Arthropods
Characteristics
• Between 70-85% of all names species are
Arthropods
• The majority of arthropods are insects
• Segmented invertebrates with bilateral
symmetry and a coelom body cavity
Segmentation
• Arthropods exoskeleton is broken into 2-3
segments
– Head
– Thorax
– Abdomen
• Head is for feeding and multiple types of eyes
Segmentation
• Thorax is the middle body region, in many
arthropods legs or wings are attached.
• Abdomen is the posterior end which has additional
legs and contains digestive and reproductive
structures
Segmentation
• Cephalothorax- is where the head and thorax
are fused together into one piece
Exoskeleton
• Arthropods have an exoskeleton made out of
chitin
• The thickness and hardness of the exoskeleton
can vary among species
Jointed appendages
• Arthropods have paired appendages
• Appendages are structures such as legs and
antennae, that grow and extend from the
animals body
Molting
• Because the exoskeleton is made out of
nonliving material it can not grow so they
must shed it.
• The process of shedding the exoskeleton is
called molting
Body structure
• Arthropods have complex organs systems that
enable them to live in diverse habitats
– Respiratory
– Circulatory
– Excretory
– Nervous
Feeding and digestion
• The mouthparts of most arthropods include a
pair of appendages called mandibles that can
bite and chew material
• They have a complex one way digestive tract
Respiration
• Arthropods obtain oxygen by using one the
three following structures
– Gills (aquatic arthropods have these- lobsters)
– Tracheal tubes
– Book lungs
Respiration
• 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
Muscle
movement
brings air
in/out
through
SPIRACLES
(openings in
abdomen and
thorax)
Respiration
• Book lungs
– spiders and relatives
– air filled chambers with leaf-like plates
– stacked plates
are arranged
like pages
of a book
Circulation/excretion
• Most arthropods don’t rely on their circulatory
system to move oxygen but to move nutrients
and waste
• Cellular waste is removed from the blood
through malpighian tubules
Response to stimuli
• Vision- Most arthropod have compound eyes
that have many facets
• Hearing- Tympanum is a flat membrane used
for hearing
Response to stimuli
• Chemicals- many insects use pheromones to
communicate with one another.
Movement/reproduction
• Insects have well developed muscular systems
– Some insects can carry 50 times their own body
weight
• Most reproduce sexually and have individual
sexes some are hermaphrodites
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
Reproduction – Sexual and Asexual
• Asexual reproduction
– PARTHENOGENISIS
• a new individual develops from an unfertilized
egg
• seen with ants, aphids and bees
Arachnids
• spiders (largest group), ticks, mites, and
scorpions
• 2 body regions: cephalothorax and abdomen
• 6 pairs of jointed appendages – 12 total
appendages!
Arachnids
• 1st pair - chelicerae, are near the mouth
chelicerae
• modified into pincers (hold food) or fangs (inject
poison)
Arachnids
• 2nd pair – pedipalps, for handling food and
sensing
pedipalps
Arachnids
• 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
Dust mites
Arachnids
• Scorpions have many abdominal body
segments
• Enlarged pincers
• Long tail with
venomous stinger
at the tip
Crustaceans
• crabs, lobster, shrimp, crayfish, barnacles
• Only arthropods with 2 pairs of antennae
• mandibles – move
from side to side
• 2 compound
eyes
Crustaceans
• 5 pairs of walking legs
• 1st pair are claws for defense
claw
legs
Crustaceans
• Most are aquatic and use gills
• pill bugs (roly-polies) live on land, but must
have moisture to aid in gas exchange
Yes! This is a
crustacean!
Centipedes and Millipedes
• 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
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
Insecta
• mate once in lifetime
• internal fertilization
• some exhibit
parthenogenesis
• large number of eggs
to increase survival rate
Insecta
Nymph
Molt
Eggs
Nymph
Molt
Adult
• insect embryos
develop inside
eggs, eggs
hatch
• some look like
miniature
adults
– will molt
several times
until adult size
Insecta
• INCOMPLETE METAMORPHOSIS
– 3 stages: egg, nymph, adult
• Nymphs can’t
reproduce
• Nymph
gradually
becomes an
adult
Insecta
• Some undergo
COMPLETE
METAMORPHOSIS
Adult
Egg
– 4 stages: egg, larva,
pupa, adult
• Metamorphosis is
controlled by
chemical substances
in the insect
Larva
Pupa
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
Origins of Arthropods
• 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