greek joint foot animal having external skeleton body paired form includes insects crustaceans characterized limbs cuticle calcium body plan consists segments pair rigid cuticle growth replace periodically versatility enabled species rich members ecological environments million described species making described living animal species unlike animals successful dry environments range size microscopic japanese spider crab primary internal cavity internal organs blood open systems like internal organs generally built repeated segments nervous system ladder like paired ventral nerve running segments forming paired segment heads formed fusion varying numbers segments brains formed fusion segments respiratory systems vary depending environment belong vision relies combinations compound eyes pigment pit species detect direction light coming compound eyes main source information main eyes spiders form images cases swivel track prey wide range chemical mechanical sensors based modifications project methods reproduction development diverse terrestrial species use internal indirect transfer sperm ground direct injection aquatic species use internal external lay eggs birth live young eggs hatched inside mother vary miniature adults lack limbs eventually undergo total produce adult form level maternal care varies prolonged care provided evolutionary ancestry dates period group generally regarded analyses support placement constituent overall basal relationships resolved likewise relationships groups actively debated contribute human food supply directly food importantly crops specific species known spread severe disease humans livestock crops etymology word comes greek joint foot leg mean leg description invertebrates bodies limbs consists cuticle crustaceans calcium diversity estimates number species vary million account known living animal species number species remains difficult determine census modeling assumptions projected regions order scale counts specific locations applied world study estimated species animals plants costa_rica important members marine freshwater land air ecosystems major animal groups adapted life dry environments living members reptiles birds mammals sub group insects species rich member ecological land fresh water environments insects weigh heaviest weigh grams living crustaceans larger example legs japanese spider crab span metres heaviest living american topping embryos built series repeated common ancestor living probably consisted series segments pair functioned limbs known living fossil grouped segments segments limbs specialized ways appearance insect bodies appearance spiders result grouping fact external signs body elements repeated pattern segments ahead mouth rear eyes mounted original structure probably upper branch acting gill lower branch walking segments known modified example form gills mouth parts antennae collecting information claws like knives equipped unique set specialized tools vanished regions body particularly common abdominal disappeared highly modified conspicuous segments head major groups includes spiders crabs etc breathe channels bodies includes insects extinct heads formed combinations segments missing specialized different ways addition extinct belong groups heads formed particular combinations segments specialized working evolutionary stages different combinations appeared difficult long known head problem hoped solved trying work solutions fun cuticle non cellular material vary details structure generally consist main layers thin outer coat moisture layers gives protection consists hardened proteins consists proteins known body segment limb section hardened cuticle joints body segments limb sections covered flexible cuticle aquatic crustaceans calcium extracted water terrestrial crustaceans developed means storing mineral land rely steady supply dissolved calcium generally affects outer recent evolution groups animals propose provides tougher defensive armor allows animals grow larger stronger providing rigid skeletons case mineral organic composite cheaper build organic comparable strength cuticle growing special cells varied form function example sensors detect air water currents contact objects aquatic use feather like increase surface area swimming filter food particles water aquatic insects air use thick felt like coats trap air extending time spend water heavy rigid serve defensive spines use muscles attached inside limbs use hydraulic pressure extend system inherited pre ancestors example spiders extend legs generate pressures times resting level stretch growth replace old growing new hardened cycles run nearly continuously reaches size initial phase animal stops feeding releases fluid mixture old cuticle phase begins new protect new old cuticle stage complete animal makes body swell taking large quantity water air makes old cuticle split weaknesses old commonly takes minutes animal struggle old cuticle point new soft animal support finds difficult new formed animal continues pump stretch new cuticle possible new excess air water end phase new formed eat discarded cuticle reclaim materials unprotected nearly new cuticle hardened danger trapped old cuticle attacked predators responsible deaths internal organs bodies internally nervous muscular systems repeated components come lineage animals membrane lined cavity gut body wall internal organs strong limbs eliminate need main ancestral functions skeleton muscles order change