Chapter 28:
Arthropods
Section 1:
Introduction to Arthropods
Introduction to Arthropods
• Arthropod – animal having a segmented body,
an exoskeleton containing chitin, and a series
of jointed appendages; belong to the phylum
Arthropoda
– More than 1 million species have been
described
– Vary enormously in size, shape, and habits
Diversity and Evolution in Arthropods
• Arthropods are divided into four subphyla:
– Trilobita
• Oldest subphylum of arthropods
• Trilobites were dwellers in ancient seas
– Extinct
– Chelicerata
• Chelicerates include spiders, ticks, mites, scorpions, and
horseshoe crabs
– Crustacea
• Crustaceans include such familiar (and edible) organisms
as crab and shrimp
– Uniramia
• Uniramians include most arthropods: centipedes,
millipedes, and all insects—including bees, moths,
grasshoppers, flies, and beetles
Spotted Cleaner Shrimp
Flat Rock Scorpions
Form and Function in Arthropods
• All arthropods exhibit several key features
– 3 most important features
• Tough exoskeleton
• Series of jointed appendages
• Segmented body
– Other characteristics
• Brain located in dorsal part of head
• Ventral nerve cord
• Open circulatory system powered by a single
heart
California Spiny Lobster
Deer Tick
The Arthropod Body Plan
• The exoskeleton is a system of external supporting
structures that are made primarily of the
carbohydrate chitin
• Provide excellent protection from physical damage
• Waterproof (terrestrial)
– Prevents water loss
• Helps arthropods move efficiently and adapt to their
environment
• Disadvantages
– Cannot grow as the animal grows
– Movement can occur only at the joints of the
“armor”
The Arthropod Body Plan
• All arthropods have jointed appendages that
enable them to move
• Over millions of years, appendages have
evolved into marvelously versatile adaptations
to different environments
– Antennae
– Claws
– Walking legs
– Wings
– Flippers
• All arthropods have segmented bodies
Feeding
• Appendages enable these animals to eat almost
any food you can imagine
• Exhibit every mode of feeding
– Herbivores
• Some eat any plant, others are more
selective
– Carnivores
• Some catch and eat other animals, others
feed on animals that are primarily already
dead
Feeding
– Parasites
• External parasites drink blood or body
fluids from animals, internal parasites
passively absorb nutrients through
body walls
– Filter feeders
• Use comb like bristles on mouthparts
or legs to filter plants and animals from
water
– Detritus feeders
Lubber Grasshopper-Herbivore
Praying Mantis-Carnivore
Respiration
• Have evolved three basic types of respiratory
structures—gills, book gills and book lungs,
and tracheal tubes
– Most have only one of these respiratory
structures
• Aquatic arthropods have gills just under
the cover of their exoskeleton
–Are formed from part of the same
appendages that form mouthparts and
legs
Respiration
• Book gills (horseshoe crabs) and book lungs
(spiders and their relatives) are unique to these
arthropods
– Several sheets of tissue are layered like the
pages of a book
– Increase the surface area for gas exchange
• Tracheal tubes (terrestrial arthropods)
– From spiracles, long branching tracheal tubes
reach deep into the animals’ tissues
– Supplies oxygen by diffusion to all body
tissues
– Work well in only small animals
Fiddler Crab--Gills
Internal Transport
• A well-developed heart pumps blood through
an open circulatory system
• When the heart contracts, it pumps blood
through arteries that branch into smaller
vessels and enter the tissues
• There the blood leaves the vessels and moves
through spaces in the tissues called sinuses
• Eventually, the blood collects in a large cavity
surrounding the heart, from which it re-enters
the heart through small openings and is
pumped around again
Excretion
• Undigested food becomes solid waste that
leaves through the anus
• Most terrestrial arthropods dispose of nitrogencontaining wastes by using a set of Malpighian
tubules
– Bathed in blood inside the body sinuses
– Remove wastes from the blood, concentrate
them, and then add them to undigested food
before it leaves via the anus
Excretion
• Terrestrial arthropods may have small excretory
glands by their legs instead of Malpighian tubules
• Aquatic arthropods dispose of cellular wastes by
diffusing them from the body into the surrounding
water
– Also eliminate nitrogen-containing wastes
through a pair of green glands located near the
base of the antennae
• Are emptied to the outside through a pair of
openings in the head
The diamond beetle uses
Malpighian tubules to get rid of
nitrogen-containing wastes,
whereas the hermit crab uses its
green glands and gills.
