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.
The antennae are often called 'feelers' because the insect waves them around. This is a wrong name because they are not only used for touch. The antennae are actually the insects 'nose' - they are used for the sense of smell.
The paired antennae are made up of a number of individual joints. This means they can be very mobile. The basic form of antenna is filiform. In this type there are many segments that are more or less equal in size. Filiform antennae are seen in a wide variety of groups, such as Dragonflies, Grasshoppers and Crickets, Book Lice, Biting Lice etc. The length and number of joints varies much between them.
This is the most basic form of insect antennae.
Modification of filiform antennae:
This basic structure is modified in a wide variety of ways. This means that a number of different types may be recognized. The main ones are as follows:-
Setaceous - There are many joints. The antenna tapers gradually from the base to the tip e.g. Bristletails, Cockroaches, Mayflies, Stoneflies , Caddisflies etc.
Moniliform:The round segments make the antenna look like a string of beads e.g. Beetles.
Serrate - the segments are angled on one side giving the appearance of a saw edge e.g. Beetles.
Pectinate - The segments are longer on one side. This gives the appearance of a comb e.g. Sawflies (related to wasps) and Beetles.
Clavate - the segments become wider towards the tip of the antenna. This may be gradual along its length, or a sudden increase and therefore mainly affecting the last few joints and giving the appearance of a club e.g. Butterflies & Moths and Beetles.
Lamellate - the segments towards the end are flattened and plate-like. This gives the appearance of a fan e.g. Beetles
Geniculate - there is an abrupt bend or elbow part of the way along the antenna e.g. Ants and Beetles.
Plumose - the segments each have a number of fine thread-like branches. This gives the appearance of a feather e.g. Flies.
The antennae of the Lepidoptera, Neuroptera, Hymenoptera and Coleoptera are very variable. These groups show examples of a number of different antenna types.
Insects Mouthparts & Modifications
Insects exhibit a range of mouthparts, adapted to particular modes of feeding. The earliest insects had chewing mouthparts. Specialization has mostly been for piercing and sucking, although a range of specializations exist, as these modes of feeding have evolved a number of times .In this chapter the individual mouthparts and their special modifications are described thereafter.
The mouthparts of insects, mites and snails have adapted over time to better suit for the food source and feeding style of the organism. Mouthparts differ from insect to insect and the damage that they cause is useful in the classification and identification of the pest. Being able to identify the type of damage caused by the various types of mouthparts also helps differentiate insect damage from mite damage and helps to determine the pest control techniques.
There are two main categories of mouthparts:
Chewing mouthparts are the most primitive type of mouthpart and are often referred to has mandibulate mouthparts. There are many different variations on the basic type of chewing mouthparts depending upon their feeding habit.
Examples of chewing insects include dragonflies, grasshoppers ,beetles etc. Some insects that do not have chewing mouthparts as adults do as larvae, such as moths and butterflies. The main parts of chewing types mouthparts are as follows:
The labrum is an upper lip of the insect articulating with the mandibles. It serves to hold food in place during chewing by the mandibles.
Figure: Typical mouthparts structure of Cockroach
Chewing insects have two mandibles, one on each side of the head. The mandibles are positioned between the labrum and maxillae. They are typically the largest mouthparts of chewing insects, being used to masticate (cut, tear, crush, chew) food items. In carnivorous chewing insects, the mandibles can be modified to be more knife-like, where-as in herbivorous chewing insects, they are more typically broad and flat on their opposing faces (eg, caterpillars). In male stag beetles, the mandibles are modified to such an extent that they do not serve any feeding function, but are instead used to defend mating sites from other males. In ants, the mandibles also serve a defensive function (particularly in soldier castes). In bull ants, the mandibles are elongate and toothed, used as hunting (and defensive) appendages. Structure of mouthparts gives clues to food type and insect habits.