Biology 320Biology 320Invertebrate ZoologyInvertebrate Zoology

Fall 2005Fall 2005

Chapter 16 – Introduction to Chapter 16 – Introduction to Phylum ArthropodaPhylum Arthropoda

Phylum ArthropodaPhylum Arthropoda

► Horseshoe crabs, Horseshoe crabs, crustaceans, arachnids, crustaceans, arachnids, insects, and millipedesinsects, and millipedes

► One million described speciesOne million described species 80% of all known animals80% of all known animals

► Another 30 million Another 30 million undescribed species of undescribed species of insects, estimatedinsects, estimated

► Only birds, bats, and insects Only birds, bats, and insects have the ability to flyhave the ability to fly

► Amazing adaptive diversityAmazing adaptive diversity

► Thought to be closely Thought to be closely related to annelidsrelated to annelids Recently thought to Recently thought to

be more closely be more closely related to nematodesrelated to nematodes

► Two major extant Two major extant taxataxa Chelicerata – Chelicerata –

horseshoe crabs and horseshoe crabs and arachnidsarachnids

MandibulataMandibulata►Crustacea – crabs, Crustacea – crabs,

barnacles, and water barnacles, and water fleasfleas

►Tracheata – insects and Tracheata – insects and centipedescentipedes

SegmentationSegmentation

► Metamerism is the technical term for segmentationMetamerism is the technical term for segmentation

► Individual segments are termed somitesIndividual segments are termed somites

► Certain body regions of arthropods display a fusion Certain body regions of arthropods display a fusion of several segmentsof several segments

► Arthropods display tagmosis (tagmatization)Arthropods display tagmosis (tagmatization) Segments are organized into regions called tagma Segments are organized into regions called tagma

(tagmata)(tagmata) Insects posses a head, thorax, and abdomenInsects posses a head, thorax, and abdomen Arachnids and crustaceans (typically) possess a Arachnids and crustaceans (typically) possess a

cephalothorax and abdomencephalothorax and abdomen

CephalizationCephalization

► Highest degree of Highest degree of cephalization of any cephalization of any phylum we have phylum we have discussed to this pointdiscussed to this point

► Head possessesHead possesses Concentration of ganglia Concentration of ganglia

(brain)(brain) Concentration of sensory Concentration of sensory

structuresstructures► Eyes and antennaeEyes and antennae

Mouth, feeding Mouth, feeding appendages, and legs (in appendages, and legs (in those with a those with a cephalothorax)cephalothorax)

AppendagesAppendages

► Segmented with Segmented with articulationsarticulations

► Specialized for specific Specialized for specific functionsfunctions Some posses more than Some posses more than

10 types10 types Enormous varietyEnormous variety

► May be simple, or have May be simple, or have multiple branchesmultiple branches

Body WallBody Wall

► Cuticle (exoskeleton) is most superficial layerCuticle (exoskeleton) is most superficial layer Made of chitin and proteinMade of chitin and protein Provides strength and protectionProvides strength and protection

► EpidermisEpidermis MonolayeredMonolayered Secretes cuticleSecretes cuticle

► Thin connective tissue layerThin connective tissue layer

► Specialized individual muscles, rather than Specialized individual muscles, rather than sheetssheets

ExoskeletonExoskeleton

► Cuticle has hard areas and Cuticle has hard areas and soft areas soft areas Allows for movement at Allows for movement at

articulationsarticulations

► Some areas are sclerotizedSome areas are sclerotized Proteins crosslink causing Proteins crosslink causing

cuticle to harden and darkencuticle to harden and darken Typical of arachnids and Typical of arachnids and

insectsinsects

► Some areas are mineralizedSome areas are mineralized Calcium salts are added to Calcium salts are added to

cuticlecuticle Crustaceans supplement Crustaceans supplement

sclerotization with sclerotization with mineralizationmineralization

► Two main layers of Two main layers of exoskeletonexoskeleton

Epicuticle – thin outer Epicuticle – thin outer layer that is water / layer that is water / gas resistantgas resistant►Waterproofed by Waterproofed by

different types of lipidsdifferent types of lipids►Useful in dry Useful in dry

environmentsenvironments► Important for Important for

osmoregulation in FW osmoregulation in FW environmentsenvironments

Procuticle – thick Procuticle – thick portionportion

►Must be periodically Must be periodically molted in order for molted in order for growth to occurgrowth to occur Rigid exoskeleton cannot Rigid exoskeleton cannot

stretch stretch Process known as ecdysisProcess known as ecdysis Regulated by the steroid Regulated by the steroid

hormone ecdysonehormone ecdysone

► Even the lining of the Even the lining of the foregut, hindgut, foregut, hindgut, tracheae, etc. are shedtracheae, etc. are shed

► Has been thoroughly studied in Has been thoroughly studied in decapodsdecapods

► When animal prepares to moltWhen animal prepares to molt Old cuticle begins to detachOld cuticle begins to detach New cuticle begins to formNew cuticle begins to form Layer of molting fluid Layer of molting fluid

