Layman D. Biology Demystified: A Self-Teaching Guide

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CHAPTER

The Arthropods: No

Jointed Backbone,

but ‘‘Jointed Feet’’

Chapter 10 introduced us to the invertebrates – the animals ‘‘without’’ (in-)

‘‘jointed backbones’’ (vertebr). Technically, the backbones are called the

vertebrae (VER-tuh-bray). Humans and other vertebrates contain such

jointed backbones that support their body weight.

This chapter now introduces another group of invertebrates – the arthro-

pods (AR-thruh-pahds). Arthropods, like other invertebrates, contain no

jointed backbones. But they are set distinctly apart, however, by their

‘‘jointed’’ (arthro-) ‘‘feet’’ (pod)!

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Common Characteristics of the Arthropods

As Figure 11.1 shows, the general body plan shared by all arthropods can be

represented by the anatomy of a lobster. The arthropods are a huge phylum of

invertebrates with bilateral symmetry whose bodies are clearly divided into

jointed segments, and whose legs are hollow and also divided into jointed

segments. Possessing bilateral symmetry, the arthropods have pairs of match-

ing right–left antennae (an-TEN-ee) or feelers, legs, and in many insects,

wings.

THE SEGMENTED BODY

The lobster is covered by a hard exoskeleton (eks-oh-SKEL-eh-ten) – an

‘‘outer’’ (exo-) ‘‘hard dried body’’ (skeleton). Since the exoskeleton is on

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Fig. 11.1 The lobster body as a typical arthropod.

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the surface of the animal, it is also called the cuticle (KYOO-tih-kl) or ‘‘little

skin.’’ The exoskeleton (cuticle) can even be thought of as a ‘‘coat of mail,’’ a

type of armor! Why is this? The reason is that the lobster exoskeleton con-

tains large amounts of chitin (KY-tin). Chitin also covers the surfaces of

crabs, beetles, and crickets. Chitin is a tough, horny substance providing

protection to the invertebrate body, much as if the lobster were an ocean-

dwelling knight wearing a ‘‘coat of mail’’ (chitin)!

The chitin-armored lobster body has two pairs of antennae (short and

long), two feeding mouthparts, as well as two large pincers, attached to either

side of the head. A thorax (THOH-racks) or ‘‘chest’’ piece forms a second

segment behind the head. The head and thorax are often lumped together by

biologists and named as a single large segment called the cephalothorax (sef-

uh-low-THOR-aks) or ‘‘head’’ (cephal)-and-‘‘chest’’ piece. Four hollow, seg-

mented legs are attached to either side of the cephalothorax and allow the

lobster to easily walk over the ocean ﬂoor.

Behind the thorax is the third body segment – a much-longer abdomen

(AB-duh-mun). The abdomen is literally the long ‘‘belly’’ of the beast. But for

we lobster-eating humans, it is usually just considered the tail! The exoske-

leton protecting the abdomen is subdivided into a number of hinged plates,

allowing the lobster to use its swimming appendages (uh-PEN-dih-jes) or

‘‘hangers-on’’ to scoot its body rapidly along.

AN OPEN-ENDED CIRCULATORY SYSTEM

Arthropods do have hearts, but these hearts pump a ﬂuid called hemolymph

(HE-moh-limpf), rather than blood. The circulatory system is an open one.

This means that the heart pumps the hemolymph into vessels and sinuses

which are not closed oﬀ. Nevertheless, the hemolymph still provides the body

tissues with an adequate supply of oxygen and other nutrients.

A DEPENDENCE UPON MOLTING

Because many arthropods have a rigid exoskeleton, yet they continue to grow

in size throughout most of adulthood, they are forced to periodically molt

(MOHLT) or shed. After the exoskeleton is molted (shed), the lobster or

other arthropod is temporarily soft-bodied and vulnerable to hungry preda-

tors. But the head of the lobster bears two eyes, each perched upon a movable

stalk, that lets it readily see approaching predators (and hopefully escape

being eaten!).

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Major Types of Arthropods

There are several major types or categories of arthropods. The ﬁrst question

a person might ask, however, is: ‘‘The arthropods are all invertebrates. So,

why weren’t they included within Chapter 10, which described the inverte-

brates?’’ [Study suggestion: Take a careful look at Figure 11.2. What parti-

cular feature really sticks out at you? How do you think this feature explains

why arthropods are placed in a separate chapter of their own?]

Using a traditional approach to taxonomy, there is a single phylum

Arthropoda (ar-THRAH-pah-da), subdivided into ﬁve smaller classes.

