10-Arthropod (2014_07_01 18_28_38 UTC)

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Phylum Arthropoda

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Arthropod Taxonomy: Overview

The arthropods evolved along four main lines, which most

zoologists recognize as 4 distinct subphyla

1. Trilobita - extinct trilobites

2. Chelicerata - horseshoe crabs, spiders, ticks, mites, and someextinct groups

3. Crustacea - crabs, lobsters, shrimps, barnacles

4. Uniramia - insects, centipedes, millipedes

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The Arthropod Exoskeleton

• Epidermis secretes an external skeleton called the exoskeleton • Advantages of possessing an exoskeleton:

– provides strong support

– provides rigid levers that muscles can attach to and pull

against – offers protection

– serves as a barrier to prevent internal tissues from drying out;

important because many arthropods live on land

– serves as a barrier to prevent infection

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Structure of the Exoskeleton

• Composed of the polysaccharide chitin and protein - glycoprotein

• Outer surface called the epicuticle; contains waxes

• The thicker portion is called the procuticle:

• exocuticle

• endocuticle

• In the exocuticle, the glycoprotein chains are cross linked;

process is called tanning

epicuticle

exocuticle

endocuticle

epidermis

procuticle

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Molting

• In order to grow the arthropod must shed its exoskeleton,

and secrete a new and larger one - molting or ecdysis.

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Jointed Appendages • Exoskeleton divided into a number of plates and cylinders

• At the junction point between plates and cylinders, the exoskeleton remains

thin and flexible; these are the joints

• Jointed appendages allows arthropods to move efficiently and quickly• Muscles are integral to arthropod movement; they attach to the inner side of

the exoskeleton; they often function as a lever system

Vertebrate joint

Arthropod joint

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Specialized Arthropod Segments: Reduction in Metamerism

• The evolution of the arthropods witnessed a reduction in

metamerism• The arthropods evolved modified groups of segments (e.g.,

segments became lost, some fused together

•The fusion of groups of segments into functional groups is called

tagmatization • In so doing, various appendages on segments became specialized

for functions other than locomotion, e.g. prey capture, filter feeding,

sensing various kinds of stimuli, gas exchange, copulation, etc.

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Arthropod Respiratory Advances

• Special respiratory structures allow the arthropods to

metabolize more efficiently and thus move rapidly

•

High metabolic rates require rapid oxygen delivery, andarthropods can accomplish this with respiratory organs

that have a large surface area for collecting oxygen

quickly

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Gills

• Many aquatic arthropods (crabs and lobsters) have gills, which are

typically modifications of appendages or outgrowths of the body wall- folds of tissue with a large surface area

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Tracheae

• Gas exchange organs among terrestrial arthropods is usually

internal; invaginations of the integument

• Insects have

tracheae, branching

networks of hollow air

conducting tubes such

that air is sent to everycell in every tissue

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Book Lungs

• Spiders have book lungs, chambers with leaf-like plates for

exchanging gases; air flows over the plates and blood flows throughthem

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Acute Senses

• Arthropods have a well-developed nervous system that is of the

same overall design as the annelids; anterior brain and a double,

ventral hollow nerve cord.

• The sensory receptors of arthropods are usually associated with

modifications of the chitinous exoskeleton

• The head usually bears various kinds of sense organs (e.g.

antennae) with extreme sensitivity

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Acute Senses cont.

•Many arthropods have compound eyes - eyes that are composed of

many visual units called facets (ommatidia); capable of color vision

and detecting the slightest movements of prey or predators• Some eyes are simple eyes with only a few photoreceptors; however,

they are capable of forming crude images

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Digestive System Divided into 3 main regions: foregut, midgut, and hindgut

• Foregut and the hindgut are lined with chitin

• Foregut is involved with ingestion, mechanical breakdown, and

storage

• Hindgut is involved with water absorption and formation of the feces

• Midgut is not lined with chitin; involved with digestion and

absorption

• Outpockets (e.g. digestive glands) increase the surface area fordigestion and absorption

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Internal Transport and Excretion

• Open circulatory system

• Many crustaceans possess an excretory organ

called the green gland (antennal gland), which

filters fluid from the blood

• Most insects and spiders have a excretory

system called malpighian tubules

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Reproduction

• Sexes are separate; fertilization is external in aquatic forms,

internal among the terrestrial forms

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Arthropod Diversity

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Subphylum Trilobita

• A group of extinct marine arthropods.

• Ranged in size from a few millimetersup to 75 centimeters.

• Body divided into three parts:

– cephalon (head), a single plate

made up of several fused segments – thorax, consists of a number of

segments hinged together

– pygidium (tail), also segmented,

but like the head, fused together

into a single plate.

