In the strict sense, cerci (singular, cercus) are paired elongate sensory appendages having their origin on abdominal segment 11 of the most primitive hexapods, such as silverfish. Since segment 11 is highly reduced or absent in most orders, the cerci are more commonly articulated on segment 10. Only in the Ephemeroptera are they retained in their primitive state, but they are also prominent in Plecoptera. In other orders they are reduced or absent or sometimes associated with structures of the male genitalia or female oviscapt. Present only in silverfish, bristletails, and mayflies, the terminal filament is an elongate, medial, sensory appendage also associated with segment 11. In some mayflies, the terminal filament may be short or rudimentary and in some stoneflies the cerci also may be rudimentary.

Cerci & terminal filament
Click to enlarge