These are the Crustaceans (krus-TAY-shuns), Arachnids (ah-RACK-nids),

Chilopods (KY-luh-pods), Diplopods (DIP-luh-pods), and Insects. In Figure

11.2, the relative size of each oval shape representing these arthropod classes

visually pictures the number of known species belonging to each one.

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Fig. 11.2 The ﬁve major classes of arthropods.

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ANCIENT TRILOBITES AND THE MODERN CRUSTACEANS

Let us begin our examination of the major arthropod classes with the crus-

taceans – invertebrates whose bodies are surrounded by hard ‘‘shells’’ (crus-

tace). In the modern world, the crustaceans include crabs, lobsters, crayﬁsh,

and shrimp. Besides their hard shells (rigid exoskeletons), these crustaceans

have jointed bodies and legs, as well as gills that allow them to exchange their

hemolymph gases with the external gases in the surrounding water.

There are now about 40,000 known species of crustaceans, most of them

living within either freshwater or saltwater environments. Yet, biologists

have repeatedly observed an early ancestor (long extinct) of the modern

crustaceans in the Fossil Record. This extinct group of arthropods is the

trilobites (TRY-loh-bites). The trilobites consisted of about 4,000 species,

and they were very abundant in the sea during the Paleozoic Era (Chapter

3). The trilobites had bodies comprised of ‘‘three’’ (tri-) vertical ‘‘lobes’’ (lob)

subdivided into many horizontal segments (Figure 11.3). Sprouting from

either side of the body segments were numerous jointed legs. But unlike

the modern lobster and other crustacea, these ancient marine invertebrates

had appendages running along either side of their body that showed very

little diﬀerence or specialization. For instance, they lacked the protective

pincers of present-day crabs and lobsters.

The trilobites rather suddenly became extinct about 250 million years ago,

near the end of the Paleozoic Era (shortly before the dinosaurs appeared

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Fig. 11.3 The three-lobed body of the ancient trilobite.

during the Age of Reptiles). Because they never reappear at later stages in the

Fossil Record, the limited time period of trilobite existence strongly supports

the concept that once a group of animals becomes extinct, they generally stay

extinct! There is a deﬁnite Biological Order or recognizable sequence of body

forms and patterns that seem to precede or follow one another during the

long time-scale of evolution.

THE ARACHNIDS: SPIDERS AND THEIR RELATIVES

The arachnids are literally ‘‘spiders or webs’’ (arachn). Besides the spiders,

however, the arachnids include scorpions, ticks, and mites as well. The

arachnid group includes about 57,000 species. The basic anatomy of the

arachnids is displayed within Figure 11.4. The spider body has two segments

– head and thorax (cephalothorax). Situated farthest in the front of the head

are the pedipalps (PED-uh-palps) and chelicera (kuh-LIS-er-ah). The

pedipalps are a pair of ‘‘feeling’’ (palp) ‘‘feet’’ (pedi) – small foot-like appen-

dages that help sense when prey is present. The chelicera or ‘‘claws’’ (cheli)

then come into play. The two chelicera serve much like fangs, injecting

poison from a gland just below the four eyes. The chelicera inject digestive

juices, as well as poison, into captured prey, which softens the attacked body

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Fig. 11.4 The amazing body of a spider.

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as well as killing it. The spider ﬁnally sucks the liqueﬁed food into its diges-

tive system.

Along each side of the cephalothorax, spiders have four pairs of walking

legs. Other special characteristics include a stack of book lungs, as well as a

silk gland with spinnerets (SPIN-uh-rets) – small organs used for spinning

webs.

CENTIPEDES AND MILLIPEDES: ‘‘SO MANY LEGS!’’

The Chilopod class literally consists of arthropods with ‘‘lip’’ (chil) ‘‘feet’’

(pod), obviously named for their appearance. There are approximately 2,000

species in this group. The main Chilopods are the centipedes (SEN-tuh-peeds)

or organisms with a ‘‘hundred’’ (centi-) ‘‘feet’’ (ped). Centipedes are thin,

worm-like arthropods with segmented bodies having many pairs of legs

(but certainly not 100!). The centipede has very long antennae and poison

claws that catch smaller insects to eat (Figure 11.5, A).

The Diplopod class includes those creatures with ‘‘double’’ (dipl) ‘‘feet’’

(pod). About 7,000 species of Diplopods exist. True to their Class name, each

body segment bears two legs with feet. The primary Diplopods are the milli-

pedes (MIH-luh-peeds) – translated to mean arthropods with a ‘‘thousand’’

(milli-) ‘‘feet.’’ The millipede body, of course, has far fewer than 1,000 feet

(Figure 11.5, B)! Millipedes feed on moss, decaying leaves, and a variety of

other types of plant matter.