(containing proteases and (containing proteases and chitinases) forms between the chitinases) forms between the two cuticlestwo cuticles

Epicuticle of new cuticle Epicuticle of new cuticle protects it from enzymesprotects it from enzymes

Calcium from old cuticle is Calcium from old cuticle is reclaimed reclaimed

► During ecdysisDuring ecdysis Animal is very vulnerableAnimal is very vulnerable Usually molts inside burrow, Usually molts inside burrow,

shelter, etc.shelter, etc. Molting related complications / Molting related complications /

deaths are common deaths are common

EcdysisEcdysis

► After moltingAfter molting Animal takes up water or Animal takes up water or

air, and swells as a resultair, and swells as a result May ingest old cuticle to May ingest old cuticle to

reclaim calciumreclaim calcium Sclerotization and Sclerotization and

mineralization mineralization (crustaceans) occur(crustaceans) occur

Excess water or air is Excess water or air is eliminatedeliminated

Animal now has room for Animal now has room for growthgrowth

► Some molt periodicallySome molt periodically

► Others have a fixed Others have a fixed number of instars (life number of instars (life stages) separated by stages) separated by moltsmolts

MusculatureMusculature

► Lack sheets of musclesLack sheets of muscles

► Small individual muscles Small individual muscles that are specializedthat are specialized FlexorsFlexors ExtensorsExtensors

► Often arranged Often arranged antagonisticallyantagonistically

► Control one articulation or Control one articulation or one scleriteone sclerite

► Striated, for rapid Striated, for rapid contractioncontraction

LocomotionLocomotion

► Arthropods posses a Arthropods posses a large variety of large variety of appendages, that appendages, that serve different serve different purposespurposes

Walking legsWalking legs

Paddles – swimmingPaddles – swimming

Wings - flightWings - flight

CoelomCoelom

► Arthropods have an Arthropods have an exoskeleton, and exoskeleton, and therefore do not rely therefore do not rely heavily on a hydrostatheavily on a hydrostat

► Coelom is reduced Coelom is reduced compared to that of the compared to that of the annelidsannelids

► Hemocoel is the Hemocoel is the functional body cavityfunctional body cavity

► No segmental divisionsNo segmental divisions

Hemal SystemHemal System► Transports nutrients, hormones, Transports nutrients, hormones,

wastes, and gasses (in some)wastes, and gasses (in some)

► Open system in mostOpen system in most

► System consists of:System consists of: Large dorsal heartLarge dorsal heart

► Tubular and contractileTubular and contractile► Possesses ostiaPossesses ostia

Arteries that branch off of heartArteries that branch off of heart Sinuses are separated by Sinuses are separated by

perforated diaphragmsperforated diaphragms► Pericardial – dorsalPericardial – dorsal► Perivisceral – middlePerivisceral – middle► Perineural (in some) - ventralPerineural (in some) - ventral

Blood Blood ► Crustaceans and chelicerates Crustaceans and chelicerates

have hemocyaninhave hemocyanin HemocoelHemocoel

► Muscles and organs are housed Muscles and organs are housed herehere

►Generalized blood flowGeneralized blood flow Oxygenated blood leaves heart via arteriesOxygenated blood leaves heart via arteries

Travels to sinuses and hemocoel where Travels to sinuses and hemocoel where tissues and organs are bathed in bloodtissues and organs are bathed in blood

Deoxygenated / waste laden blood travels Deoxygenated / waste laden blood travels to respiratory structures (in all but to respiratory structures (in all but tracheates) and gas exchange occurstracheates) and gas exchange occurs

Blood enters heart via ostiaBlood enters heart via ostia

ExcretionExcretion► Two main excretory Two main excretory

structuresstructures Saccate nephridiaSaccate nephridia Malpighian tubulesMalpighian tubules

► Saccate nephridiaSaccate nephridia Mainly found in aquatic Mainly found in aquatic

speciesspecies Not actually used for Not actually used for

excretion, as wastes excretion, as wastes (ammonia) diffuse across gills(ammonia) diffuse across gills

Maintain ion balance and fluid Maintain ion balance and fluid volumevolume

Go by many name, such as Go by many name, such as “green glands” in crayfish“green glands” in crayfish

Consists of a tube and an end Consists of a tube and an end sac which is bathed in bloodsac which is bathed in blood

► Malpighian tubulesMalpighian tubules Primary excretory organ Primary excretory organ

of terrestrial speciesof terrestrial species

Stringy tubules located Stringy tubules located at the junction of the at the junction of the midgut and hindgutmidgut and hindgut

Create uric acid and Create uric acid and guanine and transfer to guanine and transfer to the gut lumenthe gut lumen

Wastes form a Wastes form a precipitate in this low pH precipitate in this low pH environmentenvironment

Nitrogenous wastes are Nitrogenous wastes are removed with fecesremoved with feces

Adaptation to terrestrial Adaptation to terrestrial existenceexistence

Gas ExchangeGas Exchange► Small arthropods conduct gas Small arthropods conduct gas

exchange across body exchange across body surfacessurfaces

► Larger varieties make use of Larger varieties make use of specialized structures that specialized structures that must be moistmust be moist

