This document provides an overview of arthropods, which account for over three quarters of all known animal species. It discusses their key defining characteristics, including bilateral symmetry, segmented bodies, jointed legs, and hard exoskeletons. The document explores the diversity and success of arthropods, from their ancestral origins over 500 million years ago to the over one million insect species identified today. It also summarizes the major subgroups of arthropods like insects, spiders, crustaceans, and myriapods.

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“To a good
approximation,
all species are
insects.”
-Robert May

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4.  Chances are that you are interacting with arthropods on a daily basis — in your garden, in a
nearby stream, in your home, on your dinner table, and perhaps even on your pet! Arthropods abound!
 All of the animals pictured here (and many more!) are arthropods. In fact, more than 83% of all
described animal species are arthropods. In this investigation, you will delve into the evolutionary
history of arthropods to find out how they have evolved over time. You will learn what traits
arthropods inherited from their common ancestor, how arthropods got their start over 500 million
years ago, and how their lineage has diversified in some ways and been constrained in others. But first,
let’s find out what arthropods are up to today. In the next few pages, you’ll get a taste of the enormous
variety that arthropods have evolved. Arthropods are found all over the world and make their livings in
all sorts of ways.

5.  Let's get an idea of just how dominant arthropods are, by looking at how many of them there are.
 About a septillion (1,000,000,000,000,000,000) insects are alive at any time. That's about 160
million insects for each person on Earth!
 At any moment, there are about 1,000,000,000,000,000 ants alive on the
planet. If each ant were half a centimeter long, they would form a chain
long enough to circle the Earth 125,000 times!
 Tiny crustaceans called copepods, less than a centimeter long, are the most abundant animals in the ocean.
There are so many of them, that they outweigh all the whales on Earth!
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75% of all known species

6.  Throughout their evolution, arthropods have invaded new geographic areas and habitats.
Wingless fly
Gammarus
Himalayan Jumping Spider
Yeti Crab
June Beetle
Specter Stick Insect

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 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.
About 1,000,000,000,000,000,000
 They 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.

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 We're going to take a closer look at this amazing group of animals called arthropods. You are
familiar with a wide variety of arthropods from seeing them in your daily life. Have you thought about
what they all have in common?
 Arthropods are a lot more than just
delicious feasts and disgusting pests.
Before we go too far, we need to
figure out exactly what an arthropod
is.
 In this section, we will explore these
key questions:
 What is an arthropod?
 Why do all arthropods share
certain key characteristics?
 OK. Let’s dive right in!

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 Bilateral (left/right) symmetry
 Segmented body
 Hard Exoskeleton
 Jointed legs
 Many pair of limbs

10. 1. Bilateral symmetry
• One character inherited by all
arthropods is bilateral
symmetry.
• 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).
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2. Segmented body
• Another character inherited by all
arthropods is a body divided into
segments that are often grouped into
larger functional units.
• 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 abdomen.

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3. Hard exoskeleton
• We've seen that arthropods all have
bilateral symmetry and segmented
bodies. Another character inherited by all
arthropods is a hard exoskeleton.
• 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.

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4. Jointed legs
• We've seen that arthropods all have
bilateral symmetry, segmented bodies,
and hard exoskeletons. Another character
inherited by all arthropods is jointed
legs.
• Having a hard exoskeleton introduces a
problem for arthropods: flexibility.
How can an animal with a rigid body
covering move its legs? Evolution solved
this problem with joints. All arthropods
(arthro = joint, pod = foot) have jointed
limbs. In most of the leg, the exoskeleton
is hard, but at the joints it is softer and
bendable, allowing movement in the
same way that a suit of armor does. The
limb can be controlled by contracting
muscles connected to the exoskeleton on
both sides of the joint.

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5. Many pair of legs
• We've seen that arthropods all have
bilateral symmetry, segmented bodies,
hard exoskeletons and jointed legs.
Another character inherited by all
arthropods is many pairs of
limbs.
• The ancestral arthropod had many body
segments with one pair of limbs on each
segment. All arthropods inherited
multiple limbs from that ancestor.
However, in many arthropods, some
limbs have even been lost completely as
they evolved to be smaller and smaller,
while others have evolved into new
shapes. Crayfish, for example, have limbs
specialized for different functions.