animal shape enable reduced small areas reproductive systems place largely taken cavity runs length body blood flows open systems short open ended crustaceans blood carries oxygen tissues use separate system crustaceans use respiratory assist oxygen transport common respiratory pigment copper based crustaceans crustaceans insects use iron based respiratory pigment invertebrates respiratory generally dissolved blood rarely enclosed heart typically muscular tube runs length contracts run rear pushing blood forwards sections heart muscle expanded elastic small muscles case connecting heart body wall heart run series paired non return allow blood enter heart prevent leaving reaches wide variety respiratory systems small species high ratio surface area volume enables simple body surface supply oxygen usually gills modified book lungs systems branching tunnels run openings body walls deliver oxygen directly individual cells insects living paired main nerve running bodies gut segment form pair sensory motor nerves run parts segment pairs segment appear physically fused connected relatively large nerves nervous systems characteristic ladder like appearance brain head mainly consists fused foremost segments form head total pairs antennae connected head segments close brain function insects head combine pair spiders process step incorporated occupy space super segment different types systems aquatic end product reactions nitrogen toxic needs possible water eliminated membrane mainly gills crustaceans use system high consumption water responsible relative lack success crustaceans land animals groups terrestrial independently developed different system end product nitrogen acid dry material system filters acid waste blood materials expelled aquatic terrestrial ones organs called little extract senses optical stiff block information outside world penetrated sensors connections sensors nervous system fact modified elaborate sensors touch sensors respond different levels force strong contact weak air currents chemical sensors provide taste smell means pressure sensors form membranes function connected directly nerves antennae include packages monitor humidity moisture temperature sophisticated visual systems include usually compound eyes pigment cup little eyes cases capable detecting direction light coming shadow cast walls cup main eyes spiders pigment cup capable forming images jumping spiders rotate track prey compound eyes consist thousand independent columns usually cross section independent light sensitive cells lens compound eyes wide field view detect fast movement cases light hand relatively large size makes images coarse compound eyes shorter sighted birds mammals severe disadvantage objects events centimetres important color vision insects studied detail example bees contain green ultra violet lack balance acceleration sensors rely eyes tell way self behavior triggered pressure sensors underside feet report pressure crustaceans provide sort information balance motion sensors inner ear sensors report force exerted muscles degree bending body joints understood little known internal sensors reproduction development organs sexes individuals species remain sex entire lives species insects crustaceans reproduce example mating especially conditions favor population explosion rely sexual reproduction species revert sexual reproduction conditions favorable aquatic breed external example frogs internal remain female body sperm inserted known terrestrial use internal crustaceans use modified transfer sperm directly female male terrestrial produce sperm females bodies species rely females find deposited ground cases males deposit complex courtship rituals look likely successful lay eggs viviparous produce live young eggs hatched inside mother noted prolonged maternal care newly born diverse forms insects cover range extremes hatch apparently miniature adults direct development cases feed helpless insects hatch limbs hardened adult forms entering inactive phase tissues broken build adult body larvae typical limbs water jaws crustaceans commonly hatch tiny larvae segments pairs evolution common ancestor common ancestor reconstructed organism module covered armor plate bearing pair limbs ancestral limb far settled debate ventral mouth pre oral antennae dorsal eyes body non sediment processing sediment came way food fossil record proposed animals million years ago small like shells found early fossil beds dating million years ago china earliest fossils million years old class diverse worldwide suggesting time examination 1970s fossils million years ago identified assigned known groups intensified debate explosion fossil provided earliest clear evidence earliest fossil crustaceans date million years ago fossil shrimp million years ago apparently formed tight procession fossils common period onwards remained entirely aquatic possibly developed systems conserve water provide earliest identifiable fossils land animals million years ago late terrestrial tracks million years ago appear pre adapted land existing provided protection support gravity means dependent water time aquatic like largest long metres oldest known million years ago period million years ago period bears earliest known silk producing lack means true spiders appear late million years ago cretaceous periods provide large number fossil spiders including representatives modern families fossils aquatic gills appear periods earliest