Response
• Have well-developed nervous systems
• All have a brain that consists of a pair of
ganglia in the head
• Have a nerve cord with several other pairs of
ganglia
– Serve as local command centers to
coordinate the movement of legs and wings
• Why insects can move after their heads
are cut off
Response
• Have simple sense organs such as statocysts and
chemical receptors
• Have compound eyes that contain more than 2000
separate lenses that can detect color and motion
• Have well-developed sense of taste
– Receptors located on mouthparts, antennae, and legs
• Insect ears are found in strange places
– Eardrums are normally located behind their legs
Response
• Sense organs help it detect and escape predators
• Some also have venomous stings and bites
• Some arthropods trick predators by creating a
diversion
– Drop a claw or a leg
• Body part keeps moving to distract the predator
while the rest of the animal scurries away
• Use visual trickery to fool predators
• Hide through camouflage
• Others imitate the warning coloration of poisonous or
dangerous species—mimicry
The Band-Eyed Brown Horsefly
has huge compound eyes
A harlequin beetle uses its
long antennae as “feelers.”
The red hourglass on the
abdomen of the black widow
spider warns of the spider’s
venomous bite.
Movement
• Have well-developed muscle systems that are
coordinated by the nervous system
• Muscle generate force by contracting, then
transfer that force to the exoskeleton
• At each joint, some muscles are positioned to
flex the joint and other muscles extend to it
– Allows arthropods to beat their wings
against the air to fly, push their legs against
the ground to walk, or beat their flippers
against the water to swim
Reproduction
• Reproduction is simple
• Males and females produce sperm and eggs
• Fertilization takes place inside the body of the
female
• The male uses a special reproductive organ to
deposit sperm inside the female
• EXCEPTION: In spiders and some
crustaceans, the male deposits a small packet
of sperm that the female picks up
Growth and Development in
Arthropods
• Exoskeletons present a problem in terms of
growth
• Arthropods must replace their exoskeletons
with larger ones in order to allow their
bodies to increase in size as they mature
• In order to grow, all arthropods must molt,
or shed, their exoskeletons
– Controlled by hormones
Growth and Development in
Arthropods
• When molting time is near, the epidermis
digests the inner part of the exoskeleton and
absorbs much of the chitin
• After its secretes a new exoskeleton inside the
old one, an arthropod pulls completely out of
its old skeleton
– Often eat what is left of the old exoskeleton
– Must then wait for the new exoskeleton to
harden
• Can take a few hours to a few days
Growth and Development in
Arthropods
• Molt several times between hatching and
adulthood
• In most cases, the process of growth and
development involves metamorphosis, or a
dramatic change in form
• As the young grow, they keep molting and
getting larger until they reach adult size
• Along the way, they gradually acquire the
characteristics of adults
Growth and Development in Arthropods
• Many insects undergo complete metamorphosis—a
four stage process where the eggs of insects hatch into
larvae that look nothing like their parents
• As they grow, they molt repeatedly
• When it reaches a certain age, it sheds its larval skin
one last time and becomes a pupa
– During this stage the insects body is completely
rearranged
– After metamorphosis, the animal emerges as a fully
grown adult with both internal and external body
parts that are completely different from what it had
before
Incomplete
Metamorphosis
Complete
Metamorphosis
Chapter 28:
Arthropods
Section 2:
Spiders and Their Relatives
Spiders and Their Relatives
• Belong to the subphylum Chelicerata
– Arthropods that are characterized by a
two-part body and mouthparts called
chelicerae
– Lack sensory feelers
– Body is divided into two parts
Spiders and Their Relatives
• Cephalothorax
–Anterior end contains the brain, eyes,
mouth and mouthparts, and
esophagus
–Posterior end carries the front part of
the digestive system and several pairs
of walking legs
• Abdomen
–Contains most of the internal organs
Spiders and Their Relatives
• All chelicerates have two pairs of
appendages attached near the mouth that
are adapted as mouthparts
– 1st pair – chelicerae
– 2nd pair – pedipalps
A jumping spider captures its prey by
pouncing on it, rather than by
catching it in a web
The internal structures of a typical spider
Horseshoe Crabs
• Among the oldest chelicerates
• Not true crabs
• Appeared in the Ordovician Period (more than
430 million years ago)
• True “living fossils”
• Heavily armor-plated, have 5 pairs of walking
legs, and long spike like tails
• Can grow up to 60 cm long
• Newly hatched horseshoe crab larvae are
called trilobite larvae because they look much
like their extinct distant relatives
A horseshoe crab’s tiny pincher like
chelicerae and five pairs of walking legs
are visible when the animal is turned on
its back
Arachnids
• Most familiar chelicerates are the
arachnids
– Includes spiders, scorpions, ticks, and
mites
– All adult arachnids have four pairs of
walking legs on their cephalothorax
– Many are carnivores and are adapted
for capturing and holding prey and
also biting and sucking out soft parts
Spiders
• Usually feed on insects
• Capture their prey in a variety of ways
