h oug h t h e structure o f w i n g pu l sat il eor g ans i svar i e d ,t h e y a l wa y s operate by suc ki n gh emo ly mp h o ut of the posterior win g veins (Pass, 1998, 2000). In some Coleoptera and Lepidoptera, tidal flow of hemol y mph occurs in the win g s; that is, hemol y mph flows into or out of all v e i ns s i mu l taneous l y . In apterygotes an d may fli es h emo l ymp hfl ow i s bidi rect i ona l (F i gure 17.1B). Anter i or toava l ve l ocate di nt h e h eart at a b out t h e l eve l o f t h ee i g h t h a bd om i na l segment, h emo l ymp h flows forward toward the head, while behind the valve the hemol y mph is pushed backward alon g arteries that terminate at the tips of the cerci and median filament (Gereben-Krenn and Pass, 2000). Reversal of heartbeat may also occur and is characteristically seen in pupa e an d a d u l ts o f Lep id optera an d D i ptera . In some act i ve l y fl y i ng i nsects, f or examp l e, l ocusts, b utter fli es, saturn iid mot h s, an d possibl y some H y menoptera, as well as in diapausin g lepidopteran pupae, hemol y mph m ovements are closel y coordinated with the ventilation movements for g as exchan ge ( Chapter 15, Sections 3.2 and 3.3). Abdominal pumping not only improves gas exchang e wi t hi nt h e trac h ea l system b ut a l so b r i ngs a b out t id a lfl ow (osc ill at i ng c i rcu l at i on) o f h emo l ymp h .Inot h er wor d s, h emo l ymp hfl ows b ac k an df ort hb etween t h ea bd omen an d the anterior part of the bod y . Tidal flow of hemol y mph into the abdomen is aided b y revers e p eristalsis of the dorsal vessel (Miller, 1997). Control of circulation is especiall y important in lar g efl y in g insects such as bumble bees, d ragon fli es, an d n i g h t- fl y i ng mot h st h at t h ermoregu l ate. T h ermoregu l at i on a ll ows t h es e i nsects to warm up t h e i rw i ng muscu l ature at l ow am bi ent temperatures an d to di ss i pat e h eat pro d uce dd ur i n g fligh tat high temperatures. At l ow am bi ent temperatures, t h e h eart b eat i s weak and contraction of the ventral diaphra g m infrequent, so that heat produced b y pre - fli g ht contraction of win g muscles is retained within the thorax. As the thoracic temperature b ecomes su i ta bl e f or fli g h t, h eart b eat rate an d amp li tu d e i ncrease, as d ot h e f requency an d strengt h o f contract i ons o f t h e ventra ldi ap h ragm, ta ki ng h eat away f rom t h et h orax t o prevent over h eat i ng (M ill er, 1997) . 3 . 2 . Heartbea t Contract i on o f t h e h eart (systo l e) i s f o ll owe d ,as i not h er an i ma l s, b yap h ase o f re l ax - at i on ( di asto l e) d ur i ng w hi c h musc l ece ll mem b ranes b ecome repo l ar i ze d .At hi r d p h ase, diastasis, ma y follow diastole, when the diameter of the dorsal vessel suddenl y enlar g e s because of the influx of hemol y mph. Diastole in man y insects seems to be passive, that is, the result of natural elasticity of the heart muscle. Though alary muscles may be quite well d eve l ope di n suc h spec i es, t h ey apparent l y h avenoro l e i nt h ere l axat i on process. T h ey h av e b een s h own to b ee l ectr i ca ll y i nexc i ta bl e i n l ocusts an d coc k roac h es, an d cutt i ng t h em h a s n oe ff ect on t h e rate an d stren g t h o f t h e h eart b eat. In a f ew spec i es structura li nte g r i t y o f the heart and alar y muscles is vital, and cuttin g the alar y muscles terminates the heartbeat. In most pter yg otes, where hemol y mph flow is unidirectional, contraction of the dorsal v esse lb eg i ns at t h e poster i or en d an d passes f orwar d as a per i sta l t i c wave. Exper i men - ta ll y contract i on can b e i n d uce d at any po i nt a l ong t h e l engt h o f t h e vesse l an di n di v id ua l sem ii so l ate d se g ments (port i ons o f t h e h eart w i t h ter g um st ill attac h e d ) cont i nue to b eat 521 THE C IR CU L A T O R Y S Y S TE M rh y thmicall y . These observations su gg est that the heartbeat is normall y coordinated b ya pacemaker located posteriorl y . In adul t M anduca sexta in