Arthropods are invertebrates that have an exoskeleton. The exoskeleton is their integumentary system. The skin is hard, and made up of a tough polymer called Chitin. It acts like an armor, and protects them against predators. It also prevents their bodies from drying up, which helps them to survive in extreme dry conditions like the deserts.
The most common arthropods are insects, lice, mites, spiders, crustaceans and centipedes.
The exoskeleton has four layers, namely epicuticle, procuticle, epidermis and basement membrane. The topmost layer, or the epicuticle, serves to lock in water, while the procuticle is the layer that gives strength to the exoskeleton. The third layer, or the epidermis, is responsible in secreting the procuticle.
The muscles of the arthropods are attached to the exoskeleton. The exoskeleton is divided into head, thorax and abdomen. In spiders, their head and thorax are fused and is called Cephalothorax, as the exoskeleton is divided into cephalothorax and abdomen.
The exoskeleton has one disadvantage. It does not grow with the rest of the body. So, it has to molt or shed its skin when it gets too small for the body. Before the shedding process, the cuticle separates from the epidermis in the process called as Apolysis. Then the epidermis secretes a new cuticle, which is soft and takes time to harden. Hardening occurs with dehydration of the cuticle. The new skin will be pale, and it darkens as it becomes harder. In this molting stage, the arthropod has to be careful as it is vulnerable to attacks from predators.
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