Chapter 30Arthropods

Section 1Features of Arthropods

Jointed AppendagesPhylum Arthropoda = joint foot

AppendageStructure that extends from thearthropod’s body wallArthropod joints are able to bendLegs, antennae and mouth parts

Like annelids, arthropods have a

segmented body

Well-preserved fossil specimens

Arthropod DiversityTotal # of arthropod species is more than all other animal species combined.

Thought to be more than 5,000,000 species

Range in size from 1mm – 12 ft.

Arthropods divided into 2 groups:JawsFangs or pinchers

Arthropod Body PlanAll arthropods share numerous internal and external features

Not all arthropods share all of the same features below:

Jointed appendagesSegmentationDistinct head w/ compound eyesExoskeletonRespiration by gills, tracheae, or book lungsOpen circulatory systemExcretion through Malpighian tubulesWings

SegmentationOften exists during the larval stage

Butterfly

Segments usually fuse to form 3 body regions:

HeadThorax

Mid-body regionAbdomen

The head may fuse

with the thorax to

form the cephalothroax

Compound EyeComposed of multiple visual units

Each unit contains:Lens and retina

Allow them to sense

motion very quickly

Some arthropods also contain a single-lens eye that can only distinguish light from dark

Horizon detectors

ExoskeletonExternal skeleton (shell) composed of chitin

Thin and flexible where joints

are located

Chitin is a tough material, but

can be brittle

As arthropods grow their

exoskeletons become thicker to

withstand the pull of larger muscles

Molting

Periodic shedding of the exoskeletonAlso called ecdysisAllows arthropod to grow and develop

Triggered by hormone release

• New exoskeleton hardens with a few hours to a few days

Respiration

• Terrestrial arthropods– Utilize tracheae

• Network of fine tubes– Air enters body through spiracles which

passes it on to tracheae

• Valves of spiracles allow air in and help prevent water loss– Key adaptation to live on land

Excretion• Arthropod excretion conserves water and

eliminates metabolic waste

• Malpighian tubules– Finger-like extensions from the arthropod’s gut

that are bathed in blood

• Process of excretion:– Water and other particles move through the

tubules and into the gut– These particles are reabsorbed into body tissues– Waste remains in the gut until expelled from the

body

Section 2Spiders and Other Arachnids

Arachnid Modifications• Spiders, Scorpions, Ticks, Mites, Daddy longlegs

• Generally do more good than harm

• Chelicerae– Mouthparts modified into pinchers or fangs– 1st pair of appendages

• Pedipals– Modified to catch and handle prey– 2nd pair of appendages

• All except mites are carnivores

• Only able to consume liquid food

Spiders• Chelicerae modified into fangs

• Poison glands secrete toxins

through fangs– Toxins kill or paralyze fangs

• Enzymes are then injected to

digest tissues into liquids

• 2 dangerous Species in the U.S.:– Black widow– Brown recluse

• Spinnerets– Sticky strands of silk

Scorpions and Mites

Chelicerae and pedipalps are modified differently

Scorpions

• Long, slender, segmented abdomens– Abdomen ends in a venomous stinger

• Used to stun prey

• Pedipalps– Large, grasping pinchers– Used for capturing food– Used during reproduction

Mites• Largest group of arachnids

– Includes chiggers and ticks

• Head, thorax and abdomen are fused– Unsegmented bodies

• Aquatic mites– herbivores

• Terrestrial mites– carnivores

• Mites can affect plants and animals– Mites can pass viral and fungal infections to plants– Lyme disease is spread by infected deer ticks

Section 3Insects and Their Relatives

Insect Diversity• Terrestrial arthropods have chewing

mouthparts (Mandibles) – Insects, millipedes, and centipedes

• Insects are the largest group on earth– 700,000 named species– Many to still be discovered in the tropics

• 50% of all animal species are insects

Insect Body Plan• Head

– Mandibles (specialized mouthparts)– 1 pair of antennae– Large compound eyes

• Thorax– 3 fused segments– 3 pairs of jointed walking legs– 1 or 2 pairs of wings

• Abdomen– 9-11 segments– No legs– No wings

Insect Life Cycle• Metamorphosis

– The last molt a young insect undergoes– Dramatic changes

• Complete metamorphosis– Eliminates competition between adults and young– Chrysalis

• Protective capsule that encloses the wingless, wormlike larva of a young insect

– Pupa• Stage passed through while inside the chrysalis as it changes to

adult form

• Incomplete metamorphosis– Egg hatches into juvenile– Nymph

• Small wingless adult• Develops through several molts

Flight• Insects were first animals to have wings

– Allowed for greater evolution

• Wings composed of chitin– Protrude from thorax– Strengthened by veins– Fold over abdomen

• Some insects are wingless– Fleas and lice

• Usually only 1 pair of wings are used for flight

Social Insects

• Hymenoptera (ants, bees and wasps) and Isoptera (termites) have elaborate social systems– Many genetically related animals living

together

• Caste– Role played by a particular member

Insect Relatives• Centipedes and millipedes have similar

bodies to insects– Head region followed by segments

• Differ in number of legs

• Centipedes– Carnivores

• Millipedes– Herbivores

Section 4Crustaceans

Crustacean Habitats• Found throughout the world’s waters (oceans)

– “Insects of the sea”– Fresh or saltwater

• Naupilus– Larval stage of many crustaceans– 3 pairs of branched appendages– Undergoes a series of molts before becoming adult

• Mandibles

• Differences– p. 680 – Table 2

Terrestrial Crustaceans

• Pill bugs and sow bugs– Only true terrestrial crustaceans

• Found in leafy ground litter– Gardens and wooded areas

• Land crabs slightly terrestrial– Life cycle tied to the ocean

Aquatic Crustaceans

• Key food source in many food chains– Humans and animals

• Krill– One of the main food sources for many

marine species

Decapods• Shrimps, lobsters, crabs, crayfish

• Five pairs of legs

• Head and thorax are fused– cephalothorax– Protected by carapace

• Chelipeds– pinchers

• Appendages called swimmerets

• Uropods– Paddle-like appendage at end of abdomen

Sessile Crustaceans

• Barnacles– Sessile as adults– Hard plate protects adults– Feathery legs stir food into the mouth– Hermaphrodites

• Do not fertilize their own eggs