Etymology - From
the Greek Arthron a
joint and Pous for
Arthropod, animal with a hard, outer skeleton and a
jointed body and limbs. Arthropods make up a phylum of
invertebrates that includes insects, such as
ants, beetles, and butterflies; crustaceans, such as
lobsters, shrimps, and crabs; and arachnids, including
scorpions, spiders, and ticks. In terms of sheer numbers
and the variety of niches they fill, arthropods are the
most successful animals on Earth. More than one million
arthropod species have been identified—more than 20
times the number of known
fish, amphibian, reptile, bird, and mammal species
combined. This figure is considered a low estimate of
the phylum's actual size because many arthropod species
have yet to be discovered and documented. Some
scientists suggest the number of arthropod species in
tropical forests alone may approach six million to nine
Arthropods: A success story
•Almost any way you look at them, arthropods are
successful: They have been around for more than 500 million
years and are still evolving.
•They live on Earth in overwhelming numbers.
•There have come in all shapes and sizes.
•They have evolved to fill a variety of ecological niches —
from tiny internal parasite to giant bird-eating predator.
But what is it about arthropods that has made them so
successful? Let's begin our investigation by reviewing what
an arthropod is.
Bilateral (left/right) symmetry
Two Matching Sides
Many animals have a body form that is symmetrical, meaning that
it could be divided into matching halves by drawing a line down
the center. In this respect, arthropods are built like humans are;
the right half of an arthropod is a mirror image of its left half —
this is called bilateral symmetry (bi = two, latus = side). Other
animals have symmetry like a snowflake’s — there are many
different ways to carve it into matching halves, and all of these
lines meet in the middle, dividing it up like a birthday cake. That
is called radial symmetry. Other animals are not symmetrical at
all — their bodies cannot be divided into similar halves with a
Like Cars in a Train
At some point in their lives, all arthropods have bodies
that are internally and externally segmented. These
segments piggyback on one another, like a series of cars
in a train. Each body segment tends to repeat the same
suite of structures (for example, a pair of legs, a set of
breathing organs, and a set of nerves), often with slight
variations down the length of the animal. Usually, sets
of segments are grouped into a larger unit, such as the
The bodies of arthropods are supported, not by internal
bones, but by a hardened exoskeleton made of chitin, a
substance produced by many non-arthropods as well. In
arthropods, the nonliving exoskeleton is like a form-
fitting suit of armor. It is produced by the "skin" and
then hardens into a protective outer-covering. This
exoskeleton is handy in some ways (it provides
protection and prevents water loss), but is limiting in
others. In order to grow, all arthropods must shed the
exoskeleton and produce a new, larger one.
•Body cavity a true coelom.
•Most possesses a through straight gut with an anus (in
•Body possesses an external skeleton (in most cases).
•Nevous system includes a brain and ganglia.
•Possesses a respiratory system in the form of tracheae
and spiracles (in most cases).
•Possesses an open circulatory system with a heart (in most
•Feed on everything.
•Body are divided into three, head, thorax, and abdomen
The five branches of the
Modern arthropods include insects, spiders,
centipedes, shrimp, and crayfish. All arthropods
are the descendents of a single common ancestor.
Just as you and your biological cousins can trace
your ancestry back to a common set of
grandparents, all arthropods can trace their
ancestry back to a common arthropod ancestor.
Any species descended from that ancestor is an
arthropod. And any species not descended from
that ancestor is not an arthropod.
Life Cycle and Reproduction
In most instances, both sexes of arthropods
occur separately. Males commonly pass sperm to
females in sealed packets called
spermatophores. The males lay these packets on
the ground, and the females later pick them up,
or the male deposits them into the female's
genital opening. Among crustaceans, millipedes,
spiders, mites and some insects, males transfer
sperm directly to females.
Fertilized arthropod eggs hatch after days, weeks,
months, and even years. Most species deposit their
eggs externally, but some species hatch their eggs
internally, bearing live young. In many species, such
as spiders, the hatchlings look like miniature adults.
