Download Chapter 28

GUIDE FOR READING
After you read the following
sections, you will be able to
28-1 Introduction to Arthropods
• Discuss the characteristics and
classification of arthropods.
• Explain how arthropods perform
CHAPTER
essential life functions.
28-2 Spiders and Their Relatives
• Discuss the distinguishing
characteristics of chelicerates.
Arthropods
28-3 Crustaceans
• Discuss the distinguishing
characteristics of crustaceans.
28-4 Insects and Their Relatives
• Describe and give examples of
three classes in the subphylum
Unifamia.
• Discuss the anatomy of a typical
insect.
• Explain ways insects
communicate.
28-5 How Arthropods Fit into the
World
Arthropods display a wide range of forms and habit*
• Describe how arthropods interact
with other living things.
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Journal Activity
YOU AND YOUR WORLD
beneath
a^
^ in^the
COntr°'0,3
b^ow
beneath a cypress
tree.^And
heart of a Florida swamp,
43
^~ZZZ:.~Z;Z
Bugs! Creepy crawly insects. The
mere thought of them makes some
people cringe. But not all insects
are harmful. Pretend that you are a
press agent for insects. Write a short
press release informing the public
about the benefits of sharing our
world with insects.
28-1 Introduction to Arthropods
Guide For Reading
¦ What are the four subphyla of arthropods?
¦ What are three important arthropod features?
¦ How are arthropods adapted for performing
life functions?
¦ What is metamorphosis?
To describe even a fraction of the living arthropods would
take several books. More than a million arthropod species have
been described, and scientists are certain there are many more
that have not as yet been found! As you will soon learn,
members of this phylum vary enormously in size, shape,
and habits.
Diversity and Evolution in Arthropods
You already know about many common arthropods. In fact,
you have probably even eaten a number of them! Today most
biologists divide arthropods into four subphyla:
• Trilohita This is thought to be the oldest subphylum of
arthropods. Trilobites (TRlGH-loh-bights) were dwellers in
ancient seas. They are now all extinct.
• Chelicerata Chelicerates (keh-LlHS-er-ayts) include spiders,
ticks, mites, scorpions, and horseshoe crabs.
• Crustacea Crustaceans (kruhs-TAY-shuhnz) include such
familiar (and edible) organisms as crabs and shrimp.
8 Uniramia IMramians (yoo-nih-RAY-mee-ahnz) include
most arthropods: centipedes, millipedes, and all insects—
including bees, moths, grasshoppers, flies, and beetles.
Why are there so many different kinds of arthropods? One
reason is that they have been evolving on Earth for a long time.
The first arthropods appeared in the sea more than 600 million
years ago. Since that time, these animals have experienced sev¬
eral adaptive radiations. Some arthropods have remained in
polar regions to bene pools on the equator. Some arthropods are
ong e most destructive animals on Earth. Others are beneficial
even essentml. to the survival of other organisms.
What are arthropods? What common characteristics do they
the answers tQ the'^ ^ ^ ^ Y°u ^ ^over
answers tg,these questions In the pages that follow.
Figure 28-1 The scorpion, a
chelicerate, is feeding on a hawk
moth, a uniramian (left). The whitebooted shrimp is a crustacean
(right).
607
Figure 28-2 The velvet worm is traditionally placed in its own
phylum along with other organisms that have characteristics
of both annelids and arthropods. Such animals support the
hypothesis that modern annelids and uniramian arthropods
descended from a common ancestor. However, some molecular
biologists now feel that these animals should be placed in
the phylum Arthropoda.
the water, where they have colonized all parts of the sea and
most freshwater habitats. Others were among the very first
members of the animal kingdom to colonize the land. The de¬
scendants of those pioneers were on hand when the first flow¬
ering plants appeared millions of years later.
The roots of the arthropod family tree are cloaked in mys¬
tery because the ancestors of the arthropods were soft-bodied
animals that left few fossils. But by studying both living and
fossil invertebrates, researchers have accumulated many clues
to arthropod evolutionary history. Insects, centipedes, and mil¬
lipedes seem to have evolved from ancestors that were closely
related to the ancestors of modern annelid worms. Living evi¬
dence for this line of descent can be found in wormlike animals
that live in the tropics. See Figure 28-2. Other arthropods, in¬
cluding crustaceans, spiders, and the extinct trilobites, evolved
from more ancient and more distantly related ancestors.
The body form of the earliest arthropods is thought to be
similar to that of the trilobites. A typical trilobite's body had a
thick, tough outer covering and was composed of many seg¬
ments, each of which bore a pair of appendages. Each appen¬
dage was divided to form two branches, one a walking leg and
one a featherlike gill. See Figure 28-3.
Body
segments
Figure 28-3 The dorsal side of a fossil trilobite clearly shows
the three lengthwise body lobes that give the animal its name
(inset). An artist's rendering of the ventral side of a trilobite as
it might have appeared when the animal was alive reveals
numerous similarly shaped appendages.
Most living arthropods exhibit two evolutionary trends
away from the trilobite form. First, many have far fewer body
segments. The many segments found in their embryos fuse into
larger segments during development. Second, arthropod ap¬
pendages have become increasingly specialized for feeding,
locomotion, and other functions.
Figure 28-4 Some arthropods, such
as the spiny lobster (center) and the
tick (top), have extremely hard,
tough exoskeletons. The mouthparts
of a tick (inset) are adapted for
biting and hanging onto a host.
Other arthropods, such as the
emperor gum moth caterpillar
(bottom), have flexible, leathery
exoskeletons.
Form and Function in Arthropods
Although living arthropods are quite different from one an¬
other, all arthropods exhibit several key features. The three
most important arthropod features are a tough exoskeleton,
a series of jointed appendages, and a segmented body. Other
characteristics of arthropods include a brain located in the
dorsal part of the head, a ventral nerve cord, and an open cir¬
culatory system powered by a single heart.
THE ARTHROPOD BODY PLAN The exoskeleton (exomeans outside) is a system of external supporting structures
that are made primarily of the carbohydrate chitin (KlGH-tihn).
Some exoskeletons, such as those of most insects, are leathery
and flexible. Others, such as those of ticks and lobsters, are ex¬
tremely hard. These exoskeletons provide excellent protection
from physical damage. The exosketetons of many terrestrial
arthropods are waterproof. This adaptation restricts the loss
of water from the body and makes it possible for arthropods to
live in extremely dry environments such as deserts. The exo¬
skeleton also helps arthropods move efficiently and adapt to
their environment in many other ways.
Although the exoskeleton protects an arthropod's body
like a suit of armor, it has the disadvantages you might expect
from such a covering. Because an exoskeleton is a solid coat¬
ing, not a living tissue, it cannot grow as the animal grows. (You
will learn how arthropods deal with this problem shortly.) And
movement can occur only at the joints of the "armor."
609
iM,
All arthropods have jointed appendages (arthro- means
joint; -pod, which literally means foot, refers to the appen¬
dages) that enable them to move. In primitive arthropods, such
as trilobites, every body segment carries a single pair of ap¬
pendages. But in species in which body segments are fused to¬
gether, some appendages have been lost. Over millions of
years, the remaining appendages have evolved into marvelously versatile adaptations to different environments. Arthro¬
pod appendages include antennae, claws, walking legs, wings,
flippers, and other specialized structures.
All arthropods have segmented bodies. Some, such as mil¬
lipedes and centipedes, have long, wormlike bodies with many
visible segments. Others, such as insects, spiders, and crabs,
have lost some segments in the course of evolution or had the
segments fuse together to form a few large body parts.