• Compound eye, composed of radially

arranged visual units

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C i ( )

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Class Meristomata (e.g., Horeshoe crabs)

• Marine chelicerates, common off of the Atlantic and Gulf coasts

• Large dorsal carapace bearing compound eyes

• Possess chelicerae, pedipalps, and 4 pair of walking legs (all but

the last pair are chelate

• Abdomen terminates in a long tail called the telson; used to turn

the animal right side up

• Possess a series of gill plates called book gills

Cl A h id

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Class Arachnida • Among spiders, the cephalothorax and abdomen shows no external

segmentation; tagma are joined by a narrow pedicel

• Respiration is accomplished via book lungs, tracheae, or both

• Usually have 8 simple eyes; at the very least they detect motion; for some ofthe predatory forms, they are capable of forming crude images

• Many species have evolved poison glands associated with the chelicerae

Black widow

Brown recluse

Class Arachnida cont

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Class Arachnida cont.

• Many of the spiders and mites

are capable of producing silk ;

produced by silk glands thatopen to the exterior part of the

abdomen through spinnerets

• Silk is used to build webs for

trapping prey, nests which serve

as retreats, and egg cases; it is

also used to form a dragline

• Most spiders are predaceous

and have all kinds of sensory

hairs and relatively well-developed eyes for motion

detection

Orb web construction

P t th id

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Prey capture among the spiders

• Some species are cursorial predators, those

that stalk and ambush their prey; they usually

have well-developed eyes

•Others are web building spiders, those

that construct various kinds of webs made

of silk to trap their prey

• Eyes are not as well developed as cursorial

predators, but they have a battery of sensory

hairs for detecting vibrations

Grass spider

Jumping spider

S bPh l C t

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SubPhylum Crustacea • Possess 2 pair of antennae: First pair is homologous to those of

insects; second pair is unique to the crustaceans

• Second antennae have various functions, including sensory,

locomotion or feeding.

• The head bears a pair of compound eyes and 3 pairs of mouthparts: a

pair of mandibles, and 2 pairs of maxillae; used for food handling

• Trunk varies considerably among

classes

• Primitively, the first three pairs of

thoracic segments are maxillipeds;

function in handling food

• Also, there are usually 5 pairs of

appendages strengthened for

walking (walking legs) and

protection (chelipeds, pincer-like

claws)

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SubPhylum Crustacea cont

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SubPhylum Crustacea cont.

•Primitively many of the appendages of the crustaceans are

biramous: there is an outer exopod and an inner endopod

• They usually have an extremely hardened exoskeleton, which is

impregnated with calcium carbonate - carapace

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C t Di it

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Crustacean Diversity

Copepod

Ostracod

Water flea

Fairy shrimp Fish louse

SubPhylum Uniramia

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SubPhylum Uniramia

• Single pair of antennae

• First pair of feeding appendages are mandibles

• There are one or 2 pairs of maxillae

• Number of legs vary from 3 pair to many pairs; they are unbranched

or uniramous

SubPhylum Uniramia cont

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SubPhylum Uniramia cont.

Class Chilopoda (Centipedes)

• Serial segmented, flattened body and each segment has a pair of jointed

appendages• Active predators, killing their prey with poison claws, which are

modified legs on first segment

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Class Insecta

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Class Insecta • Body is divided into 3 parts: the head, thorax and the abdomen.

• Head has one pair of antennae, a pair of compound eyes and

several sets of simple eyes

• Mouthparts: a pair of mandibles, and two pairs of maxillae

• One pair of maxillae are fused together to form a lower lip -

labium

• An upper lip - labrum - formed from an extension of the head

Class Insecta cont.

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Class Insecta cont.

• Mouthparts are highly modified depending on the group you are

discussing

• Mosquitoes have pointed mouthparts for piercing and sucking;

grasshoppers have mouthparts that are well adapted for chewing; butterflies for siphoning; flies for sponging

Class Insecta con’t

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C ss sec co

• Thorax is composed of 3 segments and each one has a pair of legs;

the last two segments also have a pair of wings.

• Wings of insects are modified portions of the exoskeleton

• 1st pair is usually tough and leathery and fold over the inner pair for

protection.

• Abdomen does not have appendages; terminal portions do harbor the

reproductive structures

Most insects undergo metamorphosis

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Most insects undergo metamorphosis

Incomplete Metamorphosis (Hemimetabolous)

• Early developmental stages are very similar to the adults; only the

wings and the reproductive structures gradually develop• The immature stages are called nymphs

• Thus development is egg----> nymphs ----> adult

Complete Metamorphosis (Holometabolous)

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Complete Metamorphosis (Holometabolous)

• Each of the

developmental stages is

structurally andfunctionally very

different

• The egg develops into

an immature larva; eats

voraciously

• Followed by a

transitional stage -

pupa , contained within

cocoon• Metamorphosis occurs

within the pupal

exoskeleton, yielding a

ll t d lt

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10-Arthropod (2014_07_01 18_28_38 UTC)