Cercus

Not to be confused with circus.
An earwig with large cerci.

Cerci (singular cercus) are paired appendages on the rear-most segments of many Arthropoda, including insects and Symphyla. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs of copulation.[1] In many insects they simply may be functionless vestigial structures.

In basal arthropods, such as silverfish, the cerci originate from the 11th abdominal segment. As segment 11 is reduced or absent in the majority of arthropods, in such cases, the cerci emerge from the 10th abdominal segment.[2] It is not clear that other structures so named are homologous. In the Symphyla they are associated with spinnerets.[1]

Short cerci on abdomen of a species of Pamphagid grasshopper
Long sensory cerci on Ctenolepisma, flanking the median cerciform appendage and paired styli
Two forms of Diplura, one species of Campodea and one of Japyx, illustrating cerci with sensory glandular function, as contrasted with forcipate forms of cerci used in predation

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.[3]

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.[3]

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, silverfish and bristletails have an accompanying third central tail filament which extends from the tip of the abdomen. This is referred to as the terminal filament.[2]

Aphids have tube-like cornicles or siphunculi that are sometimes mistaken for cerci.

Evolutionary origin

Like many insect body parts, including mandibles, antennae and styli, cerci are thought to have evolved from what were legs on the primal insect form;[3] 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.[4]

References

  1. ↑ 1.0 1.1 Tiegs, O. W. The post-embryonic development of Hanseniella agilis (Symphyla). Printed at the University Press, 1945.
  2. ↑ 2.0 2.1 "CERCI AND TERMINAL FILAMENT". Entomological Glossary. University of Minnesota. Retrieved 3 June 2014.
  3. ↑ 3.0 3.1 3.2 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.
  4. ↑ David Grimaldi (16 May 2005). Evolution of the Insects. Cambridge University Press. ISBN 978-0-521-82149-0.

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