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Insect Respiration
Philip G. D. Matthews
Cetonischemaaeruginosa
InsectRespira-on
• Outcomes:
–Beabletoiden-fythedifferentpartsofthetrachealsystem–Understandhowinsectsexchangeoxygenandcarbondioxidebetweentheir-ssuesandtheirenvironment
Hi.Welcometothismini‐lectureoninsectrespira-on.Thistalkwillcoverthestructureandfunc-onoftheinsect’srespiratorysystem.Itwillcoverboththeanatomyoftheinsect’srespiratorysystemandtheprocessesinsectsusetoobtainsufficientamountsofoxygenfromtheatmospherewhileelimina-ngcarbondioxide
Respiratorysystem:Vertebrate
1.Ven'la'on 2.Perfusion/circula'on
3.Diffusion
Lung Circulatorysystem
Capillaries Tissues
O2
CO2
Beforewegettoinsects,let’sstartbylookingathowrespira-onworksinsomethingweareallfamiliarwith–avertebrate.Firstly,thetaskofanyrespiratorysystemistosupplyoxygentothevarious-ssuesinthebody,whileatthesame-meremovingcarbondioxide.Inavertebrate,theexchangeofoxygenandcarbondioxidewiththeenvironmentcanbedividedintothreeparts.First,ven-la-on,wheretheanimal’sgasexchangeorgan,inthiscasethelungs,areven-latedwithatmosphericair.Bloodcontainingarespiratorypigment,inthiscasehaemoglobin,is loadedwithoxygenasitpassesthroughthelungwhilereleasingcarbondioxideithascarriedformthe-ssues.Fromthelungsthebloodisthenac-velycirculatedaroundthebodybytheheartthroughbloodvessels.Asbloodmovesthroughthecapillaries,itunloadsitsoxygenwhichthendiffusesacrossthewallsofthecapillariestothe-ssueswhereitisconsumedwithintherespiringcells.SotherearethreemainstepstogetoxygeninandCO2out:Ven-la-onofagasexchangeorgan,perfusionofacirculatorysystemandfinallydiffusionfromthecirculatorysystemtotherespiring-ssues
Respiratorysystem:Insect
Tissues
Trachealsystem
O2
CO2
Spiracles
2.Diffusion1.Ven'la'on
Nowlet’slookataninsect’srespiratorysystem.Firstly,youcanseethattheinsectdoesn’thavelungs,oracirculatorysystemwhichpumpsbloodcontaininghaemoglobin.Insteadwhatyoufindisanetworkofsilveryair‐filledtubesrunningthroughoutallpartsoftheinsectsbody.Theseair‐filledtubescomprisethetrachealsystem,andtheyopentotheatmospherethroughholesinthecu-clecalledspiracles.Theair‐filledtracheaearelargenearthespiracles,buttheybranchandramifyingintosmallerandsmallertubeswithintheinsect.Thefinestbranchesofthetrachealsystemarecalledthetracheoles,andtheyarethe-psofthetrachealtree:theyareblindendingandlieinamongthe-ssues.Soyoucanseethattheinsectrespiratorysystemallowsoxygenandcarbondioxidetobeexchangedwiththeatmospherethroughtheirspiracles,whilethetrachealsystemprovidesanair‐filledpathwayforthemovementofgasesfromthespiraclestothe-psofthetracheolesinamongthe-ssues.Thisarrangementisquitedifferenttothatofavertebrateasitallowsoxygenuptakeandcarbondioxideexchangetooccurwithoutneedingacirculatorysystemorrespiratorypigmentslikehaemoglobin.Itissortoflikeoxygenisbeingdelivered“whole‐sale”withoutthemiddle‐manofacirculatorysystem.
