Phylum Arthropoda

3/4 of all animal species
Insects, spiders, lobsters, crabs, millipedes and centipedes

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Characteristics

Jointed appendages

Extensions of body (legs and antennae)
Segmented body with paired appendages
Lost or reduced in size in some species

Exoskeleton

Hard external covering
Protects internal organs
Provides structural support
3 layers -> Versatility and strength

Outer layer

Composed of proteins and chitin (carbohydrate)
Waxy outer layer repels water and prevents desiccation

Middle layer

Hardest; provides primary protection
Strengthened by materials (calcium)

Inner layer

Flexible joints -> Free movement

All 3 layers secreted by epidermis

Ventral nervous system
Open circulatory system
Digestive system
Specialized sensory receptors

Movement

Muscles occur in bundles
Attach to inside of exoskeleton on either side of joint
Alternate contraction and relaxation of muscles move body parts and appendages
Extensors and flexors

Growth

Exoskeleton cannot enlarge
Must be shed periodically (molting/ecdysis)
Tissue growth -> Pressure on exoskeleton walls -> Molting hormone released -> Epidermal cells -> Secrete enzymes which digest inner exoskeleton -> Epidermis synthesizes new exoskeleton (using digested exoskeleton) -> Outer layer of old exoskeleton loosens, breaks apart and sheds -> New exoskeleton stretches to fit enlarged body
Numerous molts -> Growth
Disadvantages

Vulnerable to predators (lack of hard shell)
Susceptible to desiccation
Must hide during molting

Evolution and Classification

Evolved more than 600 million years ago
Current studies suggest 4 separate lines of evolution
Differ in embryological development and morphology (appendages and mouth parts)
Ancestral arthropods

1 pair of appendages/segment
Many segments
Most modern forms have fewer

Tendency towards less segmentation
Segments fused -> Larger specialized structures

Subphyla Trilobita

Trilobites (extinct)
UCMP - Trilobita

Subphyla Crustacea

Shrimp, lobsters, crabs, barnacles, cladocerans, ostracoids, crayfish, water fleas, and copepods
Characteristics
- Branched antennae
- Mandibles (chewing mouth parts)
Encarta Online - Crustacea

Subphyla Chelicerata

Spiders, scorpions, ticks, mites, sea spiders, and horseshoe crabs
Characteristics
- Lack antennae
- Chelicerae (pincerlike mouth parts)
UCMP - Chelicerata
Encarta Online - Chelicerata

Subphyla Uniramia

Insects, centipedes, and millipedes
Characteristics
- Antennae
- Mandibles
- Unbranched appendages
UCMP - Uniramia

CRUSTACEA

Characteristics

Calcium carbonate exoskeleton
25,000 species
Crayfish, lobsters, pill bugs, sow bugs, water fleas and barnacles
Mostly aquatic
Encarta Online - Crustacea

Crayfish

External structure

Crayfish- external form
Encarta Online - Crayfish
Cephalothorax

Fused head and thorax
13 segments covered by carapace

Abdomen

7 segments
7th called telson
Flat triangle section at tail

Locomotion

Pair of appendages attached to each segment

Antennules

Appendages with receptors for touch, taste, and equilibrium

Antennae

Specialized for touch and taste

Mandibles

Move up and down to crush food

Maxillae

Move side to side to tear food

Maxillipeds

Jaws to hold food

Chelipeds

Claws that capture food and serve as defensive weapons

Walking legs

Enable the crayfish to walk slowly

Swimmerets

Create water currents
Function in reproduction

Uropods

Propel crayfish through water

Internal Structures

Crayfish- internal structure
Digestive

Food trapped by chelipeds -> Torn with maxillae and maxillipeds -> Chewed with mandibles ->
Esophagus-> Stomach (ground to fine paste with chitinous teeth) -> Mixed with digestive juices -> Nutrients absorbed by digestive glands -> Undigested wastes -> Intestines -> Anus
Crayfish- digestive and excretory systems

