Arthropods!
Ch 28 Presentation
Common Characteristics of all
Arthropods
• Typical arthropod:
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–
–
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Segmented
Coelomate with Bilateral symmetry
Invertebrate with exoskeleton
Jointed structures called appendages
• Appendage—any structure (e.g. leg, antenna) that grows out of
the body of an animal
• In arthropods, appendages are adapted for walking, sensing,
feeding, mating, etc.
• Arthropods are earliest known invertebrates to exhibit jointed
appendages
– Joints allow for flexibility of rigid exoskeleton (think knight in suit of
armor)
– Joints enable many uses for various appendages (e.g. mating in spiders,
stinging in scorpions)
Exoskeletons Support and Protect
• Exoskeleton—hard, thick outer covering of protein and
chitin
• In some species, exoskeleton is continuous over entire
body, in others it is separate plates held together by
hinges
• Exoskeleton supports internal tissues, provides places
for muscle attachment
• In many terrestrial species, exoskeleton covered with
waxy layer to minimize water loss
• In many aquatic species, exoskeleton reinforced with
calcium carbonate
Oh no, it’s MOLTING!
• Why molt?
– Exoskeletons are heavy and thick (the larger the muscles, the
heavier/thicker must the exoskeleton be), so weight is limited for
arthropods
• Terrestrial and flying arthropods have thinner, lighter (but more fragile)
exoskeletons but are less protected
• Molting—shedding the old exoskeleton (new soft skeleton
formed underneath first)
• Animal takes in air/water and contracts muscles, which swell
and split old skeleton
• Most arthropods molt 4-7x during lifetime before becoming
adults
• Mid-molt, animals are vulnerable to predators, so many hide
or remain motionless until their new skeleton hardens
•
http://youtu.be/2A1i10ZIB-w
http://youtu.be/ffo37DlH5DM
http://youtu.be/4QIgW639Oog
Segmentation
• Most arthropods are segmented but do not have as
many segments as worms
• In most groups, segments fused into three zones:
– Head, thorax, abdomen
• Sometimes head and thorax
fused together into
cephalothorax
Breathing
• Arthropods have efficient gas exchange—generally quick, active animals
with high oxygen requirements
• Three types of respiratory structures:
– Gills
– Tracheal tubes
– Book lungs
• In some arthropods, gas exchange occurs across exoskeleton (so it must be
thin and permeable)
• Aquatic arthropods use gills
• Land arthropods use either tracheal tubes or book lungs
• Most insects use tracheal tubes (branching networks of hollow air passages
that carry air throughout body)
• Air enters and leaves body through openings on thorax and abdomen called
spiracles
• Spiders and their relatives use book lungs, air-filled chambers with leaf-like
plates (maximize surface area for gas exchange)
• Tracheal Tubes and
Spiracles
•
•
Book Lungs
Book Lungs
Acute Sensory Perception
• Why can arthropods respond so quickly to their
environment?
– Strong muscular contraction
– Detect movement, chemicals and sound with
antennae
• Antennae have odor and sound receptors that are used in
communication with other members of species (spiders lack
these)
• Pheromones—chemical odor signals given off by animals
(signal animals to engage in variety of behaviors)
– Accurate vision: EYES
• Most arthropods have one pair of large compound eyes and
3-8 simple eyes
• Simple eyes- single lensed-structures for detecting light
• Compound eyes—visual structures with many lenses
– Each lens registers a tiny portion of the field of view, so the total
image is made of thousands of parts (panoramic view)
Eyes
What structures allow this damselfly to sense its
environment?