Thetrachealsystem
Componentsofthetrachealsystem:•Spiracles•Tracheae•Tracheoles•Airsacks(expanded,soW‐walledtracheae)
MillerP.L.1981TheAmericanCockroach
Thoracicspiracle
Thoracicspiracle
Abdominalspiracles
Abdominalspiracle
Dorsallongitudinaltrunk
Dorsaltransverseconnec-ves
Nowletslookatthetrachealsysteminabitmoredetail.Thisisthetrachealsystemofacockroach.Itgivesagoodoverviewofallthebasiccomponentsoftheinsect’strachealsystem.TheleWsideistheventralviewandtherightsideisthedorsalviewofthedissectedcockroach.Sothemaincomponentsofthetrachealsystemarethespiracles,whichyoucanseeassociatedwitheachsegment,andthesethenleadtolongitudionaltrachealtrunkswhicharethesetrachealtubeswhichrunthelengthoftheinsect.Branchingfromtheselongitudinaltrunksaretracheoles–thefinerbranchesofthetrachealsystem,andairsacks,theselargeexpandedtrachealbranches.Thelongitudinaltrunkswhichlinkupalltheinsect’sspiracles,andtheair‐sacksplayavitalroleinven-la-ngthetrachealsystem–whichissomethingI’lltalkaboutlater.
Thetrachealsystem
Waterboatman1stinstar:Agraptocorixaeurynome
Thisisjusttoshowyouanactualtrachealsystemwithintheinsect.Thisisadorsalviewoftheheadandthoraxofanaqua-chemipteran,abugcalledawaterboatman.Thisisanewlyhatchedfirstinstar.Itscuc-cleistransparent,andyoucanclearlyseethelongitudinaltrunksbranchingintothefinertracheoles.
Thetrachealsystem
Tracheoles
Tracheae
Waterboatman1stinstar:Agraptocorixaeurynome
Zoominginhere,youcanseetheseblind‐endingtracheolesbranchingofffromthemaintrachealtrunkandlyinginamongsttheinsect’scells.
Spiracles
Thoracicspiracle(speckledcockroachNauphoetacinerea)
Abdominalspiracle(RhinocerosbeetleXylotrupesulysses)
Hereareacoupleofotherpicturesshowingsomedifferenttypesofspiracularstructures.OntheleWthereisapictureofathoracicspiracleofacockroach–youcanseeitprotrudesfromthesideoftheinsectlikeamouth.Andontherightyoucanseeanabdominalspiraclefromarhinocerosbeetle.Youcanseequiteclearlythatthespiracleconsistsoftwosegmentswithanopeningbetweenthem.Musclesareabachedtooneofthesesegments,andbycontrac-ngitcanpullthesegmentsapart,openingthespiracleandallowinggasexchangebetweenthetrachealsystemandenvironment.Thisishowinsectscontroltheirrespira-on:byopeningandclosingtheirspiracles.Youcanalsoseethattherearelotsofhairseffec-velycoveringthespiracle.Thisispartofafiltra-onmechanismtopreventdirt,dustandpoten-allyparasites,fromcloggingupthespiracle.Mostinsectsactuallypossessaspiracularatrium–ahairlinedpit,withthespiraclelocatedatthebobom.These arepar-cularlywelldevelopedininsectsfromaridareaswheredustislikelytobemoreofaproblem,andinsectswhichparasi-seanimals,astheyaremorelikelytoencounteroilsandlotsoffinepar-culatemateriallikedander(effec-velyjustanimaldandruff)astheycrawlaroundananimal’sskin.Someaqua-cinsectshaveevenmoreeffec-vespiracleprotec-on,sincetheyrisktheirspiraclesfloodingwhileunderwater.Somanyaqua-cinsectshaveahydrophobicsieveplate–effec-velyaporousplatewhichsitsabovethespiracle,therebyprotec-ngthespiraclevalvebeneath.