which heartbeat reversal occurs , d istinct pacemakers exist for the anteriorl y and posteriorl y directed contractions (Dulcis e ta l ., 2001) . Wh et h er or not an i nsect h eart i s i nnervate d , i ts b eat i s myogen i c, t h at i s, t h e b eat ori g inates in the heart muscle itself (Jones, 1977; Miller, 198 5 , 1997). This contrasts wit h t he situation in Crustacea and Arachnida, which have neuro g enic hearts. For innervated insect hearts, it is g enerall y assumed that, as in vertebrates, control of the rate and amplitude of the heartbeat resides in the cardiac neurons. However, as Miller (198 5 ) p ointed out, suc h regu l at i on h as b een d emonstrate di non l ya f ew cases. T h e rate at w hi c h t h e h eart b eats var i es w id e ly b ot h amon g spec i es an d even w i t hi nan individual under different conditions. In the p u p aof A nagasta k uhniella ¨ , for exam p le, the h eart beats 6–11 times p er minute. In larva l Blattella germanic a r ates of 1 80 – 3 1 0 beats / mi n h ave b een recor d e d (Jones, 1974). Many f actors a ff ect t h e rate o fh eart b eat. Genera ll y, t h ere i sa d ec li ne i n h eart b eat rate i n success i ve j uven il e stages, an di nt h e pupa l stage t h e h eart b eats s l ow ly or even ceases to b eat f or l on g per i o d s. In a d u l ts t h e h eart b eats at a b out t h e rate observed in the final larval sta g e. Heartbeat rate increases with activit y , durin g feedin g , w ith increase in tem p erature or in the p resence of carbon dioxide in low concentration, but i s d epresse di n starve d or asp h yx i ate di nsects. Hormones, too, may a ff ect h eart b eat rate . Aut h ors h ave reporte d aw id e range o f car di oacce l erat i ng an d car di o i n hibi t i ng f actors, i n- cludin gj uvenile hormone, neurosecretor y peptides, octopamine, and 5 -h y drox y tr y ptamine . However, in man y instances, an effect of these substances on the metabolism of the insec t ma y cause the chan g e in heartbeat rate observed. As note d ,t h e ventra ldi ap h ragm an d accessory pu l sat il e organs may or may not be i nnervate d .T h us, i t may b e ant i c i pate d t h at, as w i t h t h e d orsa l vesse l , contract i on o f t h ese structures may b e contro ll e d neura ll yan dh ormona ll yor b y h ormones a l one. P h armaco l og- ical studies have shown that a ran g e of amines and small peptides can modulate contractio n of these structures. However, immunohistochemistr y has identified amine- and peptide- releasing neurons terminating at these structures, tending to cloud the picture with respect t ow hi c h system i sregu l at i ng t h e i r act i v i ty (Herte l an d Pass, 2002; Ko l a di c h e ta l ., 2002) . 4 . Hemolymp h H emo l ymp h , lik et h e bl oo d o f verte b rates, i nc l u d es a ce ll u l ar f ract i on, t h e h emocytes , an d a li qu id component, t h ep l asma, w h ose f unct i ons are b roa dl y compara bl ew i t h t h os e found in vertebrates. Several of the features of hemol y mph, however, contrast markedl y w ith what is seen in vertebrates. First, associated with the evolution of a tracheal s y stem, h emolymph has no gas-transporting function, except perhaps in some chironomid larvae. I n a ddi t i on, t h e compos i t i on o fh emo l ymp h (espec i a ll y i nt h e more a d vance d en d opterygotes) i s b ot h very diff erent f rom t h at o fbl oo d an di s muc h more var i a bl eona d ay-to- d ay b as i s . Amon g the trends seen in the evolution of the hi g her endopter yg otes are substitution o f or g anic molecules for the predominant inor g anic ions (sodium and chloride), an increase i n t he p ro p ortion of divalent to monovalent cations, and an increase in the im p ortance ( q uan- ti tat i ve l y) o f organ i cp h osp h ate. Furt h er, as note di nt h e prev i ous c h apter, monosacc h ar ide sugars are genera ll yo fli tt l e i mportance i nt h e h emo l ymp h an d are rep l ace db yt h e di sac - charide trehalose. The use of this molecule rather than the reducin g su g ar g lucose appear s t o be an adaptation to overcome problems of osmotic pressure and chemical reactivit y that 522 CHAPTER 17 w ould result if the monosaccharide were the ma j or form of fuel in the hemol y mph (Wheeler, 1 989). In man y insects the hemol y mph osmotic pressure is held reasonabl y constant over a ran g e of environmental conditions. In other species, the osmotic pressure chan g es in paral - l e l w i t h t h eenv i ronmenta l con di t i ons, yet t h e b o d yce ll s are a bl etoto l erate t h ese c h ange s ( C h apter 18, Sect i on 4). 