The larvae of other arthropod species have little or
no physical resemblance to adults. For example,
caterpillars are quite distinct from the butterflies
that they become as adults. These larvae, in the
process of metamorphosis, pass through a series of
distinct phases to become adults. Larvae may also
inhabit different environments and eat different
foods than their parents. The life spans of
arthropods range from a few weeks to several
The evolutionary origins of modern arthropods are
unclear and complex. It is generally accepted that the
phylum is polyphyletic—that is, derived from several
separate ancestral lines. The ancestors of arthropods
were ancient aquatic segmented worms, similar to
present-day annelids, although the fossil evidence is
sketchy. The ancient seas of the Cambrian and
Ordovician periods (570 million to 435 million years ago)
were teeming with aquatic arthropods, especially
During the Silurian and Devonian periods (435
million to 360 million years ago), the arthropods
were among the first animals to leave the water
and colonize the land. When they emerged from
the water, they had few, if any, competitors.
They swiftly adapted to the demands of
terrestrial life, occupying new niches as
predators, plant eaters, parasites, and
decomposers. The earliest terrestrial arthropod
fossil is of a scorpion-like arachnid. The earliest
insect fossils come from a few million years later.
These early terrestrial arthropods sometimes
reached great size, much larger than any land
arthropods known today.
Scientists have found evidence that Earth’s
atmosphere had more oxygen (perhaps as
high as 40 percent compared to 21 percent in
our modern world). Higher oxygen pressure
would have enabled animals with relatively
primitive breathing systems, such as those
found in land arthropods, to be active even in
very large animals. By the end of the
Carboniferous period (360 million to 290
million years), four-legged vertebrates
(amphibians and reptiles) dominated the land
and used arthropods as a major source of
Crustacean, common name for any of a group of
mainly aquatic arthropod invertebrates, including
the crab, lobster, and shrimp. They are among the
most successful animals, abundant in the sea much
as insects are on land. The majority of animals in
the world are marine crustaceans that belong to the
copepod subclass of the crustacean subphylum.
Crustaceans are also successful in fresh water, and
a few, such as sow bugs, are common in moist land
environments. Although most crustaceans are
small, they vary widely in form and include such
large members as lobsters up to 60 cm (24 in) long
and a spider crab with a leg span of 3.6 m (12 ft).
The subphylum contains about 26,000 known
Some Characteristics of Crustaceans:
•A hard exoskeleton made of calcium - no internal skeleton.
•The head has two compound eyes, two pairs of antennae,
and three pairs of mouthparts.
•A pair of green glands excrete wastes near the base of
•The abdominal segments have swimmerets (swimming legs)
•The sexes are separate. Eggs are attached to the
swimmerets (swimming legs) of the female. The first pair is
enlarged in the male (it is used to pass sperm to the
•The tail is fan-shaped, and ends in uropods and a telson.
•The circulatory system is open; there is no heart and the
"blood" is pumped by vessels into sinuses, and does not flow
in a closed loop).
•The nervous system consists of a primitive ventral nerve
cord and ganglia system (similar to those of an earthworm).
Generalized Anatomy of a Crustacean
Crustacean anatomy is characterized by an external skeleton and a
segmented body. In different crustacean species these segments and the
accompanying limbs have evolved into specialized appendages for
respiration, swimming, crawling, and feeding. The extended inner cavity
contains the digestive and nervous systems.
Usually the sexes are separate, but some parasites
and most barnacles, which have difficulty obtaining
mates, are simultaneous hermaphrodites (male and
female at the same time). This increases the
number of possible partners and may allow self-
fertilization as a last resort. Some crustaceans
switch sex as they get older. Many crustaceans
exhibit elaborate courtship behavior, and the males
may fight for the chance to mate.
The young of marine crustaceans generally pass
through one or more larval stages that are quite
unlike the adult form. Often the larvae swim in
open water to find a place to live. Freshwater and
terrestrial crustaceans skip the larval stage,
except for those that return to the sea to spawn.
After fertilization, the developing eggs are
generally cared for by the mother until they have
reached the larval or postlarval condition.