DiffusionHigh concentration low concentration
High concentrationlow concentration
O2
O2
O2O2
O2
O2 O2
O2
O2
O2 CO2
CO2CO2
CO2
CO2
CO2
CO2
CO2CO2
CO2
Sonowyouknowthestructureoftheinsect’strachealsystem,howdoesitsupplyoxygentotheinsect’s-ssueswhileelimina-ngcarbondioxide?Therearetwotransportphenomenawhichdrivetheallimportantmovementofoxygenandcarbondioxidebetweentheinsectanditsenvironment.Thefirstwe’lllookatisdiffusion.Allmoleculeswithatemperatureaboveabsolute0possesskine-cenergythatcausesthemtomovearoundandbumpintoeachother.Diffusionistherandommovementofthesemoleculeswhichoccursduetothiskine-corthermalenergy.Diffusionresultsinthenetmovementofmoleculesfromaregionofhighconcentra-ontolowconcentra-on–thatistosay,thereisnetmovementofmoleculesdownaconcentra-ongradient.Inthisdiagramyoucanseeaboxwhichisdividedbyapar--on.TheontheleWareoxygenmoleculesandontherightarecarbondioxidemolecules.Sothereexistsahighconcentra-onofoxygenmoleculesintheleWsideoftheboxandalowconcentra-onontheright,andahighCO2concentra-onontheright,andlowontheleW.Itisimportanttono-cethatthereisnopressuredifferencebetweenthetwocompartmentsasthereisanequalnumberofmoleculesonbothsides.
DiffusionEqual concentration
O2 O2
O2
O2
O2
O2 O2
O2
O2
O2
CO2
CO2
CO2CO2
CO2
CO2
CO2
CO2
CO2
CO2
Equal concentrationIfaholeisthencutinthepar--on,therandomlymovinggasmoleculeswouldbouncearound,collidingwitheachotherandthewallsofthebox,diffusingdowntheirrespec-veconcentra-ongradientsresul-nginanetincreaseinoxygenmoleculesontherightsideoftheboxandanetdecreaseontheleW,andviceversaforcarbondioxide.Givenenough-me,eventuallythemoleculeswoulddistributethemselvesequallyineachcompartmentastheyrandomlybouncedaround,un-ltheywereinequalconcentra-ononbothsidesofthepar--on,atwhichpointtherewouldnolongerbeaconcentra-ondifferencedrivingnetdiffusion.Thismeansthatwhilemoleculeswoulds-llbefreetomoveacrossthepar--on,therewouldeffec-velybenoincreaseordecreaseinoxygenorcarbondioxideoneithersideofthepar--on.Becausediffusionoccursduetoindividualmoleculesbouncingaroundandcollidingwitheachother,itisfastovershortdistances,butasthediffusiondistanceincreases,the-metakenforamoleculetodiffuseoverthatdistanceincreaseswiththesquareofthedistance:Sodiffusion-meincreasesexponen-allywithdistancetravelled.Thisisbecausethemolecularmovementisrandom,itisn’tinanypar-culardirec-on.Sowhileitmightonlytake10‐4(0.0001)secondsforanoxygenmoleculetodiffuseacrossa1micrometerdistance,todiffuseten-mesfurther(10micrometers)wouldtake100-meslonger(10squaredor.01seconds),andover1mm,or1000-mesfurther,itwouldtakeonemillion-meslonger(100seconds1000squared).Sothisshowsyouthatwhilediffusionisveryfastovershortdistances,itisreallyonlyfastenoughtobebiologicallyrelevantoverdistancesinthemicrotomillimetrerange.