– Ensnare prey in webs
– Stalk and pounce prey
• Once a spider captures its prey, it uses hollow fanglike
chelicerae to inject paralyzing venom into it
• When the prey is paralyzed, the spider’s mouth introduces
enzymes into the wounds
– Break down the prey’s tissues, enabling the spider to
suck up the liquefied tissues with it’s esophagus and a
specialized pumping stomach
Spiders
• Whether or not they spin webs, all spiders produce a
strong, flexible protein called silk
– Silk, which is produced in special glands located
in the abdomen, is five times stronger than steel
– Spiders spin silk into webs, cocoons for eggs,
wrappings for prey, and other structures by
forcing liquid silk through organs called
spinnerets
• As the liquid silk is pulled out of the
spinnerets, it hardens into a single strand
• The spiders of web-spinning species can build
intricate webs almost as soon as they hatch –
without having to learn how
The wolf spider ambushes prey from
its silk-lined burrow. Large tarantulas
are capable of catching and devouring
small vertebrates, such as lizards
Mites and Ticks
• Small arachnids, many of which are parasites
on humans, farm animals, and on important
agricultural plants
• Most are smaller than 1mm
• The chelicerae are needlelike structures that
are used to pierce the skin on their host
• May also have large teeth to help the parasite
keep a firm hold on the host
• The pedipalps are often equipped with claws
for digging in and holding on
Mites and Ticks
• Some species damage houseplants and are major
agricultural pests on crops such as cotton
• Others cause painful itching rashes in humans,
dogs, and other mammals
• A whole host of ticks parasitize humans and the
animals we raise
• Tick bites are not just annoying, they can be
dangerous
– Rocky Mountain spotted fever
– Lyme disease
Red velvet mites are similar in form
to other members of their class,
However, they are unusual in that they
are not parasites and are relatively large
Scorpions
• Scorpions are widespread in warm areas around
the world
• All scorpions are carnivores that prey on other
invertebrates, usually insects
• The pedipalps of scorpions are enormously
enlarged into a pair of claws
• The abdomen, which is long and segmented,
terminates in a venomous barb used to sting prey
• Usually, a scorpion grabs prey with its pedipalps,
then whips its abdomen over its head to sting the
prey, thus killing or paralyzing it
Scorpions
• Then chews its meal with its chelicerae
• Most North American scorpions have venom
powerful enough to cause about as much pain
as a wasp sting
– However, there are some more harmful
species of scorpions that have killed small
children who were stung accidentally
The loser of a fight between two scorpions will be stung
and eaten by the winner. Biologists can locate scorpions
at night by shining UV light on the desert floor. Under UV
light, scorpions glow brightly in the dark.
Chapter 28: Arthropods
Section 3: Crustaceans
Crustaceans
• Subphylum Crustacea contains over 35,000
species
• Primarily aquatic
• Range in size from microscopic to 6 meters in
length
• All crustaceans share a number of structural
similarities
– Hard exoskeleton
– Two pairs of antennae
– Mouthparts called mandibles
Crustaceans
• Main crustacean body parts are the head,
thorax, and abdomen
• In many crustaceans, the head and thorax have
fused into a cephalothorax that is covered by a
tough shell called the carapace
• Many large crustaceans have calcium
carbonate in the exoskeleton
– This is what makes the shells of crustaceans
such as crabs and lobsters hard and stony
Crustaceans
• In crustaceans, the first two pairs of appendages are
“feelers” called antennae
– Bear many sensory hairs
• The third pair of appendages are mouthparts that are
called mandibles
– Short, heavy structures designed for biting and
grinding food
• The appendages on the thorax and abdomen vary greatly
from one group of crustaceans to another
– May be modified for internal fertilization, carrying
eggs, spearing prey, burrowing, or many other
functions
Crayfish
• The appendages on a crayfish’s thorax and
abdomen are adapted for several different functions
– Thorax
• Pair of large claws
–Catch prey, pick up, crush, and cut food
• Four pairs of walking legs
– Abdomen
• Swimmerets
–Used for swimming
• Paddle-like appendages
Crayfish
• The paddle like appendages and the final
abdominal segment together form a large, flat
tail
• When the muscles of the abdomen contract,
the crayfish's tail snaps forward
• This provides a powerful swimming stroke that
can rapidly pull the animal backward
Pill Bug
Black Tiger Shrimp
Lobster
Hermit Crab
Homework
• Page 621 #1 – 3
Chapter 28: Arthropods
Section 4: Insects and Their Relatives
Insects and Their Relatives
• The subphylum Uniramia contains more
species than all other groups of animals alive
today
– Includes centipedes, millipedes, and insects
– Characterized by one pair of antennae and
appendages that do not branch
– Are thought to have evolved on land about
400 million years ago
– Inhabit almost every terrestrial habitat on
Earth
Centipedes and Millipedes
• Centipedes and millipedes are many-legged animals
• These two classes of arthropods are quite similar in
number
– 3000 species of centipedes
– 7500 species of millipedes
• Centipedes and millipedes are characterized by a long,
wormlike body composed of many leg-bearing
segments
