Insect Respiration - Coming Soon folder/respiration/lectur… · Insect Respiration Philip G. D. Matthews Cetonischema aeruginosa. Insect Respiraon • Outcomes: –Be able to ...

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