Cercus – An End in Itself

            I am beginning my series of insect studies not at the beginning but at the end. If you think I am writing about that magnificent beetle shown on the “Bugs” page, I will disappoint as I know nothing about it, Simply put, it is a beautiful, stunning creature and Mr. Beetle joined my blog only as an attention getter. I think he is from Mars, perhaps Venus, but certainly not from my garden!

            Let’s return to the subject of my informal dissertation today – Cercus.  One must start the study of insects somewhere, and at the posterior of most insects there are two appendages known as Cerci (plural) and as best as I can determine, they are sensory organs and can be microscopically present or extremely long as documented in several photographs. Various authors have described them as possible feelers to determine the best place to lay eggs, or some have speculated they may help in copulation (another topic for a different time). Perhaps they are a defensive tool in some insects, or an offensive threat in some that have developed pincer-like Cerci capable of inflicting crushing, stabbing, or other “911” damage. In any event, a Cercus observation may help in determining the name, species, or intentions of an insect of interest.

            Here is an excerpt from the Wikipedia article on Cercus, which introduces several terms: (https://en.wikipedia.org/wiki/Cercus, accessed 2/27/2019)

            “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, but some definitely feed on prey caught with the cerci, much as the Japygidae do.[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 and is not regarded as a cercus.

Aphids have tube-like cornicles or siphunculi that are sometimes mistaken for cerci but are not morphologically related to cerci.”

            Some new terms introduced by Wikipedia are “filiform” (threadlike) which is a subject of a future article. “Diplura” is an order very close to insects and are found in leaf litter and identified by their cerci which can be filiform or forcipate (pincers!).

Distinctive back-end “Antennae” are called Cerci.

“Oviposition” is an easy word and simply describes a process of locating the best place for laying eggs in soil or (heaven forbid!) a bean leaf. Another term introduced is “terminal filament” which is not a cercus. I suppose this third central appendage is only there to assist us in identifying certain insects or perhaps it is a “reverse feeler” (my term) to keep it aware of who or what may be sniffing, prospecting, or approaching from behind. Let’s look at a cercus or two on crickets, mayflies, silverfish, and a few others.

Note the two Cerci on the back-end of the cricket.

            There they are – even I can spot the cerci on Jiminy – now that I know what a cercus is.

A Mayfly with three Cerci longer than she is!

            Here is a Mayfly with two cerci flanking a terminal filament. I have seen thousands of these gentle insects emerge from the relatively clean waters of Lake Erie in June. They have enjoyed a comeback thanks to the invasive Zebra Mussel which has filtered out suspended toxic particles in the lake. The Mayfly is an indicator species of polluted water. Pollution equals few if any Mayflies.

Three Cerci in a Silverfish.

            Above is the familiar Silverfish with the same cerci and terminal filament structure as a Mayfly. Let’s look closely at the aphid below to determine what cerci may be present.

The Insidious Aphid!

Those two projecting nozzles, “cornicles”, on the posterior of the green aphids are not cerci! They are in the right place but serve as ejection sites for “honeydew” so ravished by ants who in a symbiotic relationship may guard the aphids against predators as shown in a favorite photo below.


Take a look at our Martian beetle on the “Bugs” page and more specifically to the Ladybug above. Do beetles have cerci?

Cerci? Where are you?

I have inspected the bottoms and posteriors of several beetles and cannot visually detect cerci. If they are there, they must be microscopic. We can use the size or absence of cerci to help identify bugs and thugs in the garden. Let’s look at a few…

Ugly but fascinating!

Earwig munching on mums and dahlias – check out those cerci!

Pinch me, I’m dreaming!

Another earwig! These can be beneficial but when populations increase they feed on plants like this purple coneflower. Cerci identify the villain!

Let me introduce you to Mary Mantis. “Ms. Mantis, may we see if you have any Cerci?”

My, Grandma, what big eyes you have!

“Thank you, Mary!”

As you can see, Cerci are present, but very small and, in fact, to a serious entomologist (not us!) the size, shape, and color of cerci help identify different Manti.

What have we learned?

We now are familiar with the term, “Cercus”, as well as its plural, “cerci”. Not all insects have cerci but those cerci that are visible might be useful in identification. Insects use cerci to perform ovipositioning, copulation positioning, rear-guarding, predation, threatening, and probably dozens of other tasks we can only imagine!

Beetles have minute cerci – if at all. Aphids are blessed with “false cerci” although somewhere in that posterior, real cerci might exist.

Best of all, we have started on a journey of learning about insect terminology.

4 thoughts on “Cercus – An End in Itself

  1. Love seeing all these bugs but now knowing some of their names and habits.

    Liked by 1 person

    1. I agree — so many and so much to learn! Thank you!


  2. Enjoy the information you have shared. Never knew what the pinchers I saw were. Am now able to call them by their actual name. Thank you!


    1. What’s really neat is seeing them on an unknown insect and using them to help identify.


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