Excretory

Green glands
- Remove wastes from blood
- Retention of salts

Circulatory

Open circulatory system
Blood flows from dorsal sinus -> Ostia (small one way valves) -> Dorsal heart -> Seven large vessels that carry it through body -> Leaves vessels ->body cavity (bathes organs and cells) -> Collects in large ventral sinus -> Carried by vessels to gills -> Dorsal sinus
Crayfish- circulatory system

Respiratory

Exchange of gases in gills
Release CO2 and absorb O2
Gills attached to walking legs
Protected in chamber under carapace
Walking causes water to move across gills
Second maxillae uses gill bailers to move water during feeding

Nervous

Includes brain and ventral nerve cord
Nerves connect brain with sense receptors in antennules, antennae, and eyes
Crayfish- nervous system

Eyes

Set on movable stalks
Contain >2,000 lenses (compound eye)
Highly sensitive to light
Detects motion
Produces crude images

Statocysts

Contain particles of calcium carbonate
Particles change position
Position monitored by nerves and interpreted by brain

Reproduction

Mate in fall
Male uses 1st and 2nd swimmerets to transfer sperm to seminal receptacle of female
Sperm remain until spring
Fertilizes eggs and are released
Eggs stuck to last three pairs of female's swimmerets
Crayfish- reproductive system

Development

Hatch after 6 weeks
Several larval stages
Emerge as small crayfish
Repeatedly molt
7 times 1st year
2 times remaining life (2-3 years)

Ever want to eat a crayfish - you need to read this

Daphnia

Daphnia- external form
Daphnia- internal structure

Other Arthropods

Subphylum Chelicerata

spider_web.jpg (3568 bytes)

Class Arachnida

100,000 species
Encarta Online - Arachnida
Spiders, scorpions, and mites
Adapted to kill prey with poison glands, stingers, or fangs
Body divided into cephalothorax and abdomen
4 pair legs; 1 pair chelicerae and 1 pair of pedipalps (chewing)

Scorpions

Greatly enlarged forward positioned pedipalps
Large stinger
Nocturnal
Seize prey with pedipalps, inject venom, tear prey with chelicerae
Scorpion- external form
Scorpion- internal structure

Mites and ticks

Fused cephalothorax and abdomen
Pests
Disease vectors
- Rocky Mountain Spotted Fever
- Lyme Disease
Ticks (Iowa State Entomology Image Gallery)

Spiders

Structure and Function

Nervous, digestive and circulatory systems similar to crustacea

Respiratory

Terrestrial with unique respiratory system
Respire through openings in cuticle (spiracles) -> Book lungs (paired sacs in abdomen) -> Enlarged surface area
Tracheae - carry air directly to tissues

Excretory

Modified for land
Malpighian tubules (hollow projections of digestive tract)

Collect body fluids
Remove wastes -> intestine
Most water reabsorbed

Coxal glands - also remove wastes

Discharged at base of legs

Nervous

Eight simple eyes
No compound eyes

Spinnerets

Posterior tip of abdomen
3 pair (composed of hundreds of microscopic tubes)
Silk gland -> fluid -> tubes -> hardens into threads
Build webs, nests, and egg cocoons

Reproduction

Male gathers sperm in pedipalp sacs
Places sperm in females seminal receptacle
Female lays eggs
Fertilized by stored sperm
Eggs sealed in cocoon
Young spiders go through 1st molt inside cocoon

Spider- external form
Spider- internal structure

Subphyla Uniramia (Myriapods)

Class Diplopoda (Millipedes)

2 pair short legs/segment
Adapted for burrowing
Coil when threatened also excrete cyanide
Body round in cross section
Live in soil, logs, and under objects
Strong olfactory sense
Poor vision
Herbivorous - feed on plants and decayed matter

Class Chilopoda (Centipedes)

1 pair long legs/segment
Voracious predator
Feed on earthworms and insects
1st body segment has clawlike venomous appendages
Centipede- external form

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