Arthropoda has a wide variety of animals. These animals occupy a wide variety of habitats on land, sea and fresh water.


Main characteristics

  • They have a segmented body.
  • Presence of an exoskeleton that is shed periodically.
  • They have jointed limbs and appendages for feeding, locomotion, and irritability.
  • They have bilateral symmetry. This is the ability the body of an organism to be divided into two identical parts.
  • They have an open circulatory system, where blood flows in open spaces.
  • They have a well-developed nervous system
  • They have a complete gut that runs from the mouth to the anus

Note:The exoskeleton is made up of chitin which is a fairly firm butflexible carbohydrate. The exoskeleton provides support to terrestrial arthropods. It also provides points of attachment for the muscles.The exoskeleton prevents the desiccation of the body by secreting wax. The exoskeleton also protects the organism from mechanical injury.The phylum comprises of classes Crustacea, Chilopoda, Diplopoda, Insecta and Arachnida.

1.Class: Crustacea

Crustacea are organisms whose body is covered by a carapace. A carapace is a hard shell. Examples of members of the class Crustacea include crabs, crayfish, lobsters, prawns, woodlice and shrimps.

Distinguishing characteristics

  • Crustacea are mainly found in marine and fresh water. Thus, they occupy aquatic habitats.
  • They breathe by means of gills through the body membrane.
  • Their body is divided into two parts. Their head and thorax are fused to form a cephalothorax. The second division is the abdomen.
  • They have a pair of compound eyes each on a raised stalk.
  • They have two antennae and small short pair called atenules.
  • They have four pairs of mouthparts namely: maxilla, mandible, labium and labrum.
  • They have five pair of limbs that are modified for swimming.

2.Class: chilopoda

Chilopoda is made up of centipedes. The centipedes are mainly found on land.


Distinguishing characteristics

  • Centipedes have a clearly defined head while the rest of the segments are similar.
  • They have a pair of antenna
  • They have one pair of mouthparts known as mandibles.
  • They have simple and compound eyes, although some lack compound eyes.
  • They have one pair of legs in each body segment.
  • They carry out gaseous exchange by means of a trachea
  • They feed on insects and worms.
  • They occupy terrestrial habitats.
  • Their body is flattened dorso-ventrally.
  • They have one pair of poison claws.

3.Class: Diplopoda

Class Diplopoda is made up of millipedes. Millipedes are common on damp places.


Distinguishing characteristics

  • Millipedes have a clearly defined head. All the other body segments are basically similar.
  • They have one pair of antenna
  • They have one pairof mouthparts, namely the mandibles.
  • They have simple and compound eyes. In some cases, the millipedes may not have compound eyes.
  • They have two pairs of legs in each segment.
  • They carry out gaseous exchange through the trachea.
  • They feed on plants.
  • They inhabit terrestrial habitats.
  • They have a cylindrical body
  • They have the ability to coil when disturbed or not active.

Note: class Chilopoda and Diplopoda used to be classified as class Myriapoda but now are classified into different classes

4.Class: Arachnida

Arachnida are terrestrial arthropods. Members of this class all have 8 legs. Examples of arachnids include spiders, ticks, scorpions and mites

Distinguishing characteristics

  • Arachnida have two body parts. The head and thorax are fused to formthe cephalothorax or prosoma (cephalothorax). The abdomen is referred to as opisthosoma.
  • They do not have antenna but have a pair of pedipalp which they use for sensory and defense purposes.
  • They do not have mouthparts. However, they have one pair of appendages for sensing prey. This pair of appendages is known as chelicerae. Thus, they have a carnivorous mode of feeding.
  • They have simple eyes.
  • They have four pairs of walking legs.
  • They carry out gaseous exchange by the lung book or trachea. A lung book consists of folds of ectoderm with slit-like openings on the surface of the abdomen
  • Arachnids do not have wings.
  • They inhabit terrestrial habitats

5.Class: Insecta

Insects are the most successful animals on earth since they possess an exoskeleton which reduces water loss from the body.Insects are the largest group of arthropods.They occupy every habitat on earth in such places as air, soil and water. However, they mainly inhabit terrestrial habitats. Examples of insects include grasshoppers, houseflies, butterflies, bees, and termites

Distinguishing characteristics

  • Insects have three body parts, namely: the head, thorax, and abdomen.
  • They have one pair of antenna.
  • They have a pair of compound eyes. In some case, simple eyes are also present.
  • They have three pairs of walking legs on the thorax. One pair of walking legs per segment of the thorax.
  • Most insects have one or two pairs of wings on the second and/ or the third segment of the thorax. Some insects have no wings.
  • They breathe by means of spiracles and carry out gaseous exchange in the tracheal system.
  • They undergo complete or incomplete metamorphosis with a larval stage.
  • Some insects such as ants are carnivorous while others such as grasshoppers are herbivorous.

They exhibit the longest level of organization in animals, i.e. social organization especially in bees, wasps and ants. They are the only invertebrates which can fly.

Their success on land is attributed to:

  • Evolution of special organs for flight. The wings which enable them to diverse andcolonize new areas.
  • Impervious exoskeleton made of chitin which has protected them from drying up in the terrestrial environment.
  • The small size has enabled them to tackle every place.
  • Excretion of toxic products as uric acid has enabled them to conserve water
  • Tracheal system has enabled them to carry efficient gaseous exchange.
  • Disposition of legs enables them to maintain swift locomotion.
  • The compound eyes that provide wide field of view for food and enemies.
  • The modified mouth parts that suit a variety of food materials.
  • The high reproductive rate that ensures enormous number of offsprings is produced

Some insects are directly beneficial to man these include pollinators like butterfly, moth and bees others are beneficial indirectly such as parasitic pest species.Harmful insects include those that directly live on man as parasite like lice, flies, mosquitoes, tsetse flies.


Metamorphosis is the gradual developmental change from the eggs to the adult stage.It occurs in insects and amphibians. Insect metamorphosis is divided into two types.

1.Complete metamorphosis(holometabolous);

This is a gradual development change where the eggs hatch into larvae and the larvae change into pupa and finally the pupa change into an adult. It involves four stages


Insects, which undergo complete metamorphosis, include butterflies, mosquitoes, houseflies, tsetse flies, bees, wasps, and beetles.

2.Incomplete metamorphosis (hemimetabolous);

This is the gradualdevelopmental change where an insect undergoes only 3 stages, when eggs hatch, they give rise to adult-like nymphs which latter change into adults. Insects showing this include locusts, grasshoppers, bedbugs, cockroaches, and termites.

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Some common orders of insects

OrderCharacteristic feather of the order
(the word ptera means wing).
DictyopteraHas hard outer wingsCockroach, beetles, weevils.
HymenopteraHas membranous wingsWasps, bees
IsopteraHas similar wingsTermites
LepidopteraHas scale wingsMoths, butterflies
DipteraHave two pairs of wings. The second pair is reduced
into halteres for balancing.
Mosquitoes, houseflies
OrthopteraHas long straight wingsGrasshopper, locusts, Preying mantis