4 . 1 . Plasm a 4.1.1. Com p osition Pl asma conta i ns a l arge var i ety o f components b ot h organ i can di norgan i cw h ose re l at i v e proport i ons may diff er great l y b ot h among spec i es an d w i t hi nan i n di v id ua l un d er diff erent p hy s i o l o gi ca l con di t i ons. Desp i te t hi svar i a bili t y , some g enera l statements ma yb ema d e . In p rimitive orders, the p redominant cation is sodium, with p otassium, calcium, an d m a g nesium present in low proportions. The ma j or anion is chloride, thou g h plasma als o c onta i ns sma ll amounts o f p h osp h ate an dbi car b onate. T h ese i norgan i c const i tuents are t he m ajor contributors to the hemolymph osmotic pressure (Figure 17. 5 A,B) . In high er or d ers certa i n tren d s can b eo b serve d .T h ere l at i ve i mportance o f so di u m decreases at the expense of potassium and, especiall y ,ma g nesium. Chloride also decreases i n importance and is replaced b y or g anic anions, especiall y amino and carbox y lic acids. Fi - n ally, the relative contribution that inorganic ions make to the hemolymph osmotic pressur e declines, and organic constituents become the major osmotic effectors (Figure 17. 5 D,E) . Super i mpose d on t h ese p h y l ogenet i cre l at i ons hi ps may b e di etary an d ontogenet ic c onsiderations, especiall y with respect to the cationic components of hemol y mph. Thus , zoopha g ous species g enerall y have a lar g er proportion of sodium in the hemol y mph, in con - trast to phytophagous species where magnesium (derived from chlorophyll) and potassiu m are the major cations (Figure 17. 5 C). The ionic composition may also change with stag e of d eve l opment, i nen d opterygotes at l east, t h oug h w h et h er t hi s i sre l ate d toac h ange o f di et w hi c h ,o f course, ma y a l so occur f rom t h e j uven il etot h ea d u l t sta g e, d oes not seem to have been considered. For example, in the exopter yg otes Aeshna cyanea ( Odonata ), F IGURE 17.5. R elative contributions to osmotic pressure of the components of hemolymph in different insect g roups. Each column represents 5 0% of the total osmolar concentration. [After D. W. Sutcliffe, 1963, The chemical c omposition of hemol y mph in insects and some other arthropods in relation to their ph y lo g en y, Comp. Ph y siol. Bioc h e m . 9 :121–135. By permission of Pergamon Press, Elmsford, NY. ] 523 THE C IR CU L A T O R Y S Y S TE M P eri p laneta american a ( Dict y optera), and Locusta migratoria ( Orthoptera) and the en - d opter yg ot e D yt i scu s (Coleoptera), the composition of the hemol y mph is similar in larva e a nd adults, but so, too, is the diet. In contrast, in a few endopter yg ote species, specificall y L ep id optera an d Hymenoptera, f or w hi c hd ata are ava il a bl e, l arva lh emo l ymp hi so f t h e hi g h magnes i um type, w h ereas a d u l t h emo l ymp hh as a muc h greater so di um content. Aga i n , h owever, t h e f act t h at t h e di et o f t h ea d u l t( if i t f ee d s) i st y p i ca lly diff erent f rom t h at o f t h e larva apparentl y has not received consideration . A s Florkin and Jeuniaux (1974) pointed out, insects whose hemol y mph contains suc h l arge quant i t i es o f magnes i um an d potass i um must h ave b ecome a d apte d so t h at p h ys i o l og i- ca l processes, espec i a ll y neuromuscu l ar f unct i on, can b e carr i e d out norma ll y b ecause t h ese ions are detrimental. Hoyle (19 5 4, cited in Florkin and Jeuniaux, 1974) suggested that th e h i g hma g nesium-potassium t y pe of hemol y mph characteristic