Fick’sLawofdiffusion
LengthRateofdiffusion=K (Conc.gradient)
Area
Area
Distance
Concentra-ongradient
Tracheae
Thatbeingsaid,thereareanumberoffactorsthataffectthespeedwithwhichdiffusionoccurs.Andthatbringsmetothisequa-on:Fick’slawofdiffusion.Basicallywhatissaysisthattherateofdiffusionispropor-onaltotheareaofthediffusionpathway(sothecross‐sec-onalareaofatracheole),inverselypropor-onaltothedistanceofthediffusionpathway(thelengthofthetracheole),andpropor-onaltotheconcentra-ongradientacrossthelengthofthetracheole.“K”isadiffusioncoefficientrela-ngdistancediffusedperunit-meperdifferenceinconcentra-on.Thisconstantisspecifictothemediumthroughwhichthemoleculeisdiffusion(thismediumisairinatracheole,andthediffusionconstantforoxygeninairisaround11cm2atm‐1min‐1.Ifthetracheoleswerefilledwithwatertherateofdiffusionwouldbearound320,000-messlower!Thisisanotherreasonwhybeingair‐filledallowsthetrachealsystemtorapidlyspplyoxygentotheinsect’s-ssues)Butthetakehomemessageofthisdiagramis:Increasingtheareaofthediffusionpathwayorincreasingtheslopeoftheconcentra-ongradientdrivingthediffusionwillincreasetherateofdiffusion.Decreasingthelengthofthediffusionpathwaywillalsoincreasetherateofdiffusion.
Diffusion
CO2 5 kPa
O2 4 kPaO2 21 kPa
CO2 0.03 kPa
PO2 Δ = 17 kPa
PCO2 Δ ≈ 5 kPa
The atmosphere At the ends of the tracheoles
Diffusion
Sohowdoesallthisphysicsapplytoaninsect?Wellaninsectisrespiringaerobically,soits-ssuesarealwaysconsumingoxygenandproducingcarbondioxide.Thismeansthatthereislessoxygenandmorecarbondioxideatthe-psofaninsect’stracheolesthanthereisoutinthesurroundingatmosphere.Sothereexistsaconcentra-ongradientdrivingthediffusionofoxygenintotheinsectdownitsconcentra-ongradient,whilecarbondioxidediffusesdownitsownconcentra-ongradientfromthetracheolesouttotheatmosphere.Andasoxygenisalwaysbeingremovedfromthe-psofthetracheoleswhileco2isadded,theseconcentra-ongradientswhichdrivediffusionalwaysexist–equilibriumisnotreached.Insteadtheatmosphereiseffec-velyaninfinitesourceofoxygenwhiletheinsectisanoxygensink,anddiffusionoccursdownthisgradient.
Convec-onEqual Pressures
Convec-onisthebulkmovementofmolecules:itisthemovementofamassofmoleculesfromaregionofhighpressuretoaregionoflowpressure.Thisiswhatyoudowhenyoubreatheinandout.Becausediffusioncouldnevermoveenoughoxygenintoyourlungsfastenoughtosupplyyourrespiratoryrequirements,youspeedtheprocessupbyven-la-ngyourlungsusingconvec-on:youbreatheairinandout.Thisisachievedbygenera-ngasub‐atmosphericpressureinyourchestwhenyourdiaphragmcontractsandrib‐cageriseswhichcausesairtomovebyconvec-on,fromthehigherpressureoftheatmospheretothelowerpressureintoyourlungs.Inthiswayyoumovealargervolumeofgasmorerapidlythanwouldbepossiblerelyingsolelyondiffusion.Andthatiswhatthisdiagramisshowing.Thereisamixedgasinthisboxwhichhasthesamepressureasthesurroundingatmosphere
Convec-onPositive Pressure
Ifaplungerthenexertsaforceonthegasinthebox,thegasiscompressedandnowhasahigherpressurethanthegasinthesurroundingatmosphere.
Convec-onEqual Pressure
Convec-on
Asmoleculesmovefromregionsofhighpressuretolowpressureinordertorestorepressureequilibrium,iftheboxisthenopened,therewouldbeamassflowofmolecules,aconvec-vemovementofgasoutoftheboxasthegasmovesfromhightolowpressure.Oncethepressurewithintheboxequalstheatmosphericpressure,theconvec-veflowstops.Sothisshowsthatinconvec-on,allmoleculesmovetogetheratthesamespeed,unlikeindiffusionwheremoleculesmoveaccordingtotheirindividualconcentra-ongradients.