► Interestingly, the epicuticle of Interestingly, the epicuticle of respiratory structures is not respiratory structures is not water / gas proofwater / gas proof

► Many types of respiratory Many types of respiratory structuresstructures Particular structures are Particular structures are

mainly dependent on the type mainly dependent on the type of habitat the animal inhabitsof habitat the animal inhabits

► Aquatic invertebratesAquatic invertebrates GillsGills

► Already discussedAlready discussed Book gillsBook gills

► Flat sheets on ventral surfaceFlat sheets on ventral surface► Possessed by horseshoe crabsPossessed by horseshoe crabs

► Terrestrial speciesTerrestrial species TracheaeTracheae

► Tubular invaginations that branch and Tubular invaginations that branch and lead directly to cells, not to the hemal lead directly to cells, not to the hemal systemsystem

► Spiracle diameter regulated by a Spiracle diameter regulated by a sphinctersphincter

► Opening / closing of spiracles is useful Opening / closing of spiracles is useful for ventilation and reducing water lossfor ventilation and reducing water loss

Book lungsBook lungs► Common in arachnidsCommon in arachnids► Invagination of exoskeleton which Invagination of exoskeleton which

contains many smaller invaginations contains many smaller invaginations (large SA)(large SA)

► Exchange occurs between surfaces of Exchange occurs between surfaces of book lungs and bloodbook lungs and blood

Digestive SystemDigestive System

► Complex, but somewhat Complex, but somewhat similar to annelid systemsimilar to annelid system

► Epithelium at junction of Epithelium at junction of foregut-midgut secretes foregut-midgut secretes a thin peritrophic a thin peritrophic membrane around bolusmembrane around bolus Protects delicate walls of Protects delicate walls of

midgut from abrasionmidgut from abrasion

► Gut lumen reabsorbs Gut lumen reabsorbs waterwater

Nervous SystemNervous System

► Similar to that of annelidsSimilar to that of annelids

► CNSCNS BrainBrain Ventral nerve cordVentral nerve cord

► PNSPNS Segmental nerves connecting to Segmental nerves connecting to

muscle and sensory organsmuscle and sensory organs

► Numerous types of sensory Numerous types of sensory structuresstructures

► Exoskeleton is modified to Exoskeleton is modified to accommodate sensory receptorsaccommodate sensory receptors

► MechanoreceptorsMechanoreceptors SetaeSetae

► Hollow extensions of cuticle Hollow extensions of cuticle w/ sensory neurons insidew/ sensory neurons inside

TrichobothriaTrichobothria► Sensitive “hairs” that Sensitive “hairs” that

detect weak air currentsdetect weak air currents

► ChemoreceptorsChemoreceptors SensillaSensilla

► Similar to setaeSimilar to setae

► EquilibriumEquilibrium StatocystsStatocysts

► Possessed by somePossessed by some

OcelliOcelli► Most use light as a point of Most use light as a point of

reference for orientationreference for orientation

► Sound detectionSound detection Tympanic membraneTympanic membrane

► Responds to air Responds to air vibrationsvibrations

► Can also produce sound Can also produce sound in somein some

► Useful for locating / Useful for locating / attracting matesattracting mates

► Moths use to hear bat Moths use to hear bat echolocationecholocation

► Temperature – Temperature – humidity detectorshumidity detectors Thermo-hygroreceptive Thermo-hygroreceptive

sensilliasensillia► Usually found on Usually found on

antennae of insectsantennae of insects

EyesEyes► Most have ocelliMost have ocelli

► Many also have compound eyesMany also have compound eyes Composed of many subunits called Composed of many subunits called

ommatidiaommatidia 15 – thousands per eye15 – thousands per eye Each unit has own lens and own Each unit has own lens and own

field of visionfield of vision Fields of vision of adjacent Fields of vision of adjacent

ommatidia overlapommatidia overlap Composite image formed, similar Composite image formed, similar

to vertebrate eyesto vertebrate eyes Are usually motion detectors that Are usually motion detectors that

have a range of about 20cmhave a range of about 20cm Many have eyestalksMany have eyestalks Some have color vision and Some have color vision and

respond to colored flowersrespond to colored flowers

ReproductionReproduction► Mostly gonochoricMostly gonochoric

► Fertilization is primarily internal in Fertilization is primarily internal in terrestrial speciesterrestrial species Can be internal or external in aquatic Can be internal or external in aquatic

speciesspecies

► Most copulate and there is often Most copulate and there is often selectivity on the part of the femaleselectivity on the part of the female

► Females often store sperm in a Females often store sperm in a seminal receptacleseminal receptacle

► Sperm transfer is direct in someSperm transfer is direct in some Penis may deliver sperm directly, or Penis may deliver sperm directly, or

serve as an intromittant organserve as an intromittant organ Spermatophores are frequently used Spermatophores are frequently used

for indirect transferfor indirect transfer

► Many brood and display parental careMany brood and display parental care