FEEDING The appendages of arthropods have evolved in
ways that enable these animals to eat almost any food you can
imagine. Every mode of feeding is seen in arthropods—herbi¬
vores, carnivores, parasites, filter feeders, and detritus feeders.
Figure 28-5 The waterflea is a tiny
freshwater crustacean that lacks
external segmentation and uses its
antennae to propel it through the
water. The round structures on the
waterflea's back are its eggs.
Although some herbivores, such as locusts, eat just about any¬
thing green, other herbivores are more selective. Some herbi¬
vores are specialized to eat specific parts of plants. Others feed
exclusively on a particular kind of plant. Some carnivores, such
as spiders, praying mantises, centipedes, and king crabs, catch
and eat other animals. Other carnivores, such as many crabs
and crayfish, feed primarily on animals that are already dead.
External parasites—such as ticks, fleas, and lice—drink the
blood and body fluids or nibble on the skin of other animals,
including humans. Some internal parasites passively absorb
nutrients through the body wall, whereas others eat away at
the host from inside. Many marine arthropods are filter feeders
that use comblike bristles on their mouthparts or legs to filter
tiny plants and animals from the water.
RESPIRATION Arthropods have evolved three basic
types of respiratory structures—gills, book gills and book
lungs, and tracheal tubes. Although most arthropods have only
one of these types of respiratory structures, a few species have
both book lungs and tracheal tubes. And some species com¬
pletely lack specialized respiratory organs.
Many aquatic arthropods, such as crabs and shrimp, have
gills that look like a row of feathers located just under cover of
their exoskeleton. These gills are formed from part of the same
appendages that form mouthparts and legs. Movement of the
mouthparts and other appendages keeps a steady stream of
water moving over the gills.
Book gills (which are found in horseshoe crabs) and book
lungs (which are found in spiders and their relatives) are
unique to these arthropods. In both these structures, several
sheets of tissue are layered like pages in a book. The many tis¬
sue layers increase the surface area for gas exchange. A horse¬
shoe crab's book gills are carried beneath its body, whereas a
spider's book lungs are contained inside a sac within the body.
An opening called the spiracle (SPlHR-ah-kuhl) connects the
sac containing the book lungs with the fresh air outside.
Most terrestrial arthropods—insects, some spiders, and
millipedes, for example—have another respiratory device
found in no other animals. From spiracles, long branching tra¬
cheal tubes reach deep into the animals' tissues. The network
of tracheal tubes supplies oxygen by diffusion to all body tis¬
sues. As the arthropods walk, fly, or crawl, the movements of
their body muscles cause the tracheae to shrink and expand,
pumping fresh air in and out of the spiracles. Tracheal tubes
work well only in small animals; large animals require a more
efficient way to deliver oxygen and remove carbon dioxide.
Figure 28-7 Although a fiddler crab
spends much time on land, it uses
gills for respiration. The male crab's
large claw is used to attract females
and to fight with other moles.
Figure 28-8 The internal structures of a representative
arthropod—a grasshopper—are shown here.
. fli
Figure 28-6 The praying mantis
(left) is a carnivore. The lubber
grasshopper (right) is a herbivore.
Head Thorax
P
Abdomen
i Digestive
I tract
Antenna
Heart j
Compound
eye
Nerve
cord
Walking legs
610
Tracheal
tube
Spiracle
Malpighian
tubules
611
Figure 28-9 The diamond beetle
. J uses Malpighian tubules to get
nd of nitrogen-containing wastes,
whereas the hermit crab (right) uses
its green glands and gills.
h^H if m abdomen. In lobsters and crayfish the
the hearTrf nt antd 1165 abOUt halfway down the body. When
the heart contracts, it pumps blood through arteries iZ
branch lnt0 Iler vessels and enter the ^
leaves the vessels and moves through spaces in the tis
ue called smuses. Eventually, the blood collects ,^ a arge cav"
Ity surrounding the heart, from which it re-enters he h i
through small openings and is pumped around agafn
spilsliiii
RESPONSE Most arthropods have well-developed ner¬
vous systems. All have a brain that consists of a pair of ganglia
in the head. These ganglia serve as central switchboards for in¬
coming information and outgoing instructions to muscles.
From the brain, a pair of nerves runs around the esophagus
and connects the brain to a nerve cord that runs along the ven¬
tral part of the body. Along this nerve cord are several more
ganglia, usually one for each original body segment. These
ganglia serve as local command centers to coordinate the
movement of legs and wings. (That's why many insects can still
walk or flap their wings after their heads are cut off!) Where
many body segments have fused together, as in insects, there
are several ganglia for each major body part.
Arthropods have simple sense organs such as statocysts
and chemical receptors. Most arthropods also have sophisti¬
cated sense organs such as compound eyes for gathering infor¬
mation from their environment. Compound eyes may have
more than 2000 separate lenses and can detect color and mo¬
tion very well. In fact, insects can see certain things better than
we can. (That's one reason it is so hard to swat flies and mos¬
quitoes!) For example, the blades of a quickly moving fan—just
a blur to our eyes—are clearly visible to a fly. And many in¬
sects can see ultraviolet light, which is invisible to humans.
Both crustaceans and insects have a well-developed sense
of taste, although their taste receptors are located in strange
places. The chemical receptors associated with the senses of
taste and smell are located on the mouthparts, as might be ex¬
pected, but also on the antennae and legs! Flies, for example,
know immediately whether a drop of water they step in con¬
tains salt or sugar. Crustaceans and insects have sensory hairs
that detect movement in the air or water (another reason in¬
sects are hard to swat or catch). As an object moves toward
them, they can feel the movement of the displaced air or water
and respond appropriately. Many insects have well-developed
ears that hear sounds above the human range. Insect ears are
often in odd places. The eardrums in grasshoppers, for exam¬
ple, are behind their legs.
Figure 28-10 The horsefly has
huge compound eyes through which
it sees poppy flowers much as they
appear here (inset).
Figure 28-11 A harlequin beetle
(left) uses its long antennae as
"feelers." The red hourglass on the
abdomen of the black widow spider
(right) warns of the spider's
venomous bite.
the body sinuses TheTh |0rgans' are bathed in blood inside
concentrate them remove wastes from this blood,
fore h leaves via'the ^ ^ t0 undigested food be-
such as lobster. ^ r g S- Many ^ic arthropods.
or to" ¦"p,"d 10"""" "»»"»»¦ per
Growth and Development in Arthropods
Figure 28-12 Can you find the
grasshopper in the photograph? The
stick grasshopper's shape is a form
of camouflage (left). The markings
and behavior of this caterpillar trick
insect-eating birds into thinking it is
a bird-eating viper (center). The
harmless hover fly mimics a stinging
honeybee (right).
Figure 28-13 Muscles attached to
the inside of an exoskeleton bend
and straighten the joints.
Exoskeleton
An arthropod's well-developed sense organs help it detect
and rrpeu predftors' The combination of these sense organs
and a tough exoskeleton is enough to protect many arthropods
But some arthropods have additional means of protection'
Scorpions, bees, and some ants have venomous stings, and
many spiders and centipedes have venomous bites. Lobsters
and crabs can attack potential enemies with powerful claws
And many insects and millipedes fight back using nasty chemh
P . arthroPods trick predators by creating a diversion
or example, some crabs can drop a claw or leg. This body part
keeps on moving to distract predators while the rest of the aniOtherCUrtheS ^ The Crab then grOWS back the lost PmbOther arthropods use visual trickery to fool predators. Some
ide through camouflage—matching the color and texture of
their surroundings so closely that they seem to disappear.
