Ento 101: the Abdomen

Long neglected, I am resuming with the Ento. 101 project. Please view the disclaimer at the bottom of the Entomology 101 page.

Previous: Ento. 101–Legs.

The Insect Abdomen (eidonomy for now, I’ll save the guts for later)

Behold, the long-suffering female conehead (most likely Neoconocephalus triops) contributes more images to the Ento. 101 cavalcade. (This conehead was lost to mold, and only the photographs remain so I cannot refer back to the specimen.) In this chapter of Ento. 101 we will learn some of the basic nomenclature of the insect abdomen.

She's dead, Jim.

She’s dead, Jim.

The abdomen of insects is for the most part, squishy.

Want to know more?

The insect abdomen is the retainer for the digestiveexcretoryreproductive and respiratory systems. Generally, the insect abdomen has less sclerotization (hence the squishy) than the head and thorax, which allows for easy expansion for food and eggs. The model insect has 11 segments, give one or take five depending on the family. Like the thorax, each segment has a sclerotized dorsal region called a tergum, a sclerotised ventral area called a sternum and a sclerotized lateral area called the pleurite. Each terga and sterna are separated by a flexible membrane, and the tergum and sternum are separated by a lateral pleural membrane, all of which allows for movement and expansion of the abdomen. The first eight segments of the pleural membrane have a spiracle on either side, which are openings for the respiratory system. This conehead specimen had no obvious pleurites, so the pleural membrane is clearly visible.

Abdominal segmentaiton. Tergites(T). sternites (S) and the pleural membrane.

Abdominal segmentaiton. Tergites(T). sternites (S) and the pleural membrane.

It is in the last three segments of the abdomen where things get really interesting. Besides carrying the naughty bits for excretion and genitalia for copulation, the last segments have adapted in various ways: as tools to lay eggs, as stingers, as shovels to dig and as claspers for mating. The cerci (sing. cercus) are sensory, but in some insects such as earwigs, they have developed into a tool-like forceps and can be used to grasp. In the case of the female conehead, there is a large ovipositor which consists of two visible pairs of valvulae (sing. valvula, sometimes called gonophysis) and one hidden enclosed pair which guide the eggs as they emerge from the gonopore. Like the name suggests, the ovipositor is for placing eggs deep into the sheaths of grass leaves. The anus is between the paraprocts and below the epiproct.

Female genetalia I

You can see a better view of the ovipositor lower paired valvulae in the ventral view in the image below:

Ventral view

Ventral view

The male genitalia (not illustrated) are designed to inseminate the female. Basically, they consist of outer claspers (parameres) and the inner aedeagus through which the sperm or spermatophore are passed to the female ovipore.

Both the female and male genitalia vary greatly between and in the orders, so you will need to consult specialist literature to get more details. (A starting point would be the Taxonomist’s Glossary of Genitalia in Insects by S. L. Tuxen  which does not seem to be available in its entirety online, as yet) The genitalia can often be the deciding factor in determining species ID in many insects, so I hope to have a closer look at this when I get into insect orders and families.

As usual, comments and corrections are welcome!

Next: probably Internal Structure, but who knows?


Grimaldi, David and M.S. Engel, Evolution of the Insects, (pp. 131-136) Cambridge University 2005.

Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, (pp.22-26) Elsevier 2003.

The Abdomen. Entomology 425. NC State University

Insect Morphology (Wikipedia)

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  1. Gary Anweiler 31 March, 2016 at 9:29 AM #

    Cool !! I like the soft images …. and I even learned stuff !! Thanks Adrian …

    • Adrian 31 March, 2016 at 9:57 AM #

      Thanks, Gary. All the colour of the specimen disappeared in the alcohol bath so I thought some toning and ‘antiquing’ of the image would be appropriate.

  2. Annie Pang 13 April, 2016 at 1:33 AM #

    Thanks Adrian. I don’t think I can remember it all, but it is quite amazing. Well done! I am in awe of your knowledge of all these details! Major wow!

    • Adrian 13 April, 2016 at 3:53 AM #

      You’re welcome, Annie. Just don’t exaggerate my knowledge, it took some time to look up the details and make sure they were right, because I don’t have the specimen to go back to. I’ll probably forget most of this in a few weeks! 🙂

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