Well-developed Nervous Systems
• Arthropods process information from sense
organs with well-developed nervous systems
• Consists of double ventral nerve cord (pearl
necklace), an anterior brain, and several
ganglia http://youtu.be/0uTdTRXNdEY
Arthropod Circulation
• Open circulatory system—blood is pumped by
a heart, through vessels that bathe the tissues
of the body, returns to heart through open
spaces
Complex Digestive Systems
• Mouth, stomach, intestine, anus, glands that
produce digestive enzymes
Diversity of Mouthparts
• Mouthparts are grouped and adapted for a vast
variety of foods eaten by arthropods
– Mandibles for chewing, pinching, holding
– Pairs of appendages for moving food around while
chewing http://youtu.be/yBHqJrKtlso http://youtu.be/ZJyeFSq3STM
• Some insects have blade- or needle-like mouthparts
for piercing for drawing blood, etc., while others
have sponging tongues to lap up food, rolled-up
proboscises for sucking up food
Insect Mouthparts
• Most mouthparts of arthropods
include one pair of mandibles
(green) http://youtu.be/QOIJPG25gr0
– These are adapted for holding,
chewing, or biting various foods
• Labrum: upper-most mouthpart
(red)
– fused plate to hold food in place
during chewing
• Maxillae (yellow): hold and
manipulate food during
mastication
– Hairs and “teeth” along inner
ridge
– Palps used for sensing the
characteristics of potential foods
– Modified into proboscis or strawlike tube for sucking up food
http://youtu.be/MYWPWTme_YI
• Labium: “floor” of the mouth,
assists during chewing (blue)
• Hypopharynx: base of labium,
assists in swallowing
Crustacean Mouthparts
• Three pairs of appendages modified into
mouthparts that manipulate and bring food into
mouth (mandibles followed by two pairs of
maxillae) http://youtu.be/25F7xMVNt-w
– Paired appendages offer opposable surfaces for
grinding and biting food
– Often followed by setae used to collect food
Arachnid Mouthparts
• Spiders and scorpions have
chelicerae—pointed appendages
used to grasp food
– In spiders, they are hollow and
contain venom glands
Other well-developed systems
• Excretory: Malpighian tubules
that remove waste from
abdomen; attached to and
empty into the intestine
• Muscular: muscle is attached
to inner surface of exoskeleton
on both sides of joints
Sexual Reproduction
• Most arthropods have separate males and
females and reproduce sexually
– Fertilization usually internal, sometimes external in
aquatic species
• A few species hermaphroditic
– Barnacles
• Parthenogenesis—asexual reproduction in which
new individual develops from unfertilized egg (ex.
Bees, wasps, ants, aphids; males with only one X
chromosome)
• Reproductive diversity one reason why more
arthropod species than ALL the other animal
species combined
Arthropod Diversity
• Arachnidae: Spiders, scorpions, mites, and ticks
• Spiders have two body sections: cephalothorax, abdomen
• Six pairs of jointed appendages
• Chelicerae: first pair of appendages located near the mouth
• Modified into pincers or fangs, spiders have no
mandibles for chewing
• Pedipalps: second pair, adapted for handling food and
sensing food
• Arachnids have no antennae
• Spiders can make elaborate webs– though all spiders spin silk, not
all make webs
• Silk secreted in glands in abdomen, then spun together into
thread by spinnerets located at rear of spider
• Ticks, mites, scorpions: only one body section (head, thorax, and
abdomen completely fused
• Mites feed on fungi, plants, animals
• Small but can expand body size up to 3x original size
• Scorpions have long tail with venomous stinger at tip,
enlarged pincers
Arthropod Diversity
• Crustacea: body sections, two pairs of antennae for
sensing, two compound eyes, mandibles for
crushing food (ex. Shrimp, lobsters, crabs,
barnacles, crawdads
– Crustacean mandibles open and close side to side
– Many have five pairs of walking legs
• Used for walking, seizing prey, cleaning appendages
• First pair of walking legs often modified into strong claws for
defense
Arthropod Diversity
• Chilopoda: centipedes, Diplopoda: millipedes
– Have Malpighian tubules for excreting wastes
– Tracheal tubules for gas exchange
– Centipedes are carnivorous with painful bites
• May have 15-181 body segments, each with one pair of legs
– Millipedes don’t bite, eat mostly plants and dead material on
forest floors
• Spray foul-smelling fluid from stink glands as defense
• May have 20 to 100+ segments, with two spiracles and legs each
Arthropod Diversity
• Merostomata: Horseshoe crabs—
”living fossils”
– Three living genera, one (Limulus)
lives along North American East Coast,
two live in Asian tropics
– Limulus fossils unchanged since
Triassic Period 220 mya
– Heavily protected by extensive
exoskeleton; live in deep coastal
waters; forage for algae, annelids,
mollusks
– Migrate to shallow water in spring,
mate at night during high tide
Arthropod Diversity
• Insecta: the most successful class of
arthropods
• 3 body segments, six legs, more
species of insects than all other
animals combined
• Reproduction: usually once during
lifetime, internal fertilization
– Parthenogenesis: in aphids all female
offspring; in bees/ants/wasps all male
– Most lay large # of eggs, increases
chances of offspring survival to
reproductive maturity
Metamorphosis
• Metamorphosis—change in body shape from egg to
adult, controlled by series of chemical changes in
animal
– EggLarvaPupaAdult
– If insect goes through all four stages: complete
– If insect goes through some stages: incomplete (nymph
stage)
– Complete metamorphosis advantageous for arthropods
because larvae don’t compete with adults for the same food
Origins of Arthropods
• Successfully covered the whole surface of Earth
more than any other group of animals because of
their ability to survive in almost every habitat
– Exoskeletons, life cycles, high reproductive output, small
size, jointed appendages
• Most likely evolved from annelid ancestor
– Evolved fused body segments that adapted for
movement, sensing environment
– More complex than annelids (eyes, nervous tissue)
– Trilobites were major group of ancient arthropods, but
have been extinct 248 million years