Others imitate the warning coloration of poisonous or dangerous species—a phenomenon called mimicry.
MOVEMENT Arthropods have well-developed muscle
systems that are coordinated by the nervous system. Muscles
genera e force by contracting, then transfer that force to the
toflexthp'i" AI i0int' SOme mUSC,es are Positioned
° th „ , an 0 er ,Tlusdes t0 exten<J it. See Figure 2813. The pull of muscles against the exoskeleton allows arthro¬
pods to beat their wings against the air to fly, push their legs
Exoskeletons, as useful as they are, present a problem in
terms of growth. As a growing student, you can understand that
problem. Imagine that you had a skin-tight suit of clothes tai¬
lored for you last year. Could you fit into it now? Probably not.
You need larger clothes as you grow. Similarly, arthropods
must replace their exoskeletons with larger ones in order to
allow their bodies to increase in size as they mature. The prob¬
lem is not a simple one, however, because the exoskeleton not
only covers all appendages and sense organs, but also lines the
gut as far down as the stomach. In order to grow, all arthropods
must molt, or shed, their exoskeletons. This complicated proc¬
ess is controlled by several important hormones, the most im¬
portant of which is called molting hormone.
When molting time is near, the epidermis (the layer of cells
that covers the outside of the body) digests the inner part of
the exoskeleton and absorbs much of the chitin in order to re¬
cycle the chemicals in it. After it secretes a new exoskeleton in¬
side the old one, an arthropod pulls completely out of its old
exoskeleton. Arthropods often eat what is left of the old exo¬
skeleton. The animal then expands to its new, larger size, and
the new exoskeleton (which is still soft) stretches to cover it.
The animal must then wait for the new exoskeleton to harden, a
process that can take from a few hours to a few days. During
this time, the new shell stays soft and the animal is quite vul¬
nerable. Thus arthropods hide from predators during molting.
Most arthropods molt several times between hatching and
adulthood. In most cases the process of growth and develop¬
ment involves metamorphosis, or a dramatic change in form.
Some arthropods, such as grasshoppers, mites, and crusta¬
ceans, hatch from eggs into young animals that look much like
the adults. However, these young animals lack functioning sex¬
ual organs and often lack other adult structures such as wings.
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. In insects, this kind of gradual change
during development is called incomplete metamorphosis.
Figure 28-14 The adult cicada
is emerging from the molted
exoskeleton of an immature stage.
Arthropods molt in order to increase
in size and also to change from one
body form to another in the process
of metamorphosis.
Figure 28-15 The grasshopper
undergoes incomplete
metamorphosis, whereas the
monarch butterfly undergoes
complete metamorphosis.
wrterto'swim.011"'1 t0 Walk' ^ beat tlleir nipperS against the
Muscle that flexes the joint
Exoskeleton
sim^eP|VMDUCT!)0r Reproduction in most arthropods is
simple. Males and females produce sperm and eggs, respecive y, and fertilization usually takes place inside the body of
e female, n spiders and some crustaceans, the male deposits
Muscle that extends the joint
second t SPerm that the female picks up- ln most insects and crustaceans, however, the male uses a special reproductive organ to deposit sperm inside the female.
Complete Metamorphosis
Incomplete
Metamorphosis
Older
larva
Newly
hatched
larva
Figure 28-16 Insect pupae are
often surrounded by a protective
covering. The bumblebee pupa in
this photograph is surrounded by a
wax case. Many caterpillars spin
cocoons of silk.
Many insects, such as bees, moths, and beetles, undergo a
four-stage process of development called complete metamor¬
phosis. Refer to Figure 28-15 on page 615 as you read about the
process of complete metamorphosis. The eggs of insects that
undergo complete metamorphosis hatch into larvae that look
nothing like their parents. As these larvae grow, they molt re¬
peatedly, growing larger each time but changing little in ap¬
pearance. When a larva reaches a certain age, it sheds its larval
skin one last time and becomes a pupa (PYOO-pah; plural:
pupae). During the pupal stage, the insect's body is totally
rearranged—adult structures grow from tiny buds and larval
structures are broken down to supply the raw materials for the
adult structures. When metamorphosis is complete, the animal
emerges as a fully grown adult with both internal and external
body parts that are completely different from what it had be¬
fore. Not only does this adult look like a totally different ani¬
mal, it acts differently too.
Metamorphosis is controlled by a complicated interaction
of several hormones, including molting hormone. In insects
that undergo complete metamorphosis, the levels of juvenile
hormone help regulate the stages of development. High levels
of juvenile hormone keep an insect in its larval form each time
it molts. As the insect matures, however, its production of juve¬
nile hormone decreases. At some point, the level of juvenile
hormone drops below a certain critical point. The next time the
insect molts, it becomes a pupa. And when no juvenile hor¬
mone is produced, the insect undergoes a pupa-to-adult molt.
Because the balance of juvenile hormone, molting hor¬
mone, and other hormones is critical in arthropod develop¬
ment, it is possible to combat insects by tampering with their
hormone levels. Certain plants defend themselves against her¬
bivorous insects by producing chemicals that prevent molting,
cause insects to develop at the wrong rate, or keep insects from
becoming functional adults. In recent years, researchers have
developed chemicals that act in a similar manner. These chem¬
icals may eventually enable people to control crop-eating in¬
sects without using dangerous poisons.
28-2 Spiders and Their
Guide For Reading
¦ What are several distinguishing
Relatives
characteristics of chelicerates?
¦ In what ways do spiders use silk?
Spiders and their relatives—horseshoe crabs, ticks, and
scorpions, for example-belong to the subphylum Chelicerata.
Chelicerates are arthropods that are characterized by a twopart body and mouth parts called chelicerae. These arthro¬
pods also lack the sensory "feelers" that are found on the
heads of most other arthropods.
All chelicerates have a body that is divided into two parts:
the cephalothorax (sehf-ah-loh-THOR-aks) and the abdomen.
The anterior end of the cephalothorax contains the brain, eyes,
mouth and mouthparts, and esophagus. The posterior end of
the cephalothorax carries the front part of the digestive system
and several pairs of walking legs. The abdomen contains-most
of the internal organs. See Figure 28-17.
All chelicerates have two pairs of appendages attached
near the mouth that are adapted as mouthparts. The first pair
of mouthparts are called chelicerae (keh-LlHS-er-ee; singular:
chelicera) The second pair of mouthparts, which are longer
than the chelicerae, are called pedipalps (PEHD-ih-palps). Both
sets of mouthparts are adapted to serve different purposes in
Figure 28-17 The internal
structures of a typical spider are
shown in this diagram. A jumping
spider (inset) captures prey by
pouncing on it, rather than by
catching it in a web.
feeding in different species.
Horseshoe Crabs
Among the oldest chelicerates are the horseshoe crabs.
This name is somewhat misleading because these animals are
not true crabs (which are crustaceans). Horseshoe crabs ap¬
peared in the Ordovician Period (more than 430 million years
ago) and have not changed much since then—they are true
"living fossils." Horseshoe crabs are heavily armor-plated,
Pumping
stomach
Heart
- |j 1 SECTION
Malpighian tubules
ll REVIEW
1. What are three characteristics of arthropods? Name the
four subphyla of arthropods.
2. Compare complete and incomplete metamorphosis.
3. Describe the different types of organs that are used in
arthropod respiration.
4. Critical Thinking—Making Inferences Terrestrial
arthropods often have valves that can open and close the
spiracles. How are these valves an adaptation to life on
land? (Hint: What is the function of the stomata on leaves?)