Convec-on
Convection
Increased pressure in the haemolymph exerts pressure on the walls of the tracheae, increasing the pressure of air inside it. Air then flows out to the atmosphere through open spiracles
+ve pressure
Insectsalsobreatheinandoutusingconvec-on,muchaswedo.Theydothisbyexpandingandcollapsingregionsoftheirtrachealsystemmuchasyouexpandandcollapseyourlungs.Theydothisbyincreasingpressureintheirbodycavitythroughmusculareffort:forexample,someinsectswillexpandandcontracttheirabdomen.Astheirabdomencontractsitincreasesthepressureoftheirbodyfluids/haemolymph,whichthenexertpressureonthewallsoftheirtrachealsystem.Thiscausessomesec-onsofthetrachealsystemtocollapse,par-cularlythelarge,soW‐walledairsacks,andtheairwithinthesestructuresisforcedoutofthetrahchealsystemthroughopenspiracles
Convec-on
Convection
-ve pressure
Haemolymph pressure drops, tracheae return to their previous shape, reinflating with air drawn from the atmosphere
Whentheirabdomenthenrelaxes,thetracheaeelas-callyre‐expandtotheirpreviousshape,andthisdrawsairinthroughopenspiracles.
Convec-on
The convective movement of air ventilates the tracheae between spiracles
Sobyperiodicallycompressingtheirtrachealsystemandinsectcanconvec-velyven-latesomepor-onsoftheirtrachealsystem.Youshouldalsonotethattheblind‐endingtracheolescannotbeven-latedconvec-vely,astheydonotdeform,andaircannotflowinthroughonebranchandouttheother,astheyaredead‐ends.Sodiffusionremainstheonlyprocessbywhichoxygenandcarbondioxidecanmovethroughthetracheoles.Ihaveputavideoontheblackboardwebsitewhichshowsconvec-veven-la-onactuallyoccurringinabeetle.Thevideowasmadebyplacingthebeetleinamachinecalledasyncrotronwhichproducesapowerful,focussedx‐raybeam.Thisallowedresearcherstosee,forthefirst-me,thatalmostallthelargetracheaecanbeexpandedandcollapsed,allowingthebeetletoconvec-velyven-latemuchofitstrachealsystem.
Unidirec-onalconvec-veven-la-on
Expira-onthroughabdominalspiracles
Inspira-onthroughthoracicspiracles
Manyinsectscanalsochanneltheconvec-veflowofairthroughtheirtrachelsystemby-mingtheopeningandclosingoftheirspiracles.Forexample,thelocustSchistocercagregariageneratesaunidirec-onalflowofairwhileitisatrestbyopeningitsthoracicspiraclesduringtheinhala-onphase,whilekeepingitsabdominalspiraclesshut.Itthenclosesitsthoracicspiraclesandopensitsabdominalspiracleswhilecompressingitstrachealsystem,thusforcingairoutthroughitsabdominalspiracles.Soby-mingtheopeningandclosingofthespiraclesonthedifferentbodysegmentswithconvec-vepumpingmovements,insectscandirecttheflowofairthroughtheirtrachealsystem.
Diffusiononly
Tracheoles
Tracheae
Waterboatman1stinstar:Agraptocorixaeurynome
Theadvantageofconvec-veven-la-onisthatitmovesfreshairthroughtheinsectstracheae,whichmeansthatdiffusiononlyoccursoververyshortdistancesinthetracheoles.Soifyoulookatthisdiagram,youcanimaginethatintheabsenceofconvec-veven-la-onoxygenwouldneedtodiffuseinthroughaspiracle,alongthetracheae,throughthetracheoles,andthenintothe-ssues.Diffusionwouldoccurquiteslowlyoverthisdistance
Convec-on+diffusion
Tracheoles
Tracheae
Waterboatman1stinstar:Agraptocorixaeurynome
Butwithconvec-veven-la-on,freshaircanbemovedenmassthroughthetracheae,sodiffusiononlyneedstooccuroverthemuchshorterdistanceofthetracheolesalone.Thismeansoxygenandcarbondioxidecanbemovedinandoutoftheinsectmuchmorerapidly.
