Phylum Arthropoda.

1. They possess a chitinous and sometimes calcareous exoskeleton, which may be rigid, stiff or flexible; shed during growth in some organisms.
2. They have jointed, paired appendages such as legs and antennae.
3. They have segmented bodies, which are arranged into regions, called tagmata comprising the head, thorax, and abdomen. The head of some arthropods such as arachnids and crustaceans is fused with the thorax to form a structure called cephalothorax (prosoma).
4. Their body plan is bilaterally symmetrical.
5. They have a ventral nervous system and open circulatory system, which is dorsally positioned.
6. They are triploblastic coelomate animals, which are metamerically segmented.
7. Their coelom is reduced and confined to cavities of excretory organs and reproductive ducts during their development and form another cavity called haemocoel.
8. They have an open circulatory system.

Arthropods are classified into five classes namely; class Crustacea, which include crabs, class Arachnida, which include spiders and scorpions, class Chilopoda, for example centipedes, class Diplopoda, for instance millipedes, and class Insecta, which include cockroaches and grasshoppers.

Crustaceans form a large group of arthropods that include familiar animals such as crabs, lobsters, prawns, shrimps, barnacles, and crayfish. Most crustaceans are marine aquatic animals, other are found in fresh water habitats, and a few are terrestrial.

Distinctive features of class Crustacea

Crustacea have distinctive features that make them unique among other arthropods as highlighted below:

1. They possess two body division, which are cephalothorax (the head fused with thorax) and abdomen.
2. They have carapace or an exoskeleton hardened with calcium salts which acts as a protective shell.
3. They have heads bearing two pairs of antennae.
4. They have a pair of compound eyes at the ends of movable stalks.
5. They have at least three pairs of mouthparts.
6. Gills are used for gaseous exchange, which are the outgrowth of the body wall or limbs.
7. They have variable number of legs, sometimes up to ten legs and can be modified for swimming. Unlike other arthopods, the legs and other appendages of Crustacea have two branches (they are biramous).

Structure of a crab

A crab is covered with a thick exoskeleton (carapace) composed of calcium carbonate. This means they are well protected against predators. The body is divided into two parts: the cephalothorax and abdomen. The abdomen is entirely hidden under the carapace; it may not be visible at all, unless the crab is turned over. The head bears two pairs of antennae. The animal is equipped with a pair of claws (pincers) which are the most important weapons with at least three functions: seizing, eating and subduing the prey. If the food is a shellfish, then the pincers can exert force to open or break the mollusc's shell. Males also use pincers for or during fighting.

The arachnids represent the second largest group of terrestrial arthropods after the insects. The class includes the animals whose bodies are organised into two tagmata called cephalothorax or prosoma (fused head with thorax) and abdomen (opisthosoma). Most of them are carnivores, except the mites, which are herbivores. Members of this class include spiders, mites, ticks and scorpions.

Distinctive features of class Arachnida

Arachnids can be distinguished from other members of the phylum Arthropoda by the following features:

1. Their bodies are divided into two regions; prosoma and opisthosoma, separated by a narrow waist-like constriction.
2. They lack antennae; alternatively, they use pedipalps to detect external stimuli.
3. They lack compound eyes, they only have simple eyes.
4. They lack true mouthparts; instead, they have two pairs of appendages born from prosoma. One pair, the chelicerae or poisonous fangs, is used for killing preys or defending themselves; the other pair called pedipalps holds the prey in place when the animal injects poison.
5. Usually they have four pairs of walking legs.
6. Their respiratory structures are book lungs or book gills or trachea.

Structure of a spider

The spider has two main body parts: a fused head and thorax, called cephalothorax and abdomen. Most external appendages are attached to the cephalothorax. The appendages are legs, chelicerae, mouthparts, and pedipalps. On the ventral part of the abdomen are two hardened plates covering the book lungs. The spinnerets, which produce silk, are also located in the abdomen.

Adaptations of spider to its mode of life

Spiders live in almost every habitat. They are adapted to different environments because:

1. They have pairs of chericerae which produce silk for capturing preys.
2. They have pedipalps for sensation.
3. They have four pairs of legs for locomotion.The hairy spiders have stings used to paralyse prey and defend themselves.

This class consists of organisms found in terrestrial environment. They are terrestrial animals abundant in moist areas, such as leaf litters, under logs or rocks. An example of chilopoda are centipedes.

Distinctive features of class Chilopoda

The following features distinguish centipedes from other members of the phylum Arthropoda:

1. They have a flattened body with a distinct head. However, other body segments are similar, the trunk is not obviously divided into thorax and abdomen.
2. They have one pair of legs per segment.
3. They are carnivores, feeding mainly on insects and worms.
4. Their first trunk segments have appendages, which are modified into a pair of poisonous fangs.

Structure of centipede

Centipedes have a head bearing eyes, well- developed one pair of mouthparts (jaws), and one pair of antennae (Figure 3.50). The body consists of many segments, each of which bears one pair of legs. The legs of the first body segment are modified into poisonous claws, and are used for defence as well as for capturing and paralysing preys. They have tracheae used for gaseous exchange.

Adaptations of the centipede to its mode of life

1. They have antennae for sensation.
2. They have poisonous claws for defence and hunting.
3. They have legs for locomotion.
4. The last pair of legs has hooks for defence.