616
Walking leg
Poison
gland
Pedipalp
Anus
Nerve cord
Spiracle
Spinneret
have five pa.rs of walking legs, and long spikelike tails The
can grow up to 60 centimeters long-about the s ze I y
Shape) of a frying pan. When they first hatch, however hors
h reh are 0n'y about 1 cent™eter long. These newf
hatched horseshoe crabs are called frilobite larvae bee™ y
they look much like their extinct distant relatives.
Arachnids
rh,rilhe ^OSt faiTliJiar chelicerates are the arachnids, which in
ude spiders, scorpions, ticks, and mites. AJI adult arachnid,
have four pans of walking legs on their cephalothor^ ^
Figure 28-18 A horseshoe crab's
tiny pincerlike chelicerae and five
pairs of walking legs are visible
when the animal is turned on its
back. The platelike structures on the
abdomen cover and protect the book
gill's "pages." The long tail is not
seen here because the horseshoe
crab has pushed it into the sand to
right itself.
arachnids are carnivores that have pedipalps adapted for can
unng and holding prey and chelicerae adapted for biting and
sucking out their soft parts. 8 d
sei.t!PIl?ERS Spiders are predators that usually feed on in
sects. However, a few large tropical spiders are capable of
Sn rie"18 anf eatl!!g Smal1 vertebrates. stJch as hummingbirds
Spiders capture their prey in a variety of ways. Some spider,
tehneSnnarre T". ^ WebS' 0thers stalk then pouTce on
the prey. And some he in wait beneath the lid of a camouflaged
Once a spider captures its prey, it uses its hollow fanglike
c e icerae to inject paralyzing venom into it. When the prey is
paralyzed, the spider's mouth introduces enzymes into toe
wound5 made by the chelicerae. These enzymes break down
tissue, with hUeS' ehabling the Spider 10 Suck up ,he ^^ficd
tissues with Its esophagus and a specialized pumping stomach
Figure 28-19 The wolf spider (left)
ambushes prey from its silk-lined
burrow. Large tarantulas (right) are
capable of catching and devouring
small vertebrates, such as lizards.
The pumping stomach then forces the liquid food through the
rest of the spider's digestive system
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. It is strong enough, in fact, to withstand the equiva¬
lent of the impact of a jet fighter every time a spider's web
traps a fly. Spiders spin silk into webs, cocoons for eggs, wrap¬
pings 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. Interest¬
ingly, the complicated behavior of web-spinning seems to be
"preprogrammed" into a spider's brain. The spiders of webspinning species can build their intricate webs almost as soon
as they hatch—without having to learn how.
MITES AND TICKS Mites and ticks are small arachnids,
many of which are parasites on humans, on farm animals, and
on important agricultural plants. Most species are smaller than
1 millimeter, but some ticks can be as large as 3 centimeters. In
many mites and ticks, the chelicerae are needlelike structures
that are used to pierce the skin of their hosts. These chelicerae
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. Some species, such as spider mites,
damage houseplants and are major agricultural pests on crops
such as cotton. Others—including chiggers, mange, and sca¬
bies mites—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. In the United States, ticks can spread Rocky
Mountain spotted fever and Lyme disease.
SCORPIONS Scorpions are widespread in warm areas
around the world, including the southwestern United States.
All scorpions are carnivores that prey on other invertebrates,
usually insects. The pedipalps of scorpions are enormously en¬
larged 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
Figure 28-20 Some spiders build
webs to capture prey.
Figure 28-21 Red velvet mites are
similar in form to other members
of their class (right). However, they
ore unusual in that they are not
parasites and are relatively large
(about 1 centimeter long). The loser
of a fight between two scorpions will
be stung and eaten by the winner
(left). Biologists can locate scorpions
at night by shining ultraviolet (UV)
light on the desert floor. Under UV
light, scorpions glow brightly in
the dark.
Abdomen ——Cephalothorax i Second^antenna
its abdomen over its head to sting the prey, thus killing or par¬
alyzing it. The scorpion then chews its meal with its chelicerae
Because scorpions like to crawl into moist, dark places, people
in areas with scorpions should check inside their shoes before
putting them on in the morning. Most North American scor¬
pions have venom powerful enough to cause about as much
pain as a wasp sting. However, the venom of one genus of scor¬
pions that lives in Mexico, New Mexico, and Arizona has killed
small children who were stung accidentally.
Carapace-^ j Heart Green gland | Brain f ^
Gonad
(testis or ovary).
Intestine.
Figure 28-23 The internal and
external structures of a crayfish are
shown here. Can you now explain
why crayfish, shrimp, lobsters, and
crabs are sometimes known as
Nerve
cord
decapods (deca- means ten)?
Mandible
Swimmerets
Digestive
SECTION
gland
Walking leg
¦ REVIEW
1. What are chelicerates? Name and give examples of the
two main groups of chelicerates.
2. What is silk? How do spiders use silk?
3. Critical Thinking—Summarizing Information How are
chelicerae modified for feeding in spiders? In ticks?
Guide For Reading
28-3 Crusty ceans
¦ What are crustaceans?
¦ How are the body parts of
crustaceans adaptations
for survival?
Figure 28-22 The pill bug is
a terrestrial crustacean. When
threatened, a pill bug curls into a
ball to protect its soft underside.
620
The subphylum Crustacea contains over 35,000 species.
Crustaceans are primarily aquatic, although there are some ter¬
restrial species. Crustaceans range in size from microscopic
water fleas less than 0.25 millimeter long to Japanese spider
crabs that are thought to grow up to 6 meters across and lob¬
sters that have a mass of more than 20 kilograms. And crusta¬
ceans vary in form as much as they vary in size!
Although crustaceans adapted to different conditions are
quite dissimilar in form, all crustaceans share a number of
structural similarities. In general, crustaceans are character¬
ized by a hard exoskeleton, two pairs of antennae, and
mouthparts called mandibles. As we examine a little of the
enormous diversity of form and function in crustaceans, we will
focus on a representative species, the crayfish. Refer to Figure
28-23 as you read about structure and function in crustaceans.
The main crustacean body parts are the head, thorax, and
abdomen. In crayfish, as in many other crustaceans, the head
and thorax have fused into a cephalothorax that is covered by a
tough shell called the carapace. Unlike most other arthropods,
many large crustaceans have calcium carbonate (limestone) in
the exoskeleton. This is what makes the shells of crustaceans
such as crabs and lobsters hard and stony.
In crustaceans, the first two pairs of appendages are
"feelers" called antennae, which bear many sensory hairs. An¬
tennae serve primarily as sense organs in crayfish, but in some
other crustaceans they are used in filter feeding. Still other
crustaceans, such as water fleas, use their antennae as oars to
push them through the water.
The third pair of appendages are mouthparts that are
called mandibles. In many species of crustaceans, including
crayfish, mandibles are short heavy structures designed for
biting and grinding food. In other species, mand.bles are
bristly structures used in filter feeding, probelike structures
used for finding and picking up detritus, or needlelike structures used to suck blood from a host.
The appendages on the thorax and abdomen vary greatly
from one group of crustaceans to another. Some such as barnacles, have delicate, feathery appendages for filter feeding,
others have legs for walking or paddles for swimming. Appen¬
dages may be modified for internal fertilization, carrying eggs,
soearing prey, burrowing, or many other functions.
" As you can see in Figure 28-23, the appendages on a cray¬
fish's thorax and abdomen are adapted for several different
functions. A pair of large claws, which are used to catch prey
and pick up, crush, and cut food, are located on the thorax.
