What are ARTHROPODS?
• Coelomate
• Segmented
• Bilateral Symmetry
• Exoskeleton – made of protein and chitin
• Jointed appendages – any structure (leg or antennae) that grows out of the body
• earliest invertebrates to exhibit jointed appendages– Jointed appendages are an advantage
because they allow more flexibility for animals with hard, rigid exoskeletons
– Joints allow powerful movements and allow appendages to be used in many ways
What are Arthropods?
What are Arthropods?
Appendages
Jointed Appendages
• made up of protein and CHITIN
• can be a continuous
covering over most
of body OR made of plates
that are held together by hinges
Exoskeletons Provide Protection
Continuous
Hinged
• Protects, supports internal tissues
• Provides place for muscle attachment
• Aquatic arthropods have exoskeleton reinforced with calcium carbonate
Exoskeleton Advantages
Exoskeleton Disadvantages
• Heavy– the larger the animal, the thicker and
heavier the exoskeleton
• Exoskeletons don’t grow– animals must molt when they get too large
for exoskeleton
Molting• Animal contracts muscles and takes in
air or water
• Body swells and causes exoskeleton to split open, usually along the back
• Most arthropods will molt 4-7 times before becoming an adult.
• increased circulation to all parts of the body cause the animal to puff up and new exoskeleton hardens leaving some “growing room”
• animal can’t protect itself, can’t move
Before the new exoskeleton hardens...
Question 1
Which of the following organisms would be most likely to have an exoskeleton reinforced with calcium carbonate?
A. Spider
B. Beetle
C. Crab
D. Dragonfly
Correct!
Question 2
• Exoskeletons are heavy. Why can aquatic arthropods grow so much larger than terrestrial arthropods?
The buoyancy of the water helps support the weight of the exoskeleton
Question 3
• What is one advantage and one disadvantage of flying arthropods having a thinner, lighter weight exoskeleton?
Disadvantage: less protection
Advantage: greater freedom to fly and jump
Question 4
• What is one advantage and one disadvantage of having a cephalothorax?
Disadvantage: less flexibility, mobility
Advantage: more protection
Segmentation
• 3 segments – abdomen – thorax – head
• Sometimes these segments can be fused together– some have head and fused thorax and
abdomen– some have abdomen
and fused head and
thorax (cephalothorax)
Segmentation
Respiration
• Efficient respiratory systems to meet large O2 demands
• Large O2 demand needed to sustain high metabolism for fast movements
• 3 types of respiratory structures– gills (aquatic arthropods)– tracheal tubes (terrestrial arthropods)– book lungs (terrestrial arthropods)
• Gills– water moves over gills– O2 from water diffuses
into gills and into
bloodstream– CO2 from body diffuses out through gills
into surrounding water
Respiration
• Tracheal tubes– branching network of hollow air passages
that take air throughout the body
Respiration
Muscle movement brings air in/out through SPIRACLES (openings in abdomen and thorax)
• Book lungs– spiders and relatives– air filled chambers with leaf-like plates– stacked plates
are arranged
like pages
of a book
Respiration
Antennae
• Acute sensing by antennae– stalk like structure that can detect changes
in the environment• movement• sound • chemicals
Used for sound and odor communication
Eyes• Compound Eyes
– visual structure with many lenses
• Simple Eyes– visual structure with one lens for detecting
light
one pair of compound eyes and 3-8 simple eyes
Nervous System
• Double ventral nerve cord
• Anterior brain
• Several fused ganglia that control the body section they are located in
Circulatory System
• Open circulatory system– blood flows away from the heart in vessels– blood flows out of vessels into tissues– blood returns to the heart through open
spaces
Digestive System• Complete digestive system with mouth,
intestine, and anus
• Mouth has 1 pair of jaws called MANDIBLES– adapted for holding, chewing, sucking, or
biting
Reproduction – Sexual and Asexual
• Sexual reproduction– separate sexes– internal fertilization for terrestrial species– external fertilization for aquatic species
• Asexual reproduction– PARTHENOGENISIS
• a new individual develops from an unfertilized egg
• seen with ants, aphids and bees
Reproduction – Sexual and Asexual
Arachnids
• spiders (largest group), ticks, mites, and scorpions
• 2 body regions: cephalothorax and abdomen
• 6 pairs of jointed appendages – 12 total appendages!