Otherwise, little parental care exists among
crustaceans. Some live in male-female pairs or are
gregarious, but they do not form well-organized
societies. The smallest crustaceans live for just a
few days, but the largest ones may live for
Krill, common name for small, shrimplike crustaceans that swarm
in dense shoals, especially in Antarctic waters. Most of the
approximately 90 species range in length from 8 to 70 mm (0.3
to 2.8 in) and use their feathery legs for straining out the tiny
diatoms on which they live; many emit a strong blue-green light
that probably helps them congregate and spawn. Krill inhabit
open seas and are an important link in the food web. They are
eaten by fish, birds, and especially baleen whales, which
consume as much as 2000 kg (4400 lb) of krill in one feeding.
Shoals of the crustaceans have densities of 20 kg per cu m (35
lb per cu yd); some species remain near the surface and others
are found as deep as 2000 m (6500 ft).
Shrimp, any of about 2,000 species of
small, aquatic animals related to crabs,
lobsters, and crayfish. Shrimp range in
size from animals not much bigger than a
fingernail to ones over 20 cm (8 in) long.
Larger species are often known as prawns,
although many people interchange the two
names for all species.
Body of a Shrimp
A shrimp has a long, thin, semitransparent body. Five
pairs of jointed legs on the abdomen are used like
paddles to swim. Rapid flicks of a fanlike tail help the
animal move quickly to avoid predators.
In shrimp reproduction, the male deposits sperm into a
gelatinous mass produced by the female and located between
her fourth pair of walking legs. The female then lays eggs, in
batches of up to 15,000 at a time. The sperm fertilizes the
eggs. Females usually care for the fertilized eggs in a brood
chamber found on the underside of her tail. In some
species, the eggs are scattered in the water and left to
develop on their own. As with crabs and lobsters, the eggs
hatch to produce drifting larvae. The larvae change shape as
they grow, developing an adult body after molting their skin
several times (see Metamorphosis). Once the change in shape
is complete (from 30 to 160 days, depending on the
species), the young shrimp are sexually mature and able to
breed. The life spans of shrimp vary among species--some die
once they have bred, but others live for six or seven years.
Lobster, common name for marine
decapod (that is, with five pairs of
appendages on the thorax) crustaceans
closely related to the freshwater
Common name for any of a group of
crustaceans characterized by a reduced
abdomen and an enlarged and broadened
anterior portion of the body. Although most
common as bottom dwellers in the sea, crabs
also occur in fresh water, and some venture
onto land. Crabs are divided into two groups:
true crabs (about 4500 species) and hermit
crabs and their allies (about 1400 species).
REPRODUCTION AND LIFE CYCLE
All crab species have separate sexes. In many species,
mating occurs only when the female has just molted and
her new shell is not yet hard. (So-called soft-shell
crabs, a delicacy, are simply crabs in this transitional
molting stage.) The eggs are held in a brood pouch and
pass through two larval stages before they hatch into
tiny larvae, which swim about in the water. In some
species the larva first appears in a form called a zoea,
which does not resemble the adult, and then in a more
crablike stage called a megalops, in which the abdomen
is still prominent. Each time the young crab molts, it
increases its size considerably, but it is exposed to
danger while it is still in its unprotected, soft state.
Lost legs and chelae can be replaced when molting takes
place. Crabs may live from 3 to 12 years.
Crayfish, also crawfish, common name for any
crustacean resembling but smaller than their
relatives the lobsters, ranging in length from 2
to 40 cm (0.8 to 16 in); the first of their five
pairs of walking legs is equipped with strong
claws. Crayfish live in freshwater rivers and
streams in temperate climates; one family is
confined to the northern hemisphere, another
to the southern hemisphere, and a third to the
Crayfish usually burrow into the banks of streams
or ponds and feed upon live or decaying animal or
vegetable matter. The male inserts sperm into a
receptacle in the female's thorax in the fall; the
eggs, laid in the spring, are then fertilized and
hatch in eight weeks or less. The young remain with
the mother for a short while; after several
molts, they reach adult size. The animals can live
for three years or longer. The white-clawed
crayfish is particularly prized for food despite its
small size. Some cave-dwelling crayfish species are
blind, such as those of Kentucky's Mammoth Cave.