of ph y topha g ous endopter y- g ote larvae mi g ht reduce, throu g h an effect on the nervous s y stem, locomotor activit y of larvae, so that they would tend to remain close to their food. The adult, in contrast, is usually muc h more act i ve an d possesses t h e more pr i m i t i ve hi g h so di um type o fh emo l ymp h . Organ i cac id s are i mportant h emo l ymp h const i tuents, espec i a ll y i n j uven il een d optery - g otes. Carbox y lic acids (citric, α -keto g lutaric, malic, fumaric, succinic, and oxaloacetic), which in the hemol y mph are anionic, are present in lar g e amounts and ma y neutraliz e a lmost 50% of the inorganic cations. They are apparently synthesized by insects (or b y sym bi ot i c b acter i a), as t h e i r l eve l s i nt h e h emo l ymp h are i n d epen d ent o fdi et. W h et h er th ese ac id s, w hi c h are components o f t h e Kre b scyc l e, a l so h ave a meta b o li c f unct i on i nt he h emol y mph is not known. Insects, especiall y endopter yg otes, also characteristicall y have h i g h concentrations of amino acids in their hemol y mph. The proportions of amino acids v ar y amon g species and within an individual accordin g to diet and developmental and ph y s - i o l og i ca l state, t h oug h g l utam i ne, g l yc i ne, hi st idi ne, l ys i ne, pro li ne, an d va li ne genera lly eac h const i tute at l east 10% o f t h e tota l am i no ac id poo l .T h eam i no ac id sma k eas i gn ifi cant contr ib ut i on to t h e h emo ly mp h osmot i c pressure, t h ou gh w h et h er t h e yf unct i on as cat i on s or as anions depends both on the pH of the fluid (usuall y between 6 .0 and 7.0) and on the individual amino acid. In addition, some have im p ortant metabolic roles. The hemolymph protein concentration is generally about 1% to 5% but varies wit h spec i es an di n di v id ua l p h ys i o l og i ca l states. For examp l e, i t i s l ow i n starve di nsects an dhi g h i n f ema l es w i t hd eve l op i ng oocytes. In en d opterygotes t h e prote i n concentrat i on o f ten i n- creases throu g h larval life, especiall y in the final instar, but then declines durin g pupation. As noted in Chapter 16 (Section 5.4), the fat bod y is the ma j or source of the man y proteins found in the hemolymph though other tissues, notably midgut epithelium, epidermis, pericardial ce ll s, an dh emocytes, a l so contr ib ute. For re l at i ve l y f ew o f t h ese prote i ns h as t h e f unct i on b een d eterm i ne d . Some, f or examp l e, l ysozyme, p h eno l ox id ase, cecrop i ns, an d attac i ns, a re important in prevention of infection. The female-specific proteins (vitello g enins) are selectivel y accumulated b y ooc y tes durin gy olk formation. Some proteins act as transport a gents for lipids and juvenile hormone. Yet others may serve simply as concentrated store s o f n i trogen t h at can b e d egra d e df or use i n growt h an d meta b o li sm. Among t h e enzymes id ent ifi e di n h emo l ymp h are h y d ro l ases (e.g., amy l ase, esterases, proteases, an d tre h a l ase) , d e hyd ro g enases an d ox id ases i mportant i n car b o hyd rate meta b o li sm, an d t y ros i nase . The principal carboh y drate in the hemol y mph of most insects is the disaccharide tre - h alose, which serves as a source of readil y available ener gy (Chapter 16, Section 5.2) . M onosacc h ar id es an d po l ysacc h ar id es norma ll y occur i non l y sma ll amounts. G l ycero l an d sor bi to l are somet i mes present i n hi g h concentrat i on i nt h e h emo l ymp h o f overw i nter i ng sta g es w h ere t h e y serve as ant if reezes. 524 CHAPTER 17 F ree lipids seldom occur in hi g h concentrations in hemol y mph, except in some specie s after feedin g , durin g fli g ht, or at metamorphosis when the y are bein g transported to sites of use or stora g e. O t h er const i tuents o fh emo l ymp hi nc l u d ep h osp h ate esters, urates, an d traces o f ot h er ni trogenous waste pro d ucts, am i no sugars suc h as acety l g l ucosam i ne pro d uce dd ur i ng di- g est i on o f t h e cut i c l e, p ig ments (o f ten con j u g ate d w i t h prote i n), an dh ormones, w hi c h ma y be trans p orted to their sites of action in combination with p rotein . 