telson; pygidium; pleon
Zoological term for the terminal segment or posterior body region of arthropods. Telson is the tail segment, pygidium is the posterior body part of certain invertebrates, and pleon is the abdomen of crustaceans. Usage is technical and field-specific.
The telson of a horseshoe crab is shaped like a sword.
hypopygium
Entomological term for the terminal abdominal segment of male insects, often modified for copulation. Highly specialized usage.
The pygidium of trilobites is an important clue for classification.
The hypopygium of male flies is specialized as a copulatory organ.