Four pairs of walking legs are also attached to the thorax. Flip¬
perlike appendages called swimmerets. which are used for
swimming, are located on the abdomen. A large pair of paddlelike appendages are found on the second-to-last abdomina
segment. The paddlelike appendages and the final abdomina
segment together form a large, flat tail. When the muscles of
the abdomen contract, the crayfish's tail snaps fo™rd- This
provides a powerful swimming stroke that can rapidly pull the
animal backward.
Figure 28-24 The abdomen
of a crab is tucked beneath its
cephalothorax. A female crab uses
its abdomen and the swimmerets
attached to it to carry its reddishbrown eggs. The blue and white
semicircle below the eggs is made up
of the last few segments of the tail.
1. What is a cephalothorax?
2. Describe the types of appendages on crayfish and give
their functions.
3. Critical Thinking—Applying Concepts Suppose you
want to catch a crayfish with a net. Should you try to
scoop it up head first or tail first? Explain.
621
Antenna
Wings
Guide For Reading
What are the characteristics
of the three classes in the
subphylum Uniramia?
What anatomical adaptations
do insects have that enable them
to survive?
How do insects communicate
with one another?
28-4 Insects and Their Relatives
The subphylum Uniramia contains more species than all
other groups of animals alive today. It includes centipedes,
millipedes, and insects. Uniramians are characterized by one
pair of antennae and appendages that do not branch (unimeans one; ramus means branch). (Recall that the appendages
in crustaceans and trilobites have two branches—usually a gill
and a leg.) These arthropods, which display a multitude of
forms and habits, are thought to have evolved on land about
400 million years ago. They inhabit almost every terrestrial
habitat on Earth. In addition, some species live in fresh water
and a few other species live in marine environments.
Centipedes and Millipedes
Figure 28-25 A centipede (top)
is a carnivore that has one pair of
legs per body segment. A millipede
(bottom) is a herbivore that has two
pairs of legs per body segment.
Centipedes and millipedes are many-legged animals. Com¬
pared to crustaceans and insects, these two classes of arthro¬
pods are quite small in number—there are approximately 3000
species of centipedes and 7500 species of millipedes. Centi¬
pedes and millipedes are characterized by a long, wormiike
body composed of many leg-bearing segments. Because they
lack closable 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 Centipedes are carnivores that have, in ad¬
dition to other mouthparts, a pair of poison claws in their head
region. These poison claws are used to catch and stun or kill
prey. Centipedes eat other arthropods, earthworms, toads,
small snakes, and even mice. The North American centipedes
that may be familiar to you are usually red-brown in color and
about 3 to 6 centimeters long. Some tropical species are
brightly colored and quite large—up to 26 centimeters long.
Despite their name, which means 100 legs (centi- means
hundred; -pede refers to legs), centipedes may have from 15 to
170 pairs of legs, depending on the species. Each segment that
makes up the body of the centipede bears one pair of legs, ex¬
cept for the first segment (which bears the poison claws) and
the last three segments (which are legless).
Insects
We know of more than 900,000 insects, and new ones are
discovered in the tropics all the time. Insects are extremely
varied in body shape and habits. However, all members of this
class share basic structural similarities. Insects are character¬
ized by a body that Is divided into three parts—head,
thorax, and abdomen—and that has three pairs of legs at¬
tached to the thorax. In addition, 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 get their name from the Latin word insectum, mean¬
ing notched, which refers to the division of their body into
three main parts; head, thorax, and abdomen. In many insects,
such as ants, the three body parts are clearly separated from
each other by narrow connections. In other insects, such as
grasshoppers, the divisions between the three body parts are
not as sharply defined.
The essential life functions in insects are carried out in ba¬
sically the same ways as they are in other arthropods. How¬
ever, insects show a variety of interesting adaptations in
feeding, movement, and behavior that deserve a closer look.
FEEDING Insects have three pairs of appendages that are
used as mouthparts, including a pair of mandibles. Mouthparts
can take on an enormous variety of shapes in species adapted
to feed on different foods. For example, a grasshopper's mouth¬
parts are designed to cut and chew plant tissues into a fine
pulp. A female mosquito's mouthparts form a sharp tube that is
used to pierce skin and suck blood. A butterfly's mouthparts
Grasshopper Moth
Head
Abdomen
Thorax
Figure 28-26 An insect is
characterized by a three-part body,
six legs, two pairs of wings, one pair
of antennae, and one pair of
compound eyes.
Figure 28-27 Although insect
mouthparts are adapted for many
different eating habits, they all
evolved from the same basic
Housefly
MILLIPEDES Although millipedes do not have a thousand
legs (milli- means thousand), they do seem to have twice as
many as centipedes. Each millipede body segment is formed
from the fusion of two segments in the embryo and thus bears
two pairs of legs. Millipedes are timid creatures that live in
damp places under rocks and in decaying logs. They feed on
dead and decaying plant material. When disturbed, many milli¬
pedes roll up into a ball to protect their softer undersides.
Some can also defend themselves by secreting unpleasant or
toxic chemicals.
623
Si~S=SH5SS
Figure 28-29 Leaf-cutter ant
workers carry pieces of leaves and
flower petals to their underground
nest. Certain chambers in the nest
are "farms" in which the ants grow
rnmmm
edible fungi on the bits of
vegetation.
ting. (Unfortunately for h^manVand ^ Clot-
saliva also contains chemicals thAt animals, mosquito
reaction. SSSSi
have a number of adAntAti^r... I ^ Honeybees
storing food. The legs and bodieT ® ™g' Processing, and
with hairs that coUect nollen Ph ^ ^ ^ COVered
change nectar inte^a more digest?ble™rmS b ^ fT helP
on the abdomen secrete wal which ^ y' d glanc[s
chambers for food and other struck: X': belhivT^
Figure 28-28 Insects, along with
birds and bats, are the only living
organisms capable of unassisted
tght. 1 he hard brown wing covers
on the may bug ore modified
forewings; in may bugs and other
for fiTghL^ ^ hindwin^ are used
during flight is k - oy these muscles
r l-jt=====-"
efficiendye^wrenhler^r^ and ~
nies'which Sctln^ fy ^ ^ ^ —
that live together Several tvr>ps TldUa'S 0f the Same SPecies
invertebrates in that thev fn 0 lnsects are un'que among
as a society In fsoc etv Z * ZZ tyPe 0f COlony known
on one another for^^fva ^3 are.dependent
624
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 castes, or types
of individuals, within insect societies. Each caste has a body
that is specialized for its functions and is therefore distinctly
different from that of another caste. The basic castes are repro¬
ductive females, reproductive males, and workers.
The reproductive females, which are called queens, lay
eggs that hatch into new individuals for the society. Most in¬
sect societies have only one queen, who is typically the largest
individual in the colony. Termite queens, for example, may be
14 centimeters long (more than 10 times longer than a worker)
and 3.5 centimeters wide. Most of a termite queen's body con¬
sists of a grotesquely swollen abdomen that contains enlarged
reproductive organs. A termite queen can produce more than
30,000 eggs a day!
The reproductive males function only to fertilize the
queen's eggs. In some insect societies, such as those of ter¬
mites, a single reproductive male stays with the queen as a per¬
manent member of the colony. In other societies, such as those
of bees, the queen receives all the sperm she needs for her
eggs after a single mating with one or more reproductive males.
The successful males die after mating, and the unsuccessful
males are ejected from the colony and soon perish.
