The telson is the posterior-most division of the body of an arthropod. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca may be present. The shape and composition of the telson differs between arthropod groups.
In lobsters, shrimp and other decapods, the telson, along with the uropods forms the tail fan. This is used as a paddle in the caridoid escape reaction ("lobstering"), whereby an alarmed animal rapidly flexes its tail, causing it to dart backwards. Krill can reach speeds of over 60 cm per second by this means. The trigger time to optical stimulus is, in spite of the low temperatures, only 55 milliseconds.
In the Isopoda and Tanaidacea (superorder Peracarida), the last abdominal body segment is fused with the telson, forming a "pleotelson".
The term telson is widely used for the caudal spine of some chelicerates . The chelicerate telson can be clearly seen in a number of fossil species (like in eurypterids) and in extant animals (like the horseshoe crab "tail" and the scorpion sting). Some authorities have urged that the usage of this word in this context be discouraged.
In millipedes, the telson consists of a legless pre-anal body segment (which may contain an posterior extension known as an epiproct), a pair of anal valves (paraprocts) or plates closing off the anus, and a plate below the anus (hypoproct), also known as a subanal scale.
In centipedes the telson is the rear-most body segment, posterior to the genital openings, bearing two anal valves.