15. Kingdom Animalia
Phylum Athropoda
Trilobites Arachnids Myriapods Crustaceans Insects
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 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.

16. tri- “three” + lobos- “lobe” + morphi- “shape”
“three-lobe shaped”
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 Among famousgroups of extinct organisms, trilobites probably rank second only
to the dinosaurs in fame. Their fossils are instantly recognizable and often strikingly
beautiful. In fact, prehistoric people liked them as much as we do. At Arcy-sur-Cure in France,
a 15,000-year- old human settlement, one of the artifacts found by archeologists was a
trilobite that had been drilled to be worn as an amulet.
 Trilobites are the most diverse group of extinct animals preserved in the fossil record. Ten
orders of trilobites are recognized, into which 20,000+ species are placed.
 Although the last few trilobites went extinct about 245 million years ago, they are one of the
best-known and most-studied groups of fossil arthropods.

18. Arakhnē- “spider”
“the spiders”
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 The land-based arachnids comprise over 98%of living chelicerate species and display a
number of marked adaptions to terrestrial existence.
 The Arachnida include the terrestrial chelicerates that everyone is familiar with, and that nearly
everyone would rather not be too familiar with: spiders (Araneae), ticks and mites
(Acari), and scorpions (Scorpiones).

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crūstaceus- “crust”
“crusty-shelled”

21.  Crustaceans are a diverse group of arthropods that include lobsters, crayfish, crabs, prawns,
shrimp, barnacles, and pill bugs. There are several key characteristics used to identify crustaceans,
including a segmented body with an exoskeleton, branched limbs, two pairs of antennae, numerous
pairs of appendages on the body for feeding, locomotion and sexual reproduction, and respiration
using gills.
 There are around 42,000 species of crustaceans, and most of them are marine. Unlike most
other arthropods, there are few species of crustaceans found on land or in freshwater.

22. Mūrias- “ten-thousand” + poda- “foot”
“ten-thousand footed”

23.  The myriapods are centipedes and millipedes , and some small relatives. Centipedes and
millipedes look similar to each other; they both look a little like worms with lots of legs.
Actually they are arthropods, they have a tough exoskeleton and jointed legs, and they are related to
insects and crustaceans. Like insects, myriapods have one pair of antennae, but they have many more
legs than insects do. All myriapods have more than 20 legs, and all the other arthropods
have fewer legs than that (most have only 6 or 8 legs).

24. hexa- “six” + poda “foot”
“six-legged”

25. 6 6 legs
 Insects don't have spines; they are invertebrates. Insects have segmented bodies. Their bodies
are divided three sections - the head, the thorax, and the abdomen. The head has
antennae and a pair of compound eyes. Insects have six legs, one pair on each of its three
segments. Some insects also have wings.
 Insects also have an exoskeleton. The exoskeleton is an outside covering that protects the insect's
internal organs. There are over 800,000 species of insects - more than all the other
animal groups combined! Insects can be found in a wide variety of environments all over the world!

26.  FACT: Arthropods are a highly successful group of
animals—they account for over three quarters of all
currently known living and fossil organisms.
 Of all animal groups, arthropods are the most diverse and
their diversity is due in no small part to the diversity of one
subgroup, the insects. Scientists approximate that there may
be as many as 30 million species of insects alive today. To date,
over one million have been identified. The success of arthropods
has been attributed to their versatile exoskeleton, the process of
metamorphosis, and their metameric body structure.
 FACT: Ancestral arthropods were the first land animals.
 The oldest known animal to have lived on land is the arthropod Pneumodesmus newmani.
Fossil evidence of this species consists of a single specimen that was discovered in 2004 in
Scotland. Pneumodesmus newmani is classified as a millipede and is thought to have lived 428
million years ago, during the Late Silurian Period.

27.  FACT: During their life cycle, arthropods undergo a transformation called
metamorphosis.
 Metamorphosis is a biological process that involves a radical transformation of body form and
physiology. The immature form of an organism, called a larva, undergoes metamorphosis to
become an adult. Since the larva and adult forms differ greatly in the way they live and what
they eat, metamorphosis enables organisms to reduce competition for resources that otherwise
would occur between the immature and adult forms.