Like other crustaceans, crayfish are capable of
regenerating lost limbs during molting.
Barnacle, popular name for members
of a subclass of sessile crustaceans.
The name was originally applied to
the barnacle goose of northern
Europe, and its transfer to these
crustaceans was due to the fable
that the bird develops from the
stalked, or goose, barnacle.
Most barnacles are hermaphrodites . All inhabit
salt water. The larvae are free-swimming, but the
adults are always permanently attached to
foreign objects, such as ship bottoms, wharf
piles, rocks, floating timbers, whales, large fish,
and shellfish. The subclass is divided into five
orders, four of which are minute forms parasitic
on other shellfish. The other order includes the
acorn barnacles, common to temperate and cold
waters, and the stalked barnacles, which are
usually found in warm waters, but, because they
attach themselves to ships, they are distributed
throughout the world.
Arachnid, term for animals in the class
including the scorpions, spiders, daddy
longlegs, mites, and ticks, and certain other
eight-legged land invertebrates. Fossils
suggest that arachnids were among the first
animals to live on land, perhaps in the early
Devonian Period, nearly 400 million years ago.
About 60,000 species are known, although
many, especially mites, remain undiscovered or
undescribed. Arachnids are found throughout
the world in nearly every habitat, but they
reach their greatest size and diversity in warm
arid and tropical regions.
The arachnid body is divided into two parts: anterior
and posterior. The anterior part, called the
cephalothorax, contains sense
organs, mouthparts, and limbs in pairs. The first pair
of limbs—the chelicerae—may form pincers or poison
fangs, and the second pair—the pedipalps—may serve
as pincers, feelers, or legs. The other limb
pairs, generally four, are used for walking. The
posterior part of the body, the abdomen, bears the
genital opening and other structures. It is usually
equipped with modified gills called book lungs. Most
arachnids are solitary except at the time of
mating, when a variety of complex behavior patterns
may be observed. Females may guard eggs or
young, which are often born live.
Scorpion, common name for arachnid having a
flat, narrow body, two lobsterlike claws, eight
legs, and a segmented abdominal tail.
Terminating in a venomous stinger supplied by a
pair of poison glands, the tail is usually curved
upward and forward over the back. About 1400
species of scorpion exist; about 40 of them
occur in the United States. Scorpions are
usually brown in color and range from about 2.5
to 20 cm (about 1 to 8 in) in length.
Spider(arthropod), any of a large group
of invertebrates (animals without
backbones) that have spinning glands
used to produce silken threads and webs.
There are about 40,000 species of
spiders. Spiders are found
worldwide, except for in the oceans, and
they live in all habitats and at most
Spiders have 8 appendages. The first pair
are used for holding the prey and
feeding. The second pair may also be used
for holding and killing their prey. The
others are used as legs for walking. Most
spiders also have 8 eyes. Spiders have
fangs that are used to inject poison to
paralyze or kill their prey. Many spiders
can produce silk threads to spin webs for
catching prey, and for building cocoons
for their eggs.
The fang-bearing appendages, or chelicerae, of most
spiders swing inward from the side to grasp prey. In
some larger spiders, however, the chelicerae swing
downwards, pinning the prey underneath.
The life cycle of the spider consists
of four stages: egg, larva, young
spider, known as a nymph or
spiderling, and adult. Like insects,
spiders grow only by molting, a
process that involves periodically
shedding their exoskeleton. In each
molting stage, young spiderlings
resemble tiny adults, a process known
as incomplete metamorphosis.
COURTSHIP AND MATING
Spiders become sexually mature after their last molt, at which
time females have developed functional ovaries and males have
mature testes. In most spider species the male courts the
female before mating occurs. After a male spider has filled its
palps with sperm cells, he begins searching for a female. A male
begins by identifying himself to a female so that she does not
mistake him for potential prey. In some spiders, such as
American tarantulas, this identification process involves the
male repeatedly touching the female. More often, a male
courting a female communicates with her over larger distances
using vibrations. For instance, a male wolf spider uses its legs to
drum on the ground. In some web spiders, the male attaches a
special signal thread to the female’s web. The male then drums
or plucks the thread in a rhythm that indicates the vibration is
caused by another spider of the same species and not by an
ensnared insect. If a female is ready to accept a courting male,
she may send signals back to him.
