Regeneration occurs in a wide variety of animals; however, it progressively declines as the animal species becomes more complex. Regeneration frequently occurs among primitive invertebrates. For example, a planarian (flatworm) can split symmetrically, with the two sides turning into clones of one other. In higher invertebrates regeneration occurs in echinoderms such as starfish and arthropods such as insects and crustaceans. Starfish are known for their ability to develop into complete individuals from one cut-off arm. Regeneration of appendages (limbs, wings, and antennae) occurs in insects such as cockroaches, fruit flies, and locusts and in crustaceans such as lobsters, crabs, and crayfish. For example, regeneration of a crayfish’s missing claw occurs at its next molt (shedding of its hard cuticle exterior shell/skin in order to grow and the subsequent hardening of a new cuticle exterior). Sometimes the regenerated claw does not achieve the same size of the missing claw. However, after every molt (a process that occurs two to three times a year) the regenerated claw grows and will eventually become nearly as large as the original claw. On a very limited basis, some amphibians and reptiles can replace a lost leg or tail.