The workers perform all the colony's tasks except for re¬
production: They care for the queen, eggs, and young; they
gather, store, and even grow food; they build, maintain, and de¬
fend the colony's home; and they perform all other necessary
jobs. In societies of ants, bees, and wasps, the workers are all
females; in those of termites, there are both male and female
workers. Bee and wasp workers are capable of performing all of
their societies' tasks for workers. Ant and termite workers are
specialized and are able to carry out only their specific tasks,
such as defending the colony or storing food.
Figure 28-30 Mature termite
queens are approximately the size
and shape of a hot dog. The large
termite next to the queen is a
reproductive male. The smaller
brown termites with the large heads
are called soldiers. The tiny white
termites are workers.
INSECT COMMUNICATION All insects use sound, visual
ofThT and 0ther lyPeS 0f Signals for commun'cati0n. Much
the communication done by nonsocial insects involves find
mg a mate. To attract females, male crickets chirp by rubbing
eir fore wings together, and male cicadas buzz by vibratin?
special membranes on the abdomen. Male fireflies turn a light
producmg organ in their abdomens on and off, producing a disct series of flashes. When female fireflies (which are
wingless and are known as glowworms) see the correct signal
ey flash back a signal of their own and the males fly to them'
view ^th ^ 3 g00d thing ^ the maIe firefly,s Point of
v.ew the carnivorous females of one genus of fireflies can
deX L S1fnal 0f anoth€r Senus and lure males to their
death. Many female moths release chemicals that attract disant males to them. These chemicals are a type of pheromone
and / SP. C chem,(:al messenger that affects the behavior
and/or development of other individuals of the same species
Communication in social insects is generally more complex
an in nonsocial insects. A sophisticated system of communiFigure 28-31 A male tana moth's
feathery antennae can detect
pheromones released by a female
several kilometers away.
insert a society. Each species of social
insect has its own language" of visual, touch, sound and
the colony.18113 that information among members of
Certafn^nhT"68 are particularly iniPortant in insect societies.
^ n Tm0T a nCti0n aS rapid messages
at signal alarm, the death of a member of the colony or the
she'head l ^ eXample' when a worker ant finds food,
heads back to the nest, dragging her abdomen along the
g ound. As she does so, she leaves behind a trail of a special
m of Pheromone. Her nestmates can detect her trail by using
Other'nh ' 0n ^ antennae and fol,ow ^ back to the food,
er pheromones act as long-term controls over the colony
or example, a queen honeybee produces a pheromone, called
queen substance, that prevents the development of rival
eggs lt ahnT" SUbtShtanCe makes worker bees unable to lay
afQl]' a. H aUSeS T t0 raiSe fema,e larvae as ^rkers, not
fhe hivp k , WeVe,i' When the amOUnt 0f q1166" stance in
diet This T f 665 feed a feW female ,arvae a sPecial
diet. This causes the larvae to develop into queens
Honeybees communicate with sound and movement as
well as with pheromones. Worker bees are able to convey infor¬
mation about the type, quality, direction, and distance of a food
coded6bv^A Jhe ,anguage of the bee's dance was deered th/t K T b,0!0glSt KarI VOn FriSCh- Von Fr'sch discov-
wTggle danc^05 ^ ^ danCeS; a ^d da- and a
In the round dance, the bee that has found food circles first
tells The othP^h11 th?H0thlr' 0Ver and 0Ver again' This dance
other bees that there is a source of food within 50
meters of the hive. The frequency with which the dancing bee
Round Dance
Waggle Dance
Figure 28-32 Bees communicate
information about food sources by
using a language of movement. The
round dance tells members of the
hive that a source of food is nearby.
The waggle dance gives information
about a more distant food source.
changes direction indicates the quality of the food source—the
more frequent the changes in direction, the greater the energy
value of the food. By smelling the dancer with the chemical re¬
ceptors on their antennae, the other bees can determine what
kind of flowers she has found.
In the waggle dance, the bee that has found food runs for¬
ward in a straight line while waggling her abdomen, circles
around one way, runs in a straight line again, and circles
around the other way. See Figure 28-32. The waggle dance tells
the other bees that the food is more than 50 meters away. Most
of the information about the food source is conveyed by the
part of the waggle dance called the straight run. The longer the
bee takes to perform the straight run and the more she wag¬
gles, the farther away the food. The direction of the straight run
indicates in which direction the food is to be found. For exam¬
ple, if the dancer runs straight up the vertical honeycomb, the
food source is in the same direction as the sun.
SECTION
REVIEW
_
1. Compare the body plans and feeding habits of millipedes
and centipedes.
2. Describe the basic body plan of an insect.
3. Give three specific examples of why and how insects
communicate.
4. Explain how the mouthparts of bees, mosquitoes, and
butterflies are adapted to different food sources.
5. How does the waggle dance of honeybees convey
information about the location of a food source?
6. Critical Thinking—Relating Cause and Effect If all
worker bees are females, why is the queen the only
egg-layer in the colony?
627
TECHNOLOGY,
AND SOCIETY
Controlling Agricultural Pests
Many plant-eating insects are enemies of
both crops and plants in nature. Yet although
cornfields can be destroyed by insects, forests
and grasslands are never wiped out. Why not?
And how can the answer to that question help
us protect food crops?
Pests rarely get out of control in nature for
three main reasons. First, they are eaten by
such natural enemies as ladybugs, spiders
parasitic wasps, and birds, and suffer from dis¬
eases caused by bacteria and fungi. Second
many plants produce compounds that taste
bad to insects, poison them, or mimic the ac¬
tion of insect juvenile hormones. Larvae that
eat these hormone mimics cannot pupate
never mature, and cannot reproduce Third
natural environments contain several plant
species mixed together, so insect pests have to
work harder to find their favorite foods.
For many years now. farmers have taken
their cues from poisonous plants, and have re-
hed on toxic sprays such as DDT to kill pests
It is clear, however, that widespread, constant
use of poison sprays has created more prob¬
lems than it solved. Many pesticides are
deadly to humans, livestock, wildlife, and the
natural enemies of pests. Some chemicals stay
m the environment for a long time, poisoning
rivers and water supplies. Furthermore, many
insecticides are no longer useful because pests
have evolved resistance to them. In 1938, there
were only 7 pests that were resistant to chemi¬
cals; by 1985 there were 447.
Clearly, different insect control methods
are needed. Researchers have therefore fo¬
cused their attention on pests' natural enemies
and on ancient agricultural practices.
A great deal has been learned in recent
years. Insect biologists are learning to breed
and collect pest-eating insects, and to release
them in fields at the right time to control
pests. Biotechnology companies are learning
to manufacture hormone mimics economi-
Mult ladybugs and their larvae feed on crop-
leT? /n ^ /0ng run' the use of natural
pest controls is safer and more efficient and
pesiHides USe ofarm^ chemical
cally. Farmers are learning to use these non¬
toxic sprays, which affect only the insects at
which they are targeted and remain useful for
a long time. (Pests cannot usually evolve resis¬
tance against hormone mimics, because that
would involve extensive modifications to its
complex internal control system.)
Finally, ecologists and anthropologists
have discovered that certain traditional agri¬
cultural practices control plant pests and dis¬
eases remarkably well. Some practices en¬
courage animals that prey on pests to live in
farmers fields. Others control harmful insects
by starving them—through planting different
crops in different years and leaving some
fields unplanted every few seasons.