• 1st pair - chelicerae, are near the mouth
• modified into pincers (hold food) or fangs (inject poison)
Arachnids
chelicerae
• 2nd pair – pedipalps, for handling food and sensing
Arachnids
pedipalps
• Silk, for webs, is secreted by silk glands in the abdomen
• as it is secreted, it is spun into thread by SPINNERETTES
• spiders are predatory and feed almost exclusively on other animals
Arachnids
• Ticks and mites have only 1 body section
• Head, thorax and abdomen are completely fused• Ticks feed on blood of other animals
Arachnids
• Mites feed on fungi, plants, and animals
• small – not usually visible
• can transmit diseases
Arachnids
Dust mites
• Scorpions have many abdominal body segments
• Enlarged pincers
• Long tail with
venomous stinger
at the tip
Arachnids
Crustaceans• crabs, lobster, shrimp, crayfish,
barnacles
• Only arthropods with 2 pairs of antennae
• mandibles – move
from side to side
• 2 compound
eyes
• 5 pairs of walking legs
• 1st pair are claws for defense
Crustaceans
claw legs
• Most are aquatic and use gills
• pill bugs (roly-polies) live on land, but must have moisture to aid in gas exchange
Crustaceans
Yes! This is a crustacean!
• Centipedes are carnivorous – eat soil arthropods, snails, slugs, and worms
• Bites can be painful• Millipedes – eats plants and dead
material on damp forest floors• Does not bite, but does
spray foul-smelling fluid
Centipedes and Millipedes
Horseshoe Crabs
• Class Merostomata
• “Living Fossils”- unchanged for 220 million years (Triassic period)
• Extensive exoskeleton
• Live in deep coastal waters
• forage bottoms for algae,
annelids and molluscs
Insecta
• Flies, grasshoppers, lice, butterflies, beetles
• 3 body segments
• 6 legs
• Very diverse - more insects than all other classes of animals combined
• mate once in lifetime
• internal fertilization
• some exhibit
parthenogenesis
• large number of eggs
to increase survival rate
Insecta
• insect embryos develop inside eggs, eggs hatch
• some look like miniature adults– will molt
several times until adult size
Insecta
EggsNymph
Molt
Nymph
Molt
Adult
• INCOMPLETE METAMORPHOSIS– 3 stages: egg, nymph, adult
Insecta
• Nymphs can’t reproduce
• Nymph gradually becomes an adult
• Some undergo COMPLETE METAMORPHOSIS– 4 stages: egg, larva,
pupa, adult
• Metamorphosis is controlled by chemical substances in the insect
Insecta
Egg
Larva
Pupa
Adult
Insecta
• Incomplete metamorphosis: grasshoppers and cockroaches
• Complete metamorphosis: ants, beetles, flies, wasps
Origins of Arthropods
• Successful because of
– varied life styles
– high reproductive output
– structural adaptations
– hard exoskeletons
– jointed appendages
• Hard exoskeletons fossilize – a lot is known about evolutionary history
• Evolved from ANNELIDS (segmented worms)
• Arthropods have more complex segments, more developed nervous systems
• circular muscles in annelids do not exist in arthropods
Origins of Arthropods
Question 5
Spiders are:
a. predators
b. scavengers
c. decomposers
d. parasites
Predators!
Question 6
Having 2 pairs of antennae distinguish _________ from other arthropods.
a. centipedes
b. millipedes
c. crustaceans
d. horseshoe crabs
Crustaceans!
Question 7• Why are horseshoe crabs called “living
fossils?”
They remain unchanged after 220 million years!
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