Locating the right female can be tricky
for a male spider. Fortunately, female
spiders produce certain chemical
substances known as pheromones that aid
spider courtship. A female may release
these pheromones through the air (like a
perfume) or she may deposit them on her
silk threads. When a male spider
encounters pheromones from a female of
the same species, he becomes
excited, even if the female is not
Spiders with better eyesight may rely
mostly on visual signals during courtship.
When a male notices a female, he starts
a zigzag dance in front of her in which
he raises his front legs, vibrates his
palps, and twitches his abdomen. Each
species uses a different courtship
dance with unique movements. A female
will only accept a male who performs a
dance with movements specific to that
When a male finds an interested female, he
inserts his sperm-containing palp into the
female’s genital opening. The process of
mating can be very brief (a matter of
seconds), or it can last several
hours, depending on the species. In most
species both sexes separate peacefully
after mating. Contrary to popular belief, the
female black widow spider does not kill her
partner after mating. Depending on the
species, a female may mate only once or she
may mate with several males during her
Typically the female lays her
fertilized eggs in a silky case called a
cocoon, which provides a protective
and insulating environment for the
developing spiders. Many females
abandon their cocoons right after
they deposit their eggs, although
they may camouflage them or hang
them in hidden locations. Other
spiders guard and defend their
cocoon until the eggs hatch.
Young Garden Spiders Hatching
All spiders lay eggs in some sort of silk-wrapped
cocoon. The female may then abandon the cocoon or
guard it from predators. In some species of spiders,
the female carries the cocoon with her until the eggs
TYPES OF WEB
The orb web has a
circular shape. This
type of web is used
to catch large flying
insects, like the
like this Australian
species manipulate a
small web between
their front legs. To
catch its prey, the
spider uses the web
as a net, stretching
it over a doomed
The web of the funnel spider is built above the
ground and oriented so it will intercept flying
insects. Funnel spiders design their webs with a
small opening, or funnel, at the bottom, in which
the spider lies in wait.
This trap-door spider emerges from its burrow through a
doorway surrounded by leaves. Trap-door spiders are
various species of large tropical spiders that live in self-
constructed underground lairs with carefully hidden
entrances. These spiders come out of their homes to eat
ants and insects, and are harmless to humans.
SYDNEY FUNNELWEB SPIDER
BROWN RECLUSE SPIDER BLACK WIDOW
Tick, common name for members of a group
of large mitelike arachnids parasitic on
mammals, birds, and reptiles. All ticks are
bloodsucking parasites. Ticks are found in
most parts of the world but are generally
limited to those habitats frequented by
their hosts—namely, woods, tall grass, and
shrubby vegetation—where they climb onto
plants and wait to jump on a passing host.
Mite, common name for some 30,000 species of minute, usually
oval-bodied arachnids. They are worldwide in distribution. Mites
resemble ticks in having the head, thorax, and abdomen fused into
one unsegmented body, but they are usually much smaller. They
often have three pairs of legs in the larval stage and four pairs of
legs in the nymph and adult stages. The mouthparts are adapted
for piercing. Like most arachnids, mites breathe by means of
tracheae (small tubes opening on the surface of the body), and
they live in both aquatic and terrestrial habitats. Many are animal
parasites; some, which subsist on vegetation, produce galls on
plants. They are economically and medically injurious, because
they carry diseases affecting livestock and humans.
Daddy Longlegs, common name for any of a group of
spiderlike arachnids, also called harvestmen or
harvest spiders. Daddy longlegs resemble true
spiders but have oval, segmented abdomens, only two
eyes, and exceptionally long, slender legs. They feed
on small insects, dead animals, or plant juices. They
have very small mouthparts and do not harm humans.
