Arthropods are invertebrate animals characterised by their segmented bodies, exoskeletons and jointed appendages. Insects, crustaceans and arachnids are all arthropods. A thick chitin material called an exoskeleton covers the body of arthropods and it serves many purposes. Benefits of an exoskeleton are found within its structure and function, but there are disadvantages due to the necessity to moult.
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On a microscopic level, the exoskeleton has many layers. The inner layer, known as the epidermis, secretes a supportive fibre membrane. Above the epidermis is the procuticle made up of chitin surrounded by a matrix of protein. This layer solidifies to form the rigid plates of the exoskeleton. The epicuticle is the outermost layer. It prevents the soft tissue of the arthropod from drying out and protects from invasion.
The exoskeleton functions to protect, prevent drying, anchor muscles and sense conditions in the environment. Perhaps the most important function of the exoskeleton is protection from predators. The outer layer is very strong and resists compression. When compromised, this layer will crack. The inner layers have tensile strength and resist tension. This combination is ideal for resisting damage from predators who exert compression force on the outer layer and tensile force on the inner layer of the arthropod's exoskeleton.
The exoskeleton of an arthropod plays a key role in the movement of the animals. Muscles attach to the structure. The exoskeleton produces better leverage for muscles than an endoskeleton. Unlike the shell of a mollusc, the exoskeleton has many flexible joints that allow greater movement. The greater leverage for muscles contributes to the overall strength of the animal. Ants are able to lift many times their body weight in part due to the exoskeleton.
Unlike the bones of a vertebrate animal, the exoskeleton does not grow with the body of the arthropod. An arthropod must moult an existing exoskeleton and produce a new one to fit a larger body. During the moulting process, the arthropod is very vulnerable to attack and injury. When the new rigid covering is grown, the body is better protected in comparison to animals with an endoskeleton.
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