4.1.2. F u n c t io n s A p art from the s p ecific functions of p articular com p onents, which were outlined in the above consideration of its composition, the plasma has some important general functions. It serves as t h eme di um i nw hi c h nutr i ents, h ormones, an d waste mater i a l s can b e transporte d to s i tes o f use, act i on, an ddi sposa l , respect i ve l y. It i san i mportant s i te f or t h e storage , usuall y temporar y , of metabolites. Plasma is the source of cell water, and durin g periods o f desiccation its volume ma y decline at the expense of water enterin g the tissues. B y virtu e o f some components (proteins, amino acids, carbox y lic acids, bicarbonate, and phosphate) i t i s a strong b u ff er an d res i sts c h anges i npHt h at m i g h t occur as a resu l to f meta b o li sm. As a li qu id , i t i sa l so use d to transm i t pressure c h anges f rom one part o f t h e b o d y to anot h er. It i s use d hyd rostat i ca lly , f or examp l e, to ma i nta i nt h e tur g or necessar yf or movement i nso f t - bodied insects, split the old exocuticle durin g ecd y sis, expand appenda g es after ecd y sis, ev e rt structures such as the p enis, and extend the labium of larval Odonata. The p lasma a l so h as an i mportant t h ermoregu l atory f unct i on i n many act i ve l y fl y i ng i nsects. Var i ou s structura l an d p h ys i o l og i ca lf eatures o f t h ec i rcu l atory system h ave evo l ve d t h at a ll ow h eat t o b e reta i ne di nt h et h orac i cre gi on (at l ower am bi ent temperatures) or to b e eas ily transporte d to the abdomen and hence dissipated when ambient temperatures are hi g h (Miller, 1985) . 4 .2. Hemocytes 4.2.1. O r i g i n, Number, and For m T he embr y onic ori g in of hemoc y tes is considered in Chapter 20, Section 7. 5 . In postem- br y onic sta g es of man y species, hemoc y tes are produced in discrete hemopoietic or g ans; i n o ther species multiplication of hemocytes takes place in the hemocoel p er s e, either as the c e ll sc i rcu l ate or as t h ey rest on t h e sur f ace o f t i ssues . In many spec i es a ll or near l ya ll o f t h e h emocytes are i nc i rcu l at i on; i n some spec i es ver y f ew hemoc y tes circulate, the g reat ma j orit y remainin g loosel y attached to tissue surfaces. In adult mosquitoes there are no circulatin g hemoc y tes (Jones, 1977). Hemoc y te counts are normall y hi g hl y variable within the same species, as well as differin g amon g species. T h us, i n Perip l aneta americana, counts ranging from 45,000–120,000 hemocytes/ µ l ma y b e measure d .Ast h e i nsect h as a b out 17 0 µ l o f h emo l ymp h , i tw ill h ave b etween 7 an d 2 0m illi on c i rcu l at i n g ce ll s. Accordin g to Crossle y (1975), there is evidence that the number of circulatin g hemo - cy tes ma y depend on the hormone titer of the hemol y mph. Both ecd y sone and j uvenile h ormone are sa id to st i mu l ate an i ncrease i nt h e num b er o f c i rcu l at i ng h emocytes, es - pec i a ll yp l asmatocytes. In some i nstances, t h e i ncrease seems to b ere l ate d to i ncrease d m o bili t y o f preex i st i n g ce ll s rat h er t h an an i ncrease d rate o f ce ll mu l t i p li cat i on, t h ou gh h o w hormones brin g about this effect is not known . [...]... hemolin, gram-negative bacteria recognition protein, and lectins (= hemagglutinins, agglutinins) In common, these are large, highly specific proteins with multivalent capacity to bind with membrane-surface carbohydrates, notably lipopolysaccharides, peptidoglycans and -1 , 3-glucans Even less is known about hemocyte-surface PRR, though there are indications that these may, in fact, be membrane-bound forms... (microparticles), and enzyme-controlled cross-linking of the plasma and hemocyte coagulogens A possible scheme for hemolymph coagulation is given in Figure 17. 7 Coagulation is initiated when coagulocytes rupture, a step that requires calcium ions This releases 529 THE CIRCULATORY SYSTEM FIGURE 17. 