Chapter 30
Arthropods
Section 1
Features of Arthropods
Jointed Appendages
Phylum Arthropoda = joint foot
Appendage
Structure that extends from the
arthropod’s body wall
Arthropod joints are able to bend
Legs, antennae and mouth parts
Like annelids, arthropods have a
segmented body
Well-preserved fossil specimens
Arthropod Diversity
Total # 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:
Jaws
Fangs or pinchers
Arthropod Body Plan
All arthropods share numerous internal and external
features
Not all arthropods share all of the same features below:
Jointed appendages
Segmentation
Distinct head w/ compound eyes
Exoskeleton
Respiration by gills, tracheae, or book lungs
Open circulatory system
Excretion through Malpighian tubules
Wings
Segmentation
Often exists during the larval stage
Butterfly
Segments usually fuse to form 3 body regions:
Head
Thorax
Mid-body region
Abdomen
The head may fuse
with the thorax to
form the cephalothroax
Compound Eye
Composed 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
Exoskeleton
External 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 exoskeleton
Also called ecdysis
Allows 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 2
Spiders 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
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
Spiders
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 3
Insects 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 4
Crustaceans
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