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Fig. 11.5 Centipedes and millipedes: So many legs!

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INSECTS: THE SWARMING HERD

Of all the arthropods, the insects represent by far the hugest number of species

– at least one million known (and maybe another million or so insect species

still undiscovered, mainly within tropical forests)! This means that there are

more species of insects than species of all other types of animals, combined!

Yet, despite their overwhelming numbers and diversity, insects share a

common body plan (Figure 11.6). The word insect actually comes from the

Latin for ‘‘cut’’ (sect) ‘‘into’’ (in-). The reason for this name is that the body

of most insects is ‘‘cut’’ (subdivided) into three major segments. Using the

example of a grasshopper, there is a head, a thorax, and an abdomen. The

insect head bears two antennae and a pair of compound eyes. In their adult

stages, many insects have three pairs of jointed legs and either one or two

pairs of wings, all attached to the thorax.

Metamorphosis: A Caterpillar Becomes A

Butterfly

No doubt you have seen a humble caterpillar crawling slowly across a leaf.

But hidden from your gaze, at the end of summer the caterpillar begins to

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Fig. 11.6 The grasshopper: An insect body plan.

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undergo a dramatic metamorphosis (met-uh-MOR-fuh-sis) – a ‘‘changing

over’’ (meta-) of its body shape or ‘‘form’’ (morph).

In most cases, metamorphosis is a mechanism whereby an embryonic stage

of an animal (such as an insect) changes into a strikingly diﬀerent adult body

form. One of the most beautiful examples of metamorphosis is the transfor-

mation of a caterpillar into a gracious, delicate-winged butterﬂy. Some of the

major steps in this process are shown in Figure 11.7 for the well-known

monarch (MAHN-ark) butterﬂy. Like a king or queen or some other type

of ‘‘ruler-alone’’ (monarch) in the world, the adult monarch butterﬂy stands

alone in its striking orange-and-black body pattern.

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Fig. 11.7 Caterpillar into monarch: a striking metamorphosis.

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The process begins with a larva (caterpillar) stage. The black-striped mon-

arch caterpillar (larva) likes to feed on the milkweed plant, named for its

white, milky juice. The caterpillar just keeps gorging itself on milkweed leaves

and growing throughout the summer, molting its skin several times. Finally,

at the end of the growing stage, the caterpillar larva ﬁrmly attaches itself to a

branch. Here it molts several more times, then encases itself within a cocoon

(kuh-KOON). A cocoon is an external ‘‘shell or husk’’ of silky material that

caterpillars spin around themselves during their preparation for metamor-

phosis.

When the larva is encased within a tough, protective cocoon, its new stage

of development is called the pupa (PYOO-puh) or chrysalis (KRIS-uh-lis). In

Latin, chrysalis literally means ‘‘golden pupa of a butterﬂy.’’ Likewise, pupa

means ‘‘girl or doll.’’ So, the pupa (chrysalis) is poetically described as a

‘‘golden girl or doll’’ that will eventually mature into a queen-like, elegant

butterﬂy! Perhaps the reference to ‘‘golden’’ is due to the rather shiny appear-

ance of the outer wall of the cocoon enveloping the chrysalis (pupa).

Within the pupa, a precisely timed genetic program is now turned on. The

tissues of the larva are broken down, then replaced by other cells that

undergo mitosis and diﬀerentiation (dif-uh-ren-she-AY-shun) – ‘‘the process

of becoming diﬀerent’’ or specialized. Certain body cells, for example,

migrate to the sides of the pupa in a bilaterally symmetrical manner. The

cells in these locations then diﬀerentiate (dif-uh-REN-she-ate) into the highly

specialized structures of the developing butterﬂy wings.

Soon after the arrival of spring, the adult begins to emerge from the

cocoon. At ﬁrst, the wings are ﬂat and wet and pressed against the sides of

the butterﬂy body. But a pumping process pushes ﬂuid out into the veins of

the wings, stiﬀening and opening them. The result is a beautiful, orange,

black-striped, bilaterally symmetrical monarch butterﬂy – one of the most

impressive displays of Biological Order known to humankind!

Insects: Inducers of Biological Order and

Disorder In the Environment

Since there are more species of insects than all other types of animals com-

bined, a convincing argument can be made that insects (not human beings)

are the real ‘‘monarchs’’ (rulers) of this planet! Because of their sheer num-

bers, then, the World of Insects exerts a critical inﬂuence upon both

Biological Order, and Biological Disorder, within the ecosystem of the exter-

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