Phylum Arthropoda

Arthropod means jointed legs. This Phylum is made up of animals that have joint appendages.

Characteristics of Phylum Arthropoda:

Characteristics of Phylum Arthropoda include; 

1. They have segmented bodies.

2. They have an exoskeleton made up of chitinChitin is the most abundant amino polysaccharide polymer occurring in nature, and is the building material that gives strength to the exoskeletons of crustaceans, insects, fish and the cell walls of... More. This is an important factor in protection and efficient locomotion in members of this phylum by providing a base to which the muscles are attached.

3. They possess “jointed appendages” or “jointed legs” used for feeding, movement, and reproduction and some serve as sensory organs.

4. They are bilaterally symmetrical.

5. They undergo periodic moulting for growth.

6. They are coelomates with coelom much reduced.

7. They are triploblastic.

8. They undergo metamorphosis.

Classification of Arthropoda:

This is the largest phylum in the animal kingdom and is divided into the following classes;

a. Crustacea
b. Insecta
c. Arachnida
d. Myriapoda: Sub-Class: Chilopoda & Diplopoda

a. Class Crustacea:

Characteristics of Crustaceans:

1. They are aquatic.

2. A crustacean’s body is made up of segments that are divided into three regions: the cephalon (head), pereon (thorax), and pleon (abdomen).

3. They have cephalothorax i.e. the head and thorax are fused together.

4. They have stalked eyes. 

5. They have two pairs of antennae. 

6. There are two pairs of maxillae on their heads.

7. Crustaceans have gills

8. A pair of mandibles which are used for eating food.

9. They have swimmerets for swimming, telson and uropods.

10. Setae on the legs of krill and other small crustaceans help them to gather phytoplanktonPhytoplankton and zooplankton are two types of plankton that are found in water. Phytoplankton are plants, while zooplankton are animals; this is the main difference between them. Larval crustaceans and krills... More.

11. They possess gills for gaseous exchange in water.

12. Examples are Barnacles, Crabs, Crayfish, Water fleas, Shrimps, Prawns, Lobsters, Woodlice, Copepods and Krill.

b. Class Insecta:

 They consist of 70% of all known species of animals on earth.

Characteristics of Insecta:

1. Their body is divided into three parts, the head, thorax and abdomen.

2. They have three pairs of jointed legs.

3. They possess a pair of compound eyes and a pair of jointed antennae.

4. They possess one or two pairs of wings

5. They possess spiracles on the abdomen, through which air enters the body.

6. The thorax has three segments: prothorax, mesothorax and metathorax.

7. Growth is by moulting or ecdysis.

8. They have a tracheal system for gaseous exchange.

9. They have Malpighian tubules for excretion.

10. They show either complete or incomplete metamorphosis.

Economic Importance of Insects:

1. They help in the pollination of flowers e.g. bees, butterflies and moths. 

2. Some are edible e.g. winged termites while some produce honey e.g. honey bees.

3. Some are parasites of plants.

4. Some are vectors of diseases e.g. mosquito, cockroaches. 

5. They cause injury by biting or stinging.

c. Class Arachnida (Spiders, Scorpions, Ticks, Mites)

Characteristics of Arachnida:

1. Arachnids have two body regions: prosoma (cephalothorax) and opisthosoma (abdomen).

2. On the cephalothorax, there are four pairs of simple eyes and segmented mouthparts.

3. They have no true jaws, on the cephalothorax is a pair of pincer-like chelicerae, which have claws like fangs with poison at the tips. They use chelicerae for feeding.

4. They possess eight simple eyes called ocelli.

5. They have four pairs of walking legs.

6. They breathe by means of lung books.

7. They have no antennae.

8. They have pedipalps as sensory organs instead of antennae. They help the spider to sense objects in its environment. Some spiders also use their pedipalps to shape their webs and help feed and capture prey.

9. Examples are spiders, ticks, scorpions, and mites.

d. Class Myriapoda:

• Sub-class Chilopoda (centipedes)
• Sub-class Diplopoda (millipedes)

i. Sub-Class Chilopoda (Centipedes):

They live in terrestrial habitats.

Characteristics of Centipedes:

1. They have two body divisions; the head and the fused trunk and abdomen.

2. The head bears a pair of simple eyes and a pair of short antennae.

3. They respire by means of tracheae.

4. Centipedes have a pair of appendages on each segment.

5. The paired legs on the first segment are modified into poisonous claws.

6. They are carnivorous.

ii. Sub-Class Diplopoda (Millipedes):

Characteristics of Millipedes:

1. They have two pairs of appendages on each body segment.

2. They breathe by means of tracheae.

3. Their head bears a pair of simple eyes and a pair of short antennae.

4. They are herbivorous.

5. They have no claws, unlike the centipedes which have claws for catching their prey.

Differences between Centipedes and Millipedes:

	Centipede:	Millipede:
i.	Body is elongated and flattened.	The body is cylindrical.
ii.	Each segment bears a pair of walking legs except the last two and the first pair.	Each abdominal segment bears two pairs of legs.
iii.	They are carnivores.	They are herbivores.
iv.	The paired legs on the first segment are modified into poisonous claws.	No poisonous claws on the paired legs.
v.	Unlike millipedes, centipedes never curl up when disturbed.	Millipedes curl up when disturbed.

Differences between Insects and Millipedes:

	Insects:	Millipede:
i.	Three pairs of legs.	Many pairs of legs, 2 pairs on each abdominal segment.
ii.	Body divided into head, thorax and abdomen.	The body of a millipede is divided into two distinct parts, the head and the trunk.
iii.	Most insects possess wings.	Do not possess wings.
iv.	Legs on thorax segment only.	Legs on thorax & abdominal segment.
v.	Compound eyes present.	Collection of simple eyes.