28-5 How Arthropods Fit
into the World
As you might expect from such a large, diverse group of an¬
imals, arthropods play many roles in the natural world. They
are a direct source of food for many carnivorous organisms—
from protists such as radiolarians to plants such as Venus' fly¬
traps to animals such as sea anemones, fishes, frogs, turtles,
birds, whales, and humans. And they are also an indirect
source of food for many other organisms.
The interactions of arthropods with other organisms are
not limited to eating or being eaten. Two thirds of the world's
flowering plants depend on insects to pollinate them. Some
plants live even more intimately with arthropods. The bull's
horn acacia tree has hollow swellings at the base of some of its
thorns that house symbiotic ants and special structures that
feed the ants. The ants protect the acacia by eating herbivorous
insects and by driving away larger herbivores with their painful
bites and stings. Animals are also involved in symbiotic rela¬
tionships with arthropods. It is amazing to see a large fish
allow a bite-sized cleaner shrimp to crawl on its body and even
into its mouth. But by allowing the cleaner shrimp to go un¬
harmed, the fish is cleaned of annoying parasites and bits of
dead tissue, and the cleaner shrimp gets a meal.
Figure 28-34 The cleaner shrimp and fish such as the queen
angelhsh, shown here, engage in a symbiotic relationship that
benefits both organisms. The cleaner shrimp gets a meal by
eating parasites on the hsh and the fish gets rid of annoying
pests. A number of fishes can sometimes be observed
congregating around a shrimp's cleaning station, waiting for their
turn to be cleaned!
Guide For Reading
How do arthropods interact with
other organisms in nature?
In what ways do arthropods
affect humans?
Figure 28-33 The damsel fly has
been trapped by a carnivorous
sundew plant. Although sundews
and other carnivorous plants are
photosynthetic, they need to "eat"
insects and other small animals to
obtain the nitrogen compounds they
need to survive.
Today, agricultural experts favor combin¬
ing all these methods into a strategy called In¬
tegrated Pest Management or IPM. IPM strives
to control pests more naturally, while using
dangerous poison sprays only in emergencies
This tactic has dramatically reduced the use of
pesticides in many places around the world.
Importantly, farmers are discovering that 1PM
•s often cheaper in the long run. This new
technology thus has the potential to improve
our environment while saving farmers world¬
wide many millions of dollars each year.
629
P IN BIOLOGY
T~^\
Humans encounter arthropods almost everywhere Th
pores of our skin are home to thousands of harmless micr 6
scopic mites that feed on dead skin and oil. And no matter how
zl
neat and clean we are, our homes, even our beds, contain mil
Figure 28-35 Millions of
microscopic dust mites live in
human homes. The mites feed on
the tiny flakes of dead skin that are
constantly being shed from the
bodies of humans.
lions of microscopic dust mites. For the most part, these tin
harmless mites are of little consequence. But many of the more
visible arthropods are of significance because they are either
useful to us or a great nuisance.
Arthropods contribute enormously to the richness of
human life. Agriculture would be impossible without the bees
butterflies, wasps, moths, and flies that pollinate crops. Bees
manufacture honey, and silkworms produce silk. In Southeast
Asia and Japan, whole shrimp and shrimp paste are important
sources of protein and major ingredients in cooking. In the
United States, shrimp, crab, crayfish, and lobster are consid¬
ered delicacies. In Africa and Asia, many people eat insects
such as grasshoppers and termites. (These insects, which are
quite nutritious, are said to taste rather good.) And many in¬
sects and spiders are predators or parasites that prey upon
harmful species.
There are many useful chemicals that may be obtained
from arthropods—far too many to list here. An extract of
horseshoe-crab blood, for example, is used to test the purity of
medications. Chitin extracted from crustacean shells is used to
dress wounds and to make thread for surgical stitches. The
chemical that makes fireflies glow is used in medical tests and
as a marker in genetic engineering.
Many new applications of arthropod chemicals are cur¬
rently being investigated. For example, chitin could be sprayed
onto fruit and frozen food to prevent spoilage and to preserve
flavor. The adhesive that barnacles use to attach themselves to
rocks, which sets quickly and hardens under water into a per¬
manent bond, could be useful in applications ranging from den¬
tistry to underwater construction and repair. Chemicals in
spider venom are being tested for potential applications as
pesticides. And scientists are currently trying to produce ge¬
netically engineered spider silk that could be used in making
products as diverse as aircraft, helmets, bulletproof vests, and
surgical thread.
Not all arthropods are beneficial to humans, however. In¬
sects (such as locusts and "medflies") and arachnids (such as
mites and ticks) cause billions of dollars in damage each year
to livestock and crops around the world. Mosquitoes inflict
n
identifying arthropods
Having heard that you are now an expert on
arthropods, some of your friends bring you the
rreatures pictured below and ask you to iden¬
tify them. Can you place each in the correct
arthropod group? Be as specific as possible.
(Hint: Check the following important features,
number of body segments, walking legs, and
pairs of antennae; presence of wings; presence
of claws. Compare the animals with the de¬
scriptions and photos in the text.)
annoying bites, and some species carry malaria and yellow
fever Biting flies carry diseases such as sleeping sickness and
rC blindness, and fleas can carry bubonic plague. Termites
cause extensive damage to wooden structures. Locusts have
destroyed crops from the time humans first began to farm. Boll
weevils are notorious for the trouble they cause cotton fanners
in the South. For years, farmers have spent billions of dollars
on poisonous chemicals to save their crops from these pests.
mm
section
1.' : R E V 1 E W
1. Why are certain insects essential to agriculture?
Figure 28-36 Some species of grasshoppers exist in two
distinctly different forms: a dull-colored solitary grasshopper or
a brightly colored gregarious locust (shown here). Locusts travel
in immense swarms that may contain as many as 50 thousand
million individuals. A swarm can devastate huge areas of crops.
One swarm destroyed 167,000 tons of growing grain—enough
to feed 1 million people for a year.
2. How are arthropods beneficial to other living things?
Give specific examples.
3 Critical Thinking—Relating Concepts Name three
dangerous or destructive arthropods and explain how
they cause problems for humans. ^
vS* I w w *
to i I
p
PROBLEM
What changes occur as
some insects grow and develop?
The key concepts in each section of this chapter are listed below to help you
review the chapter content. Make sure you understand each concept and its
relationship to other concepts and to the theme of this chapter.
MATERiALS (per group)
600-mL beaker
cheesecloth
corn flakes
dissecting tray
SUMMARIZING THE CONCEPTS
hand lens
25 mealworms
probe
rubber band
4. Put the mealworm back into the beaker of corn
flakes. Cover the beaker with a piece of
cheesecloth and secure the cheesecloth in
place with a rubber band.
5. Check the beaker every other day for a few
months by moving the probe carefully through
the corn flakes. Be careful not to injure the lar¬
vae. Look for any changes in the larvae with
respect to size or shape. Do you find any lifeess shells that look like the exoskeletons of
the larvae?
6. If you find a short, thick motionless footballshaped object among the corn flakes, carefully
slide it mto the dissecting tray with a probe
I his is the pupal stage of the Tenebrio beetle
Carefully examine the pupa with a hand lens'
noting the location and number of append¬
ages. Draw a diagram of the pupa.
7. Put the pupa back into the beaker. Continue
beet'lef 0bServations until y™ find adult
28-4 Insects and Their Relatives
• Arthropods are characterized by an exoskeleton of chitin, jointed appendages, and a
segmented body.
• Uniramians include centipedes, millipedes,
and insects.
• In order to grow, arthropods must periodi¬
cally shed their exoskeletons in a process
called molting.
PROCEDURE 1 B
1. Fill a 600-mL beaker halfway with corn flakes.
ni 25 ™al— into the beaker of corn
flakes. Observe their behavior.