The class Diplopoda consists of members with many legs; usually two pairs of legs per body segment. An example of a member of class Diplopoda is a millipede. The head bears one pair of antennae and a pair of mouthparts (jaws) while the trunk is not divided into thorax and abdomen. They are scavengers that feed on decaying organic matter, like leaves and compost.

Distinctive features of class Diplopoda

Diplopods have unique features, which differentiate them from other Arthropods.

1. They have round bodies with clearly defined head, followed by other similar segments.
2. They have many legs; with two pairs of legs per segment.
3. Most millipedes are herbivorous, feeding mainly on decaying vegetation.

Structure of millipede

The millipede has elongated and cylindrical body in which each body segment bears two pairs of legs. Millipedes are made up of a series of segments, whose number varies greatly from one species of millipedes to another (Figure 3.51). Many of the segments contain special glands that secrete a noxious chemical to repel predators. The first segment behind the head does not have legs, but the rest of the segments have legs. Towards the end, some of the segments have no legs. The number of legless segments varies from one millipede to another. Some species of millipedes have hair or spiny appendages protruding from their bodies. Its head is on one end of its body, and is about the same diameter as the adjacent segment. Also the head has a pair of antennae that helps the millipede to sense the surrounding environment. One pair of mouthparts (jaws) is on the front part of its head and the eyes are fixed above its jaws. Most millipedes have simple eyes that provide basic vision, but not all types or species of millipedes have eyes.

Adaptations of the millipede to its mode of life

Millipedes have the following features that make them adapt to their environment:

1. They have many, short and strong legs that enable them to burrow into the soil.
2. They produce special secretions, which help them to moisturize dead organic matter on which they feed upon.
3. A millipede tends to curl up into a tight flat coil for self-defence, and protect their delicate legs inside an armoured exoskeleton.
4. Millipedes produce an offensively odorous fluid (repugnatorial fluid) when provoked, this acts as a defence against predators.

The class contains all insects and is the most diverse group of organisms on earth. Members of this class are found in all environments including fresh water aquatic and terrestrial environments, but very few are found in marine habitats. They differ in morphology and feeding habits. Although members of this group vary greatly, they have some common characteristics, which make them to belong into the same class. Members of this class have three pairs of walking legs, three distinct body parts or regions and one pair of antennae borne on the head. The antennae are used as sense organs for detection of odour molecules in the air, changes in the concentration of water vapour, sounds, and gauging air speed. Examples of insects include grasshopper, butterfly, house fly, cockroach, beetle, bees, ant, wasp, and termite.

Distinctive features of class Insecta

Insects have the following features that differentiate them from other arthropods:

1. Their bodies are divided into three main regions or tagmata, namely the head, thorax, and abdomen.
2. They have three pairs of walking legs on the thorax (one pair per thoracic segment).
3. They usually have one or two pairs of wings on the thorax, some members may lack wings.
4. They use the tracheal system as their respiratory surface with segmental spiracles.
5. They undergo metamorphosis during their development through the molting process.
6. They have a pair of compound eyes and simple eyes.
7. They usually have three pairs of mouthparts, which are maxillae, mandible, and labrum.

Structure of a cockroach

The body of a cockroach is elongated and segmented, divided into a head, thorax, and abdomen. The thorax is subdivided into three parts; prothorax, mesothorax and metathorax and each segment bears one pair of legs (prothoracic legs, mesothoracic legs and metathoracic legs respectively). The thorax also bears two pairs of wings in most adults insects. The body is covered by a rigid exoskeletons (cuticle) secreted by the epidermal layer. This occurs in jointed sections or plates to allow body movement. The exoskeleton is made up by a structural protein known as chitin. It is coated with wax, which is impermeable to water. The exoskeleton provides attachment for body muscles. Cockroaches are mostly dark brown or reddish in colour. Male cockroach has a narrow abdomen an expanded tergum of the last visible segment, and has a pair of styles at the end of abdomen, while the female cockroach has a wide abdomen with a large podical plates used to carry the ootheca, and lacks styles.

1. Insects such as bees and butterflies are useful in agriculture; as they act as pollinators; many plants depend on insects for pollination.
2. They are used in industries for production of honey and wax materials; for example honey bees.
3. Insects are used in biological control of pests; in certain cases, insects have been very effective predators to reduce the number of pests. For example, lady beetles are used to kill aphids (the pest insects that transmit viruses to plants).
4. Many arthropods are used as food; they are a good source of protein for humans and other animals. Example include shrimps and lobsters, crabs, edible grasshoppers, locusts and grasshoppers, and many larvae of beetles and moths.
5. Insects are used in research and biological studies.
6. Insects such a as butterflies are valued for their beauty.
7. Many arthropods are used as indicators of the quality of environment.

1. Some insects are vectors of animal and plant diseases. For example, mosquitoes carry Plasmodium, which cause malaria, and tsetse flies carry Trypanosoma, which cause sleeping sickness to human beings. Whiteflies carry Tomato yellow leaf curl virus which cause chlorosis in plants.
2. Some arthropods such as scorpions and centipedes bite human beings, release their poisons causing severe pains.
3. Some arthropods destroy crop plants; examples include locusts and the caterpillars of butterflies and moths.
4. Some arthropods are parasitic to mammals. Good examples of such arthropods are ticks, jiggers, and bedbugs, which grip themselves on human skin and suck blood