Arthropods have characteristics of higher forms such as bilateral symmetry, triploblastic, coelomic cavity and organ systems and have reached the peak of invertebrate evolution. Two of their main achievements are the chitionus exoskeleton and locomotary mechanism. These animal can walk, swim and fly. The jointed appendages locomotary mechanism. These animals can walk, swim and fly. The jointed appendages (limbs) have been modified or diversified for various uses in the different sub-groups of arthropoda.
Chitin is non-living, non-cellular and is secreted by the under lying epidermis. It is made of polysaccharide. On the outer side of chitin, there is a waxy layer. In some Arthropoda and in certain parts in other arthropods chitin is soft and flexible, in others it is hard. In general, it is for protection but it also serves as lever fort the movement of muscles of jointed limbs. The chitin in the jaws is used for biting and crushing food. It also forms lens of the compound eyes the copulatory organ and organs of defense and offense.
In the young Arthropoda such as insect larvae, chitin us exoskeleton is shed from time to time to allow the growth of the larva. This process of shedding of exoskeleton is called molting or ecdysis. Is short the exoskeleton of chitin in the arthropods is one of the primary factors n the success of Arthropda as it helps them to adapt to a wide variety of habitat.
Arthropods share with annelids the characteristic of having the body divided into similar segments. In Arthropods however segmentation is not metameric and organs are not repeated in the different segments. Each somite typically is provided with a pair of jointed appendages. But this arrangement is often modified with both segments and appendages specialized for different functions in different habitats. However in all kinds of habitat the jointed appendages provides an efficient means of locomotion, offense and defense and also help in reproduction.
Man and insects have been at war for the same food, same place to live in. insects attack man, his domestic animals and also his crops, causing a number of diseases. They are not only a health hazard but also cause economic loss to man by destroying his property and crops. Some insects are also useful to him such as the honey bee or the silk worm. Insects are therefore of great importance to mankind.
1. Harmful insects: many types of mosquitoes, flies fleas, lice and bugs transmit disease causing organisms to man and domestic animals. We are familiar with mosquito of genus anopheles; the female of which transmits plasmodium that causes malaria in man. The Tse-tse fly of African countries transmits Trypanosoma, the cause of sleeping sickness and skin diseases. The common house fly carries disease causing organisms to contaminate food and cause cholera, hepatitis etc. some species of Trypanosoma cause diseases in cattle, also.
A number of insects lay eggs on fruits and other commercial crops such as sugar- cane, maize, cotton and also on vegetables etc. the larvae of these insects damage fruits and the crops resulting in economic loss to farmers. The locusts that move in large numbers from country to country cause damage to standing crops and other plants.
2. Beneficial insects: the useful insects are the honey bee that provides man with honey and also wax. Similarly the silk worm gives us silk. There are some insects that are predaceous on other harmful insects. Some insects are scavengers and they eat up dead animal and vegetable matter. Insect larvae are source of food for they eat up dead animal and vegetable matter. Insect larvae and source of food for fish.