In some arthropods, such as crabs, the head is fused with the thorax, which forms the cephalothorax. 

The rigid outer layer of the arthropod body is called an exoskeleton, composed of chitin.

An exoskeleton does not grow; arthropods shed their exoskeleton periodically and form a new, larger one.

Many land arthropods breathe through a network of fine tubes, each one called a trachea.

The circulatory system of arthropods is open, which means that blood is not always contained within vessels.

Different arthropod groups have varied mouthparts but all have Malpighian tubules used for excretion.

Many arthropods have a compound eye that is composed of thousands of individual visual units.

The brain receives input from these units and then composes the image of an object.

The compound eye is exceptionally good at detecting movement, which makes sneaking up on a fly difficult.

Most arthropods reproduce only sexually, though some can produce offspring from unfertilized eggs.

The tough exoskeleton protects arthropods but does not grow, so an arthropod must periodically shed their exoskeleton.

This process, called molting, allows arthropods to grow a larger body.

The total number of arthropods exceeds the number of all other animal species combined.

This is mainly due to the enormous evolutionary success of arthropods.

Arthropods‘ exoskeleton provides adaptation to life on land; arthropods are small, and they show a wide range of specialization in food sources and habitats.

The subphylum Chelicerata includes spiders, scorpions, mites, ticks, and horseshoe crabs.

The chelicerates are arthropods that have appendages called chelicerae which are specialized for feeding.

Chelicerae (kuh-lis-er-uh) are the first pair of modified appendages called pincers or fangs used in feeding. The second pair of appendages, called pedipalps, catch and handle prey.

All arachnids, except some mites, are carnivores.

Because arachnids do not have jaws, they can only consume liquid food.

The arachnid first injects its prey with powerful enzymes that turn the prey’s tissues into liquid. Then the arachnid sucks the liquid food into its stomach.

While feared and disliked, arachnids play an important role in the environment. Many spiders are major predators of insect pests.

Spiders are important predators of insects in almost every terrestrial ecosystem.

In the U.S., the black widow and the brown recluse are two species of arachnid that are poisonous to humans.

Most spiders have spinnerets, appendages at the end of the abdomen that secrete strands of sticky silk.

Some spinnerets do not produce silk.

Instead, they secrete a sticky substance that the spider can use to make silk strands adhesive.

Two other familiar groups of arachnids are scorpions and mites.

Scorpions have a long, segmented abdomen that ends in a venomous stinger that is used to stun prey.

Mites are the largest group of arachnids. Blood-sucking ticks attach themselves to a host.

Lyme disease is spread by bites from infected deer ticks.

Many crustaceans have a cephalothorax and an abdomen. Like chelicerates, crustaceans have appendages on their abdomen.

Unlike chelicerates, crustaceans have mandibles that are adapted for feeding and have two antennae. Crustaceans breathe by using gills.

There are terrestrial crustaceans, such as pill bugs and sand fleas, as well as aquatic crustaceans.

Most insects share the same general body plan, specialized mouthparts for feeding, a unique life cycle, and the ability to fly.

Most insects have specialized mouthparts for eating called mandibles.

The types of feeding among insects vary including sucking blood, chewing seeds or wood, and sucking nectar from flowers.

Insect are adapted for flight by having a lightweight body, wings, and strong muscles to power flight.

Insects have a unique life cycle compared with other arthropods. During development, a young insect undergoes metamorphosis.

Metamorphosis is a dramatic physical change, which can be complete or incomplete.

Almost all insects undergo complete metamorphosis. A chrysalis is a protective capsule where the lava passes through a pupa stage.

A smaller number of insect species develop into adults by incomplete metamorphosis.

Social insects have elaborate social systems involving specialization of labor, parental care, and communication between individuals.

The role that an individual plays in a colony is called a caste.

Caste is determined by heredity, diet, hormones, and pheromones, hormone-like chemicals that are used for communication.

Social insects include honeybees and termites.

A honeybee hive contains a queen, workers, and drone males.

Termites have kings as well as queens. Workers gather the food, raise the young, and excavate tunnels.

Myriapods (meaning "many footed") include centipedes and millipedes. Myriapods are not considered insects, though they share certain characteristics.

Each myriapod has a head region that is followed by many similar segments.

Centipedes have one pair of legs per segment and millipedes usually have two pair of legs per segment.

All adult echinoderms(i-kahy-nuh-durm) (meaning "spiny skinned") have an internal skeleton, five-part radial symmetry, a water-vascular system, and the ability to breathe through their skin.

Echinoderms have a calcium-rich endoskeleton that is composed of individual plates called ossicles.

Echinoderms have no head or brain. Most sensory perception occurs in the skin.

Echinoderms have a water-filled system of interconnected canals with thousands of tiny, hollow tube feet that make up a water-vascular system.

The tube feet are tiny legs that can be used to move, grip surfaces, or manipulate objects.

Particles and respiratory gases move freely throughout the large, fluid-filled body cavity (called a coelom).

Skin gills are small, fingerlike projections that grow among the echinoderm’s spines.

They create more surface area through which respiratory gases can be exchanged.

The living classes of echinoderms include sea stars, brittle stars, and basket stars, sea lilies and feather stars, sea urchins, and sand dollars, and sea cucumbers.

Sea stars, commonly called starfish, are perhaps the most familiar echinoderms.