Arthropods have an external jointed skeleton. it is composed of cuticle. Exoskeleton encloses the arthropods. The exoskeleton is major reason or the success of arthropods. Exoskeleton performs following functions:
1. It provides structural support.
2. It protects the body.
3. It is impermeable for water. Therefore, it prevents the loss of water.
4. It acts as a system of levers for muscle attachment and movement.
The exoskeleton covers all body surfaces. It also covers the invaginations of the body wall like anterior and posterior portions of the gut. It is nonliving. It is secreted by a single layer of epidermal cells. The outside of arthropods is covered by exoskeleton. Thus its epidermis is not directly exposed to air or water. Therefore their epidermal laver is called the hypodermis.
Composition of exoskeleton
The exoskeleton has two lavers:
1. Epicuticle: It is the outermost layer. It is made up of a waxy lipoprotein. It is impermeable for water. It also acts as a barrier to microorganisms and pesticides.
2. Procuticle: It forms the bulk of the exoskeleton. It is present below the epicuticle. Procuticle is called the endocuticle in crustaceans. The procuticle is composed of chitin and several kinds of proteins. Chitin is a tough leathery polysaccharide. The procuticle hardens by a process called sclerotization. It is sometimes impregnated with calcium carbonate. Sclerotization is a tanning process in which layers of protein is chemically cross-linked with one another. It hardens and darkens the exoskeleton. This bonding occurs in the outer portion of the procutile in insects and most other arthropods. The exoskeleton of crustaceans hardens by sclerotization and bv the deposition of calcium carbonate in e middle regions of the procuticle. Some proteins give the exoskeleton resiliency for flapping wings and jumping. The inner portion of the procuticle does not harden.
Modification in the exoskeleton
1.The hard procuticle protects the arthropods as armor. It also causes many modifications and adaptations in arthropods:
2. Invaginations of the exoskeleton rm firm ridges and bars. These ridges and bars are used for muscle attachment.
3. Exoskeleum forms joints. A flexible membrane articular membrane is present in the inner and less hardened areas. The cuticle is modified to form sensilla. Sensilla is present in the form of pegs, bristles and Lenses
4. There are modifications of the exoskeleton for gas exchange.
Molting or Ecdysis
The periodic shedding of exoskeleton for growth in arthropods is called molting process called ecdysis. The growth in arthropods is impossible without molting. Ecdysis is divided into four stages:
1. Hypodermal glands sewete enzymes. These enzymes start digesting the old procuticle and separate the hypodermis and the exoskeleton.
2. New procuticle and epicuticle are secreted.
3. The animal stretches by air or water intake. Therefore, the exoskeleton splits along ecdysal lines. Pores in the procuticle secrete additional epicuticle.
4. Finally calcium carbonate deposits and sclerotization harden the new exoskeleton.The arthropod is vulnerable (weak) ) to predators during ecdvsis. Therefore, it remains hidden. The nervous and endocrine systems control all these changes.