fossil air breathing book lungs dates early period oldest definitive insect fossil dated million years ago type found winged insects suggests earliest insects appeared period late million years ago include species gigantic modern standards indicate insects occupied main modern ecological social ants appear early cretaceous advanced social bees found late cretaceous rocks abundant evolutionary family tree zoologist argued words share common ancestor instead proposed separate groups evolved separately common worm like ancestors including spiders crustaceans consisting arguments usually bypassed evolutionary relationships class unclear proponents argued following similarities groups results evolution natural consequences having rigid groups use different chemical means cuticle significant differences construction compound eyes hard different configurations segments head evolved ancestor crustaceans limbs separate gill leg branches groups limbs single branch serves leg analysis discoveries 1990s reversed view led acceptance words share common ancestor example analyses convinced animals similar early presented evolutionary family tree showed cousins changes scope term unclear proposed wider group labelled animals limbs hardened called true contrary view presented argued sister group lost evolved features developed earliest known ate mud order extract food particles possessed variable numbers segments functioned gills legs standards time huge sophisticated predators specialized mouths fixed numbers segments specialized tail fins gills different reasoning implies legs backward pointing mouth like earliest credible closest relative suggested closely related higher family tree traditionally considered closest relatives groups bodies combination groups labelled competing proposals closely related groups remained minority views difficult specify detail relationships groups 1990s molecular phylogenetic analyses dna sequences produced coherent scheme showing members labelled animals contained excluded backed studies anatomy development animals showed features supported hypothesis showed significant differences earliest details hardly present hypothesis groups hypothesis correct evolved inherited older ancestor subsequently lost non members classification typically classified extinct group numerous marine animals disappeared extinction event decline prior killing blow having reduced order late extinction include crabs spiders related organisms characterised presence mouth appear crabs tiny claws use feeding spiders developed venom comprise relatives body segments bearing pairs legs grouped crustaceans primarily aquatic notable exception characterised having include crabs shrimp comprise insects small orders insect like animals legs grouped group called genetic evidence tends support closer relationship crustaceans aside major groups number fossil forms early difficult place lack obvious affinity main groups clear affinity recognized significantly different known groups phylogeny major extant groups area considerable interest dispute recent studies strongly suggest traditionally defined having evolved form clade position remains unclear april studies grouped forming studies grouped forming sister plus placement extinct frequent subject dispute newer originated pair evolved antennae ancestors place antennae closer international code nomenclature recognises priority rank family higher level groups referred variety different names interaction humans crustaceans crabs shrimp long human cuisine raised commercially insects meat eaten raw cooked cultures excluding european hindu islamic cultures cooked considered southern venezuela highly hairs spider main defense system removed humans unintentionally eat foods food safety regulations lay acceptable contamination levels different kinds food material intentional cultivation small animals human food referred emerging animal sound concept commercial butterfly breeding provides stock butterfly educational exhibits schools research facilities cultural events greatest contribution human food supply study examined crops lists grown food estimated economic value billion value world agricultural production human food bees produce honey basis rapidly growing industry international trade red produced central american species insect economically important region spanish control mexico second lucrative export regaining ground lost synthetic competitors blood crabs contains agent test kidney machines free dangerous bacteria detect spinal forensic uses evidence provided establish time place death human cases cause recently insects gained attention potential sources drugs medicinal substances relative simplicity body plan allowing variety surfaces land water useful models provided segments allows robots normally damaged lost numerous earth thousands species inflict relatively bites humans far effects humans diseases carried blood insects blood insects livestock diseases kill animals greatly reduce cause parasite borne diseases humans closely related humans causing intense cause diseases including hay fever species principally insects agricultural forest largest single problem faced worldwide efforts control large scale use caused long term effects human health biodiversity increasing resistance led development integrated pest management wide range measures including biological control predatory useful controlling
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