Daddy longlegs often congregate in large numbers in
caves or other sheltered places. The name is also
applied to crane flies.
Centipede, common name for the
members of a class of the
arthropod phylum. The centipedes
are long, segmented animals with
jointed appendages and a
poisonous “bite” that in some
species is dangerous to humans.
Centipedes are often confused
with millipedes, which constitute
a separate class.
The centipede body is divided into well-marked
segments, the number of which varies from 12 to
more than 100. The head, which is covered by a flat
shield above, bears a pair of antennae, usually of
considerable length and consisting of from 12 to
more than 100 joints; a pair of small, strong,
toothed, and bristly mandibles; and a pair of
underjaws, usually with palps. The next, limblike
appendages are followed by a modified pair of legs
with strong joints, terminating in a sharp claw into
which a poison gland opens. These appendages are
used for seizing and killing prey. The two legs on
each other segment are usually seven-jointed,
sometimes bearing spurs and glands, and are
Centipedes are nocturnal and remain
under stones or wood during the day.
They are all carnivorous. One genus
bears live young; the others lay
Millipede, any of about 1000 species of cylindrical, many-
legged arthropods . Found worldwide, millipedes have
segmented bodies with two pairs of legs on each of the 9 to
100 or more abdominal segments, depending on the species,
and one pair on three of the four thoracic segments. Because
of their numerous legs, the animals walk slowly, with a
wavelike motion of the legs down the body. In length they
range from about 0.2 to 23 cm (about 0.1 to 9 in). Millipedes
have a hard protective layer of calcium-containing chitin
(except in some small species), two simple eyes, one pair of
mandibles, two short antennae, and (in most species) stink
glands with secretions that repel or kill insect predators.
Another protective strategy is to curl into a spiral or a ball
when threatened. The animals live in dark, damp places and
feed on decaying plant life, sometimes damaging crops but
also enriching the soil. They grow by molting and may live for
one to seven years.
Insect, small, air-breathing animal
characterized by a segmented body
with three main parts—head, thorax,
and abdomen. In their adult forms,
insects typically have three pairs of
legs, one pair of antennae, and in most
instances, two pairs of wings.
Insect means "segmented" in Latin.
There are about a million different
types of insects and many more that
have not been discovered yet.
REPRODUCTION AND METAMORPHOSIS
A small number of insects give birth to live young,
but for most insects, life starts inside an egg. Insect
eggs are protected by hard shells, and although they
are tiny and inconspicuous, they are often laid in vast
numbers. A female house fly, for example, may lay
more than 1,000 eggs in a two-week period. As with
all insects, only a small proportion of her young are
likely to survive, but when conditions are unusually
favorable, the proportion of survivors shoots up, and
insect numbers can explode. In the 1870s, one of
these population explosions produced the biggest
mass of insects ever recorded: a swarm of locusts in
Nebraska estimated to be over 10 trillion strong.
Most insects undergo one of two varieties of
metamorphosis: incomplete or complete.
Dragonflies, grasshoppers, and crickets are
among the insects that experience incomplete
metamorphosis. In these insects, the
differences between the adults and the young
are the least marked. The young, which are
known as nymphs (or naiads in the case of
dragonflies), gradually develop the adult body
shape by changing each time they molt, or shed
their exoskeleton. A nymph's wings form in buds
outside its body, and they become fully
functional once the final molt is complete.
Insects that undergo complete metamorphosis include
butterflies, moths, beetles, bees, and flies. Among
these species the young, which are called larvae, look
completely different from their parents, and they
usually eat different food and live in different
environments. After the larvae grow to their full
size, they enter a stage called the pupa, in which they
undergo a drastic change in shape. The body of a
pupating insect is confined within a protective
structure. In butterflies, this structure is called a
chrysalis, and in some other insects the structure is
called a chamber or a cocoon. The larva's body is
broken down, and an adult one is assembled in its place.
The adult then breaks out of the protective
structure, pumps blood into its newly formed wings, and
BLUE DAMSEL FLIES IN
EGGS FRUIT FLY HONEY PUPAE
LARVA AND CATERPILLAR