7 Proposed scheme for hemolymph clotting in insects hemocyte coagulogen, the cross-linking enzyme, and microparticles... plasmatocytes More often, they are non-motile and appear to be involved in intermediary metabolism Other types of hemocytes include adipohemocytes, oenocytoids, spherule cells, and cystocytes As their name indicates, the adipohemocytes are cells whose cytoplasm normally contains droplets of lipid In addition to lipid droplets, the cytoplasm may have non-lipid 526 CHAPTER 17 vacuoles and granules that contain... The clot is then stabilized and strengthened as the plasmacoagulogen becomes cross-linked to the gel This step is enzyme-controlled and also requires calcium It is possible that the cross-linking enzyme is part of the phenoloxidase enzyme complex, promoting tanning of the proteins, as occurs in the formation of exocuticle (Chapter 11, Section 3.3) This speculation is based on the observation by Li et... substance’s role as a transmitter at the neuromuscular junction (Chapter 13, Section 2.3) Certain hemocytes (spherule cells in Diptera, oenocytoids in other insects) contain enzymes for metabolism of tyrosine, derivatives of which are important in tanning and/or darkening of cuticle (Chapter 11, Section 3.3) and have a bacteriostatic effect 530 CHAPTER 17 Various authors also have suggested that the hemocytes... is different, however, from the antigen-antibody system of vertebrates in two major respects First, the immunity is short-lived, lasting only a matter of days, indicating that the cells that produce the immunogen (the substance that reacts against the pathogen) have no “memory,” as do the antibody-producing lymphoid cells in vertebrates Second, the response is non-specific, which is in marked contrast... that collectively provide the host with broad-spectrum coverage against bacteria Further, the peptides act synergistically by targeting different parts of the bacterial cell envelope Thus, attacins and diptericins attack gram-negative bacteria, inhibiting the synthesis of the cell membrane during division Defensins kill gram-positive bacteria by inducing ion-channel formation in the membrane, hence lysis... work has shown that a number of similarities do exist For example, in insects as in mammals there are both plasma- and cellular clotting proteins (coagulogens), the latter being released as the cells rupture Interestingly, in many insects the plasma coagulogens are lipophorins, high-molecular-weight lipoproteins synthesized in the fat body that transport lipids and hormones The hemocyte coagulogen is... “plasmatocyte-spreading peptide” that induces plasmatocytes to aggregate and adhere to a foreign surface Thus, in the second phase, which takes several hours to complete, plasmatocytes surround the melanized core, become flattened, and form a multicellular sheath comparable to that seen after encapsulation (Lavine and Strand, 2002) Plasmatocyte-spreading peptide may not be the only hemocyte-aggregating...525 THE CIRCULATORY SYSTEM FIGURE 17. 6 Different types of hemocytes [After R F Chapman, 1971, The Insects: Structure and Function By permission of Elsevier/North-Holland, Inc., and the author.] Though several types of hemocytes have been recognized, which differ in size, stainability, function, and cytology (including fine structure) (Figure 17. 6), their classification and relationships have . are i mportant h emo l ymp h const i tuents, espec i a ll y i n j uven il een d optery - g otes. Carbox y lic acids (citric, α -keto g lutaric, malic, fumaric, succinic, and oxaloacetic), which in the hemol y mph. humoral receptors also reco g nize and bind to abiotic ob j ects, specificall y ,n y lon, latex, and chromato g raph y beads . A biotic tar g ets are reco g nized b y their ph y sicochemical characteristics,. carbox y lic acids. Fi - n ally, the relative contribution that inorganic ions make to the hemolymph osmotic pressur e declines, and organic constituents become the major osmotic effectors (Figure 17. 5 D,E) . Super i mpose d on