An insect's abdomen is the third functional region (tagma) of its body; the abdomen is located just behind the thorax. In most insects, the junction between thorax and abdomen is broad, but in some groups, the junction is very narrow (petiolate) giving the appearance of a "wasp-waist".
Entomologists generally agree that insects arose from primitive arthopod ancestors with eleven-segmented abdomens. Some present-day insects (e.g. silverfish and mayflies) still have all of these segments (or remnants of them), but natural selection in more advanced (or specialized) groups has contributed to a reduction in the number of segments -- sometimes to as few as six or seven (e.g. beetles and flies).
Each segment of the abdomen consists of a dorsal sclerite, the tergum, and a ventral sclerite, the sternum, joined to one another laterally by a pleural membrane. The front margins of each segment often "telecope" inside the sclerites of the preceding sement, allowing the abdomen to expand and contract in response to the actions of skeletal muscles.
In many adult insects, there is a spiracle (opening to the respiratory system) near the pleural membrane on each side of the first eight abdominal segments. Some spiracles may be permanently closed, but still represented by a dimple in the sclerite.
At the very back of the abdomen, the anus (rear opening of the digestive system) is nestled between three protective sclerites: a dorsal epiproct and a pair of lateral paraprocts. A pair of sensory organs, the cerci, may be located near the anterior margin of the paraprocts. These structures are tactile (touch) receptors. They are usually regarded as a "primitive" trait because they are absent in the hemipteroid and holometabolous orders.
The insect's genital opening lies just below the anus: it is surrounded by specialized sclerites that form the external genitalia. In females, paired appendages of the eighth and ninth abdominal segment fit together to form an egg-laying mechanism called the ovipositor. These appendages consist of four valvifers (basal sclerites with muscle attachments) and six valvulae (apical sclerites which guide the egg as it emerges from the female's body). In males, the genital opening is usually enclosed in a tube-like aedeagus which enters the female's body during copulation (like a penis). The external genitalia may also include other sclerites (e.g. subgenital plate, claspers, styli, etc.) that facilitate mating or egg-laying. The structure of these genital sclerites differs from species to species to the extent that it usually prevents inter-species hybridization and also serves as a valuable identification tool for insect taxonomists.
Other abdominal structures may also be present in some insects. These include:
- Pincers -- In Dermaptera (earwigs), the cerci are heavily sclerotized and forceps-like. They are used mostly for defense, but also during courtship, and sometimes to help in folding the wings.
- Median caudal filament -- a thread-like projection arising from the center of the last abdominal segment (between the cerci). This structure is found only in "primitive" orders (e.g. Diplura, Thysanura, Ephemeroptera).
- Cornicles -- paired secretory structures located dorsally on the abdomen of aphids. The cornicles produce substances that repel predators or elicit care-giving behavior by symbiotic ants.
- Abdominal prolegs -- fleshy, locomotory appendages found only in the larvae of certain orders (notably Lepidoptera, but also Mecoptera and some Hymenoptera).
- Sting -- a modified ovipositor, found only in the females of aculeate Hymenoptera (ants, bees, and predatory wasps).
- Abdominal gills -- respiratory organs found in the nymphs (naiads) of certain aquatic insects. In Ephemeroptera (mayflies), paired gills are located along the sides of each abdominal segment; in Odonata (damselflies), the gills are attached to the end of the abdomen.
- Furcula -- the "springtail" jumping organ found in Collembola on the ventral side of the fifth abdominal segment. A clasp (the tenaculum) on the third abdominal segment holds the springtail in its "cocked" position.
- Collophore -- a fleshy, peg-like structure found in Collembola on the ventral side of the first abdominal segment. It appears to maintain homeostasis by regulating absorption of water from the environment.