How to Bend a Hard Exoskeleton
Having a hard exoskeleton introduces a problem for arthropods: flexibility. How can an animal with a rigid body covering move its legs? Evolution solved this problem with joints. All arthropods (arthro = joint, pod = foot) have jointed limbs.
In most of the leg, the exoskeleton is hard, but at the joints it is softer and bendable, allowing movement in the same way that a suit of armor does. The limb can be controlled by contracting muscles connected to the exoskeleton on both sides of the joint.