1.0Arthropoda

Arthropoda is the largest and most diverse phylum in the animal kingdom, comprising over 80% of all known animal species. The name "Arthropoda" means "jointed legs" (from Greek: arthron = joint, pous/pod = foot).

2.0Introduction

• Arthropoda is the largest phylum of animalia, which includes insects. 
• Over two-thirds of named species on earth are arthropods. 

3.0Habit and Habitat

• They may be aquatic (marine and fresh water) or terrestrial, free living and sometimes parasitic (mosquito), solitary (centipede, millipede), colonial (bee), gregarious (Locust). 
• The body is Bilaterally symmetrical, Triploblastic with organ system level of organisation, tube within tube body plan. 
• They are metamerically segmented and coelomate animals. 

4.0Body Structure

• The body is divided into three regions: Head, thorax & abdomen. 
• In some animals (for example Scorpion, Crab) there are two body parts Cephalothorax and abdomen. (Head and thorax fused to form cephalothorax)  Unique features 
• They have jointed appendages for different functions. (arthro - jointed, poda - foot/appendages). 
• The body of Arthropods is covered by a chitinous exoskeleton. 

5.0Respiratory System

• Respiration by gills (e.g. Prawn), Book-gills (e.g. King crab), Tracheal system (e.g. Insects), Book-lungs (e.g. Scorpion). 
• Trachea carries oxygen directly to the body cells.

• In insects, gaseous exchange occurs through a network of tubes collectively known as the tracheal system. 
• The small openings on the sides of an insect's body are known as spiracles. Oxygen-rich air enters through the spiracles. 
• The spiracles are connected to the network of tubes. From the spiracles, oxygen enters the tracheae. From here, oxygen diffuses into the cells of the body. 
• The movement of carbon dioxide follows the reverse path. The CO₂ from the cells of the body first enters the tracheae and then leaves the body through the spiracles.

6.0Circulatory System

• The circulatory system is Open type i.e. blood flows in open tissue spaces and haemocoel instead of blood vessels. 
• Blood is colourless called Haemolymph (e.g. Insect). 
• Respiratory pigment absent. 
• Copper containing pigment hemocyanin is found in some arthropods (e.g. Prawn). 
• The body cavity around the viscera contains blood and the coelom filled with blood is called the haemocoel. 

Other Body Structure

• The digestive tract is complete, and they can feed upon all kinds of food substances.
• Excretory organs are – 
• Antennary or green glands (e.g., Prawn)
• Coxal gland (e.g., Scorpion)
• Malpighian tubules (e.g., Insects) opening into the gut. 
• Excretory matter is ammonia in aquatic animals and uric acid in land animals. 
• The nervous system comprises a nerve ring and a double, solid, and ventral nerve cord bearing ganglia. 
• Head is distinct [High degree of cephalization] 
• Sensory organs like simple eyes, or compound eyes or both, antennae, statocyst and anal cerci are found. 
• They are mostly dioecious. 
• Sexes are separate. 
• Fertilization is usually internal, but few aquatic forms have external fertilization. Gonads have ducts. Sexual dimorphism may be present. They are mostly oviparous (Few are viviparous e.g. scorpion).  
• Development may be direct or indirect.  
• Animals of Arthropoda are most successful invaders of terrestrial environment among invertebrates due to presence of 
• Cuticle
• Appendages
• Wings 

7.0Examples 

• Gregarious pest - Locusta (Locust)  
• Living Fossil - Limulus (King crab) 
• Others - Butterfly, Scorpion, Prawn, Spider, Centipede, Millipede 
• Peripatus (Connecting link between annelida and arthropoda) etc. 
• Lepisma-Silver fish (book louse / Book worm), wingless insect

Vectors

(a) Mosquitoes                        (b) Others 

(i) Anopheles :- Malaria           (i) Glossina :- Sleeping sickness 

(ii) Aedes :- Dengue                (ii) Sand-fly :- Leishmaniases 

(iii) Culex :- Elephantiasis 

Economically important insects

• Apis (Honey bee) 
• Bombyx (Silk worm) 
• Laccifer (Lac insect)