3. Using a probe, slide one of the mealworms
mto a dissecting tray. The mealworm is the
arvai stage of the Tenebrio beetle. Carefully
examine the larva with a hand lens, noting the
location and number of appendages. Draw a
diagram of the larva.
28-1 Introduction to Arthropods
8. Do not remove the beetles from the con
flakes, as they may fly. Instead, try to keep ;
beetle uncovered in the beaker and examine il
with a hand lens. Note the location and
adult" 0f appendages- Draw a diagram of the
OBSERVATIONS
1. What did the mealworms do when you placed
them in the corn flakes?
® Centipedes are carnivores that have poison
claws and possess one pair of legs per body
segment. Millipedes are herbivores that have
two pairs of legs per body segment.
® The process of growth and development in
arthropods often involves metamorphosis,
• Insects have a body that is divided into three
parts: head, thorax, and abdomen. They have
three pairs of legs attached to the thorax.
2.8-2 Spiders and Their Relatives
® Members of insect societies are specialized
for performing different functions.
® Chelicerates have a body that consists of two
parts—cephalothorax and abdomen. Cheli¬
cerates have chelicerae and lack antennae.
• Arachnids, such as spiders, scorpions, and
mites, are typically carnivores that have four
pairs of walking legs.
® Insects communicate. Some forms of commu¬
nication rely on pheromones.
28-5 How Arthropods Fit into the World
® Arthropods play many roles in the natural
world,
28-3 Crustaceans
® Crustaceans, such as crabs and crayfish, are
characterized by a stony exoskeleton, two
pairs of antennae, and mandibles.
® Some arthropods are of little significance to
humans; others are important because they
are useful or a great nuisance.
2' """J 'egS d0es each of the stages have?
What other appendages does each of the
stages have?
3. What happened to the sizes of the larvae over
timer
4. Compare the larval, pupal, and adult stages in
terms of appearance and behavior.
ANALYSIS AND CONCLUSIONS
1. What did the mealworms use for food during
growth and development?
2. What evidence did you find of molting? Why is
molting necessary?
3. What changes occur as Tenebrio beetles grow
sm and develop?
REVIEWING KEY TERMS
Vocabulary terms are important to your understanding of biology. The key terms
listed below are those you should be especially familiar with. Review these terms
and their meanings. Then use each term in a complete sentence. If you are not
sure of a term's meaning, return to the appropriate section and review its definition.
28-1 Introduction to
uniramian
exoskeleton
trilobite
chitin
molt
chelicerate
crustacean
pupa
arthropod
metamorphosis
28-2 Spiders and
Their Relatives
arachnid
2B-4 Insects and
Their Relatives
pheromone
28-3 Crustaceans
mandible
632
633
c o N c E P "LM AST E R Y
CONTENT REVIEW
Choose the letter of the answer that best completes each statement.
1. Which of these is an arachnid?
a. scorpion c. grasshopper
b. horseshoe crab d. lobster
2. A free-living arthropod is certain to have
a. antennae. c. jointed appendages.
b. chelicerae. d. gills.
3. insects are characterized by
a. a stony exoskeleton containing calcium
carbonate.
b. chelicerae and pedipalps.
c. three pairs of legs on the thorax.
d. many body segments, each of which
bears two pairs of legs.
4. In crustaceans, nitrogenous wastes are
5. A wormlike immature animal undergoes a
resting stage during which it changes into
an adult that has four wings and six legs.
This animal is a (an)
a. crustacean. c. chelicerate.
b. insect. d. trilobite.
6. Most spiders breathe using
a. mandibles. c. Malpighian tubules.
b. tracheal tubes. d. book lungs.
7. Which is most likely to be a herbivore?
a. spider c. tick
b. centipede d. millipede
8. Trilobites
a. are primarily terrestrial.
excreted with the help of
a. green glands c. Malpighian tubules.
c. have highly specialized appendages.
b. spiracles d. pheromones.
d. communicate by "dancing."
b. are extinct.
Useyourunders,anding of the concerts «ed chaPter to ansuter eactt
of the following in a brief paragraph.
arthropods belong to the subphylum
, Beetles undergo complete metamorphosis
Mandibulata according to this classification
' and dragonflies undergo incomplete
scheme? Explain why many experts do not
metamorphosis, Describe the major events
favor this method of grouping arthropods.
Of the life cycles of beetles and drag°""ies3. Using a crayfish as your representative
Be sure to include a comparison of their
organism, discuss the distinguishing
characteristics of arthropods.
9 Irfsom^ciassification schemes, arthropods
4 Certain chemicals bind with juvenile
' are divided into two subphyla—Chehcerata
hormone and make it inactive. Describe
and Mandibulata—based on the type of
how exposure to such chemicals would
mouthparts they possess (chelicera and
affect the development of a moth.
mandibles, respectively). Which groups of
CRITICAL AND CREATIVE THINKING
Discuss each of the following in a brief paragraph.
1. Applying concepts Explain why you
will never see spiders three stories tall
or ants big enough to eat New York (excep
True or False
Determine whether each statement is true or false. If it is true, write "true." If it
is false, change the underlined word or words to make the statement true.
1. If the level of juvenile hormone in an
insect's body is high, the insect is in the
pupal stage.
2. Arthropods have a closed circulatory
system.
3. Centipedes are herbivores that have two
pairs of legs on each segment.
4. A spider uses fanglike pedipalps to inject
venom into its prey.
5. All the members of the class Uniramia are
now extinct.
6. Horseshoe crabs are classified as
crustaceans.
7. Arthropods are characterized by an
exoskeleton composed of calcium
carbonate.
8. Arthropods must periodically molt, or shed,
their exoskeletons.
Word Relationships
some crabs are soft-shelled.
3. Relating cause and effect ^ople who
squash an annoying hornet are often
unpleasantly surprised to find themselve
suddenly under attack by dozens of
hornets. Explain this phenomenon,
4. Making inferences Instead of spray.ng
a field with chemicals, a plane disperses
tens of thousands of tiny wasps oyer he
In each of the following sets of terms, three of the terms are related. One term
does not belong. Determine the characteristic common to three of the terms and
then identify the term that does not belong.
1. tracheal tube, book lung, spiracle, pupa
2. green gland, gill, Malpighian tubule, chitin
2 Relating0'concepts Blue crabs usually
have ha'rd, stony shells. However some
blue crabs have thin, papery shells^ These
crabs are called soft-shell crabs and are a
popular food for some people, who eat
them whole-shell and all! Explain why
3. mandible, chelicera, pedipalp, walking leg
4. chelicerate, uniramian, crustacean, insect
growing plants. What is the most likefy
reason for such an action?
3 Anplving concepts At the park one day,
¦ you observe a bee flying around an open
can of soda. Soon after, you notice that
there are a lot of bees buzzing around this
can However, there are no bees on other
open cans of soda a few meters away,
a Explain how the bees probably found
the first can of soda,
b Explain why the bees do not seeni
interested in the other cans of soda.
6. Assessing concepts Which do you think
is a better arrangement for an insect
society: having workers that can each
perform all necessary tasks or having
workers that are specialized for specific
tasks? Explain your answer.
7 Using the writing process Certam crabs
• have a peculiar symbiotic relationship with
coral- They cause branches of coral to
grow around them to form a P">tect've
orison The imprisoned crab obtains fo
and oxygen from the currents of water that
flow through its coral cage. Write a short
story or play in which one of these
imprisoned crabs converses with a more
typical crustacean.