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FIND OUT MORE. Invertebrates 102

EXOSKELETON

An arthropod’s exoskeleton is a protective

case and an anchor point for muscles. As

well as being tough, it is waterproof,

helping these creatures to survive

in even the harshest habitats.

1 LOBSTER

The lobster’s hard exoskeleton supports and protects its

body. Even delicate parts, such as the legs and antennae,

are completely encased. The North Atlantic lobster is the

world’s heaviest arthropod, weighing up to 20 kg (44 lb).

HOW DO ARTHROPODS GROW?

In order to grow, arthropods have to moult

(shed their exoskeletons) every so often. They

then expand their bodies before their new casing

hardens. Arthropods are vulnerable while moulting, so

try to find a safe place to hide before they begin.

WHERE DO ARTHROPODS LIVE?

Arthropods occur in virtually every habitat, from the

cold ocean depths to the hottest deserts. They can live

through extremes that would kill most vertebrates.

Scorpions, for example, can survive being frozen solid.

Short antenna

feels objects

close to the

mouthparts

Long antenna

helps the lobster

navigate on

the sea bed

First leg,

or cheliped,

is modified to

carry a pincer

Walking legs

are jointed in

several places

Carapace

protects most of

the vital organs

Pincers,

used to grab

food and for

defence

Tailpiece
moves to propel
the lobster

4 PILL MILLIPEDE

With their tough, rounded bodies, millipedes make

difficult prey. The pill millipede has an extra trick to

deter predators – it rolls into a ball

when attacked. Some millipedes

defend themselves with poisons,

such as quinone and cyanide,

produced by glands

between their segments.

WHAT IS THE DIFFERENCE BETWEEN

A CENTIPEDE AND A MILLIPEDE?

Centipedes are active hunters, while most millipedes

eat plant matter. Also, centipedes have two legs per

body segment. Millipedes have four. Centipedes and

millipedes are collectively known as myriapods.

WHAT FEATURES DO ARTHROPODS SHARE?

All arthropods have bodies divided into segments and

covered with a hard. EXOSKELETON. This tough

casing is made of a protein called chitin, which is also

found in human fingernails. The armour is flexible at

joints on the legs, which makes arthropods nimble.

Arthropods

Centipedes, millipedes, insects, crustaceans, and

arachnids, including spiders, all belong to a super-

group of invertebrates called arthropods. Arthropods are

more numerous and varied than any other animal group.

Arthropods make up the largest phylum (major group) in the animal

kingdom. There are more than 900,000 named species divided into 13

classes: • Crustaceans • Insects • Arachnids • Centipedes • Millipedes

• Sea spiders • Pauropods • Symphylans • Springtails • Proturans

• Two-pronged bristletails • Three-pronged bristletails • King crabs

ARTHROPOD CLASSIFICATION

GIANT CENTIPEDE 1

Centipedes paralyse or kill their

prey with poison, injected by

fang-like claws just behind the

mouth. There are over 3,000

species of centipedes. The largest

live in the tropics and may grow

up to 30 cm (12 in) long – large

enough to kill a mouse.

DO ALL CENTIPEDES HAVE ONE HUNDRED LEGS?

The word centipede means “100 legs”, but some

centipedes have fewer than 100 legs, and others have

more. Similarly, the word millipede means “1,000

legs”, but in fact no millipede has more than 750 legs.

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arthropods