Rela-veimportanceofdiffusionandconvec-on
2mmlong
Unknownclownbeetle:Histeridae
>50mmlong
Insectgasexchangebestdescribedbyamixofbothdiffusion/convec-on
RhinocerosbeetleXylotrupesulysses
Sowhichismoreimportanttoaninsect:diffusionorconvec-on?Wellthatdependsonhowlargeyouare.Asmallinsectlikeafairywaspdoesn’thavemuchdistancebetweenitsrespiring-ssuesandtheoutsideatmosphere.Thismeansthatitsrespiratoryrequirementswillbeeasilymetbydiffusion.Butwhathappenswhenyouaremuchbigger,likearhinocerosbeetle?Diffusioncannotoccurrapidlyenoughoverthelongerdistancesbetweentheirspiraclesandthe-psoftheirtracheoles.Sobiggerinsectstendtorelymoreonconvec-veven-la-ontomaintaingasexchange,andyouwilloWenseelargebeetlesandgrasshoppersac-velypumpingtheirabdomensinordertoven-latetheirtrachealsystems.
Autoven-la-on
FlyingCetoniidflowerbeetle,Cetonischemaaeruginosa
Gramforgram,flyinginsectshavethehighestratesofoxygenconsump-onintheanimalkingdom:3‐foldgreaterthanahoveringhummingbird,30‐foldgreaterthanahumanathlete
Flightmuscle
Airsack(expanded)
Whileconvec-onbecomesincreasinglyimportantwithsize,italsobecomesincreasinglyimportantwithhighmetabolicrateswhichdemandlargefluxesofoxygentothe-ssues.Thisispar-cularlytrueforflyinginsectswhichcanhavephenomenallyhighratesofaerobicrespira-on,thehighestintheanimalkingdom.Toensurethattheirflightmusclesreceiveenoughoxygen,manyinsectsuseautoven-la-on:Thisisbasicallywhereairsacksneartheflightmusclesarecompressedandexpandedwiththewingstroke.Thismeansthattheflightmusclesnotonlymovethewings,butincidentallypumpairintotheinsectstrachealsystemandkeepthemselvessuppliedwithoxygen.
Autoven-la-on
FlyingCetoniidflowerbeetle,Cetonischemaaeruginosa
Gramforgram,flyinginsectshavethehighestratesofoxygenconsump-onintheanimalkingdom:3‐foldgreaterthanahoveringhummingbird,30‐foldgreaterthanahumanathlete
Flightmuscle
Airsack(compressed)
Whileconvec-onbecomesincreasinglyimportantwithsize,italsobecomesincreasinglyimportantwithhighmetabolicrateswhichdemandlargefluxesofoxygentothe-ssues.Thisispar-cularlytrueforflyinginsectswhichcanhavephenomenallyhighratesofaerobicrespira-on,thehighestintheanimalkingdom.Toensurethattheirflightmusclesrecieveenoughoxygen,manyinsectsuseautoven-la-on:Thisisbasicallywhereairsacksneartheflightmusclesarecompressedandexpandedwiththewingstroke.Thismeansthattheflightmusclesnotonlymovethewings,butincidentallypumpairintotheinsectstrachealsystemandkeepthemselvessuppliedwithoxygen.
