Why are phase changes shown in pairs on the phase change triangle?

Arthropods have partially alleviated this problem by having the exoskeleton form as individual plates on each body segment or individual cylinders on each segment of the appendages. The plates are called sclerites, and they are connected by flexible tissue to allow the animal to bend during movement.Early land arthropods evolved adaptations such as book lungs or trachea to breathe air. The exoskeleton was another important adaptation. It prevents an animal from drying out. It also provides support in the absence of buoyant water.