• Because they lack closeable spiracles and a waterproof
coating on their exoskeleton, their bodies lose water easily
• Thus they tend to live beneath rocks, in soil, or in other
relatively moist areas
Centipedes
• Carnivores
• Have a pair of poison claws in their head region
– Used to catch and stun or kill prey
• Eat other arthropods, earthworms, toads, small snakes,
and mice
• Usually about 3 – 6 cm long
• Despite their name, which means 100 legs, centipedes
may have from 15 to 170 pairs of legs
• Each segment that makes up the body of the centipede
bears one pair of legs, except for the first segment and
the last three segments
Millipedes
• Each millipede body segment is formed from the
fusion of two segments in the embryo and thus
bears two pairs of legs
• Timid creatures that live in damp places under
rocks and in decaying logs
• Feed on dead and decaying plant material
• When disturbed, many millipedes roll up into a
ball to protect their softer undersides
• Some can also defend themselves by secreting
unpleasant or toxic chemicals
Insects
• More than 900,000 insects
– New ones are discovered in the tropics all the time
– Extremely varied in body shape and habits
– Insects are characterized by a body that is
divided into three parts – head, thorax, and
abdomen – and has three pairs of legs attached
to the thorax
– A typical insect has one pair of antennae and one
pair of compound eyes on the head, two pairs of
wings on the thorax, and uses a system of tracheal
tubes for respiration
Insects
• Insects get their name from the Latin word
insectum, meaning notched, which refers to the
division of their body into three main parts
• The essential life functions in insects are
carried out in basically the same ways as they
are in other arthropods
• However, insects show a variety of interesting
adaptations in feeding, movement, and
behavior
Feeding
• Three pairs of appendages that are used as
mouthparts
• Can take on an enormous variety of shapes in
species adapted to feed on different foods
• Insect adaptations for feeding are not restricted
to the shapes of the mouthparts
• Many insects produce saliva that contains
digestive enzymes and helps break down food
Movement
• Insects have three pairs of walking legs
– Often equipped with spines and hooks for holding onto
things and for defense
– May be adapted for functions such as jumping or
capturing and holding prey
• Along with birds and bats, insects are the only living
organisms that are capable of unassisted flight
• The flying ability of insects varies greatly
• In flying insects, most of the space in the thorax is taken
up by the large muscles that operate the wings
Insect Societies
• Many animals form colonies, which are
collections of individuals of the same species
that live together
• Several types of insects are unique among
invertebrates in that they form a special type of
colony known as a society
– Separate individuals are dependent on one
another for survival
• Social insects
Insect Societies
• These insects have developed complicated
societies that may be composed of from half a
dozen to more than 7 million individuals
• Within such societies there is division of labor
– Different individuals perform the tasks
necessary for the survival of the entire group
– There are several types of individuals within
insect societies
Insect Societies
• The basic types are:
– Reproductive females
• Queens
• Lay eggs that hatch into new individuals for the
society
• Largest individual in the colony
– Reproductive males
• Fertilize the queen’s eggs
– Workers
• Perform all the colony’s tasks except for
reproduction
Insect Communication
• All insects use sound, visual, chemical,and
other types of signals for communication
• Much of the communication done by
nonsocial insects involves finding a mate
• Insects release chemical messengers called
pheromones that affects the behavior
and/or development of other individuals of
the same species
Insect Communication
• Communication in social insects is generally more complex
than in nonsocial insects
• A sophisticated system of communication is necessary to
organize a society
• Each species of social insect has its own “language” of
visual, touch, sound, and chemical signals that convey
information among members of the colony
• Pheromones are particularly important in insect societies
– Short-term messages
– Long-term controls
• Also use “dances” to convey information
– Round dance
– Waggle dance
Chapter 28: Arthropods
Section 5: How Arthropods Fit into
the World
How Arthropods Fit into the
World
• Arthropods play many roles in the natural world
– Direct food source for carnivorous organisms
– Two-thirds of the world’s flowering plants depend on
insects to pollinate them
– Agriculture would be impossible without the bees,
butterflies, wasps, moths,and flies that pollinate crops
– Bees manufacture honey
– Silkworms produce silk
– Shrimp, crab, crayfish, and lobster are a large food
source for humans
– Many useful chemicals can be obtained from
arthropods
How Arthropods Fit into the
World
• Not all arthropods are beneficial to humans
– Insects and arachnids cause billions of dollars in
damage each year to livestock and crops around the
world
– Mosquitoes inflict annoying bites, and some
species carry malaria and yellow fever
– Termites cause extensive damage to wooden
structures
– Farmers have spent billions of dollars on poisonous
chemicals to save their crops from these pests