Convec-on:thehornet’sAchillies’heel
Papachristoforou,Rortaisetal.2007Smotheredtodeath:HornetsasphyxiatedbyhoneybeesCurrentBiology17R795‐R796
PhotobyTakahasi,Wikipediacommons
Honeybeesswarmahornetina‘beeball’ Hornetwithbracedabdominaltergites
Theimportanceofconvec-veven-la-onininsectrespira-oncanalsobeseeninhowcyprianbeesdealwithanabackinghornet.Hornetspreyuponhoneybees,raidingtheirhivesandea-ngtheworkers.ButontheislandofCyprus,thehoneybeeshavedevelopedaneffec-vedefenceagainsttheiraback.Researchersno-cedthatwhenCyprianhornetsabackahoneybeehive,thebeeswouldswarmthehornet.Butinsteadofs-ngingittheywouldclusteralloverit,par-cularlyarounditsabdomen,andaWeranhourthehornetwouldbedead.Itturnsoutthatwhatthebeesweredoingwassuffoca-ngthehornet.Hornetsmustpumptheirabdomenstoconvec-velyven-latetheirtrachealsystem,andwhatthebeesweredoingwasgrabbingontothehornetsabdomenandkeepingitcompressed,preven-ngthehornetfromconvec-velyven-la-ng.Eventually,thiscausedthehornettosuffocate.Theresearchersalsoshowedthatifplas-cblockswereinsertedbetweentheabdominaltergitesofthehornet,whichiswhatthepictureontherightisshowing,thebeeswereunabletocompressthehornet’sabdomen,andsothehornetcouldsurvivebeingmobbedbythebeesforfarlonger.Sothisisjustaneatexampleofhowconvec-onisimportant,especiallyamongthebiggerinsects.
Isinsectsizelimitedduetodiffusionthroughthetrachealsystem?
Highoxygenlevels&giantinsects
Sowhilelargerinsectstendtouseconvec-onmoretoensuretheycandeliverenoughoxygentotheir-ssuesquicklyenough,therearesomepartsoftheinsectwhichcannotbeven-latedbyconvec-on,andwherediffusionmustsuffice.Forexample,insectlegsdon’tcontainairsacks,orloopsoftracheaethatwouldallowairtobepumpeddownonetracheaeandoutthroughtheother.Thismeansinsectlegsmustreceiveanadequatesupplyofoxygenbydiffusionalone.Ithasbeensuggestedthatthediffusionlimitsthelengththatthesediffusion‐onlysec-onsoftheinsectcanbe,andsothisconstrainstheuppermaximumsizeoftheinsect.Thispartlyexplainswhymostinsectsaresmall,andonlyaveryfewinsectsexistwhicharelargerthan10cen-metresorsoinlength.Thisideaofdiffusionlimi-nginsectsizeispartlybackedupbyevidencefromthegeologicalrecord.Some300millionyearsagoinaperiodcalledthecarboniferous,levelsofatmosphericoxygenweremuchhigherthantoday.Atthemomenttheearth’satmosphereisaround21%oxygen,butinthecarboniferousitwasprobablycloseto30%.Andduringthisperiodwefindfossilsofenormousinsects–proto‐dragonflieswithwingspansofover70cen-metres.Fromthiscorrela-onithasbeenarguedthatthesegiantinsectscouldgrowsolargebecausethegradientdrivingthediffusionofoxygenintotheinsectwasmuchlarger,andthusdiffusioncouldoccurrapidlyenoughinthishighoxygenatmospheretosustaintheinsect’srespira-on.
Summary
• Thetrachealsystemiseffec-vebecause:– Oxygenandcarbondioxideareexchangeddirectlywiththeatmosphere–acirculatorysystemisnotrequired
– Tracheolesprovideadiffusionpathwaydirectlytorespiring-ssues
– Gasexchangeoccursduetobothdiffusionandconvec-on.Convec-onincreasinglyimportantinlargerinsects.Diffusionimportantinblind‐endingtracheoles
– Compressionandexpansionoftrachealsystemcombinedwithcoordinatedspiracularopening/closinggeneratesdirec-onalconvec-veflowsofair