telson

The telson is the final body segment in many arthropods. In crustaceans it forms the terminal part of the abdomen and, together with the paired uropods, makes up the tail fan used for propulsion. In arachnids, the telson refers to the last segment of the metasoma and, in scorpions, bears the sting.

In crustaceans, the telson is typically a broadened, sometimes triangular plate that articulates with the preceding abdominal segments. It usually lacks walking appendages of its own. When the tail fan is deployed, the telson and uropods spread to push water backward, allowing rapid backward movement or steering.

In scorpions and some other arachnids, the telson is the final metasomal segment from which the venom-bearing aculeus projects. It encloses venom glands and, with the pedicel and movable stinger, enables puncture and injection. The structure and robustness of the telson influence sting delivery.

Across groups, the telson serves different functions and shows considerable morphological variation; it is an example of convergent use of a terminal body region for propulsion or defense rather than a homologous organ.

Origin and terminology: the word telson comes from Greek telos, "end". See also uropod, metasoma, abdomen.