Cerci (singular cercus) are paired appendages on the rear-most segments of many Arthropoda, including insects and Symphyla, but it is not clear that other structures so named are homologous. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs of copulation. In the Symphyla they are associated with spinnerets. In many insects they simply may be functionless vestigial structures.
Morphology and functions
Most cerci are segmented and jointed, or filiform (threadlike), but some take very different forms. Some Diplura, in particular Japyx species, have large, stout forcipate (pincer-like) cerci that they use in capturing their prey. The Dermaptera or Earwigs are well known for the forcipate cerci that most of them bear, though species in the suborders Arixeniina and Hemimerina do not. It is not clear how many of the Dermaptera use their cerci for anything but defense. Crickets have particularly long cerci while other insects have cerci that are too small to be noticeable. However, it is not always obvious that small cerci are without function; they are rich in sensory cells and may be of importance in guiding copulation and oviposition. Some insects such as mayflies have an accompanying third tail filament which extends from the tip of the abdomen. Aphids have tube-like cornicles or siphunculi that are sometimes mistaken for cerci.
Like a many insect body parts, including mandibles, antennae and styli, cerci are thought to have evolved from what were legs on the primal insect form; a creature that may have resembled a velvet worm, Symphylan or a centipede, worm-like with one pair of limbs for each segment behind the head or anterior tagma.
- Tiegs, O. W. The post-embryonic development of Hanseniella agilis (Symphyla). Printed at the University Press, 1945. 
- Richards, O. W.; Davies, R.G. (1977). Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. ISBN 0-412-61390-5.
- David Grimaldi (16 May 2005). Evolution of the Insects. Cambridge University Press. ISBN 978-0-521-82149-0.
- Media related to Cerci at Wikimedia Commons