Insect body segmentation , all about the parts of insect body , body parts of an insect.

Insect body segmentation , all about the parts of insect body , body parts of an insect.

1. LECTURE -5 INSECT BODY SEGMENTATION

2. Body segmentation of Insect In general, insect body is divided in to a series of segments, which in primitive arthropods are known as “ somites ” or “ metameres ”. The type of arrangement of these body segments in embryonic stage is known as primary segmentation while in adult insects is known as the secondary segmentation which differs from primary in having a sclerotized membranous intersegmental region.

3. Insect body is divided in to three regions or tagmata namely head, thorax and abdomen. This grouping of body segments in to regions is known as tagmosis. Head consists of mouthparts, compound eyes, simple eyes (ocelli) and a pair of antennae. Thorax consists of 3 segments i.e. prothorax, mesothorax and metathorax . Meso and metathorax are together known as pterothorax . All the three thoracic segments possess a pair of legs and meso and meta thorax possess one pair of wings. Abdomen has 7-11 segments with genital appendages on 8 th and 9 th segments.

4. THE HEAD Insect head is a hard and highly sclerotized compact structure. It is the foremost part in insect body consisting of 6 segments that are fused to form a head capsule. The head segments can be divided in to two regions i.e. procephalon and gnathocephalon (mouth). The procephalon represents preoral region of the head and bears two paired structures, the eyes and the antennae . The gnathocephalon represents postoral region of the head. It bears the mandibles, maxillae and labium.

5. Sclerites and sutures of head The head capsule is formed by the union of number of sclerites which are joined together by means of cuticular lines or ridges known as sutures . These sutures provide mechanical support to the cranial wall.

6. Compound eye: Large, many faceted structure , situated on each side of the head. Ocelli otherwise called simple eyes, three single faceted organs usually occur between the compound eyes. 2. Clypeus: It is situated above the labrum and is divided in to anterior ante-clypeus and posterior post-clypeus . 3. Frons: It is the facial part of the insect consisting of median ocellus. 4. Vertex: It is the top portion of the head behind the frons or the area between the two compound eyes.

7. 5. Epicraniun : It is the upper part of the head extending from vertex to occipital suture. 6. Occiput : It is an inverted “U” shaped structure representing the area between the epicranium and post occiput. 7. Post occiput: It is the extreme posterior part of the insect head that remains before the neck region. 8. Gena: It is the area extending from below the compound eyes to just above the mandibles. 9. Occular sclerites: These are cuticular ring like structures present around each compound eye.

8. 10 . Antennal sclerites: These form the base for the antennae and present around the scape which are well developed in Plecoptera (stone flies). 11. Tentorium: It is the endoskeleton of the insect head found from invagination of the body wall that provides rigidity and areas for muscle attachment . It is composed of three pairs of arms, the anterior, posterior and dorsal tentorial arms. All these tentorial arms unite in the middle forming an expanded central skeletal platform. It may simply constitute a structure resembling the X or the Greek letter, π.

9. The common sutures present in head are Clypeo -labral suture : It is the suture present between clypeus and labrum . It remains in the lower margin of the clypeus from which the labrum hangs down. Clypeo -frontal suture or epistomal suture: The suture present between clypeus and frons Epicranial suture: It is an inverted ‘Y’ shaped suture distributed above the facial region extending up to the epicranial part of the head. It consists of two arms called frontal suture occupying the frons and stem called as coronal suture. This epicranial suture is also known as line of weakness or ecdysial suture because the exuvial membrane splits along this suture during the process of ecdysis.

10. Frontogenal suture : Develops on each lateral region of the head. It starts from the junction of the clypofrontal and the subgenal suturs . Occipital suture: It is ‘ U’ shaped or horseshoe shaped suture between epicranium and occiput. Post occipital suture: It is the only real suture in insect head. As this suture separates the head from the neck, hence named as real suture. Posterior end of the head is marked by the post occipital suture to which the sclerites are attached. Occular suture: It is circular suture present around each compound eye. Antennal suture : It is a marginal depressed ring around the antennal socket.

11. The thorax is the body region between the head and abdomen. Thorax is provided with locomotory appendages, strong musculature and exoskeletal elements consisting locomotory centre of the body. It is composed of three segments, i.e , the prothorax (pro= first), mesothorax (meso=middle) and metathorax (meta=last). Each segment consists of hardened plates, or sclerites. Dorsal sclerites are called tergum or nota (singular notum), lateral sclerites are called pleura (singular pleuron), and ventral sclerites are called sterna (singular sternum). Thorax

12. pleuron notum sternum pleuron Cross section of insect thorax

13. A tergum or notum in accordance with the thoracic segments is termed as the pro-, meso- and metatargum or notum and also accordingly pro-, meso- and metasternum and pro-, meso- and metapluron with reference to a particular thoracic segment to which they present. The sclerites of the tergum, sternum and pleuron are also called as the tergites, sternites and pleurites respectively. Each of the three thoracic segments contains one pair of legs.  Wings are found only on the meso- and metathoracic segments. These wing bearing segments of the thorax collectively form the pterothorax .

14. It is the third and posterior region of body consisting of usually 10-11 segments and is highly flexiable . Abdominal segments are called uromeres . It is greatly reduced to 6 segments in collembola. Abdominal segments are telescopic in nature and are interconnected by a membrane called conjunctiva. In ants, bees and wasp, the first abdominal segment is fused with the metathorax and is called propodeum. Often the second segment forms a narrow petiole. The rest of the abdomen is called gaster . A typical abdominal segment consists of dorsal tergum, ventral sternum, lateral pleura and a spiracle on side of each segment situated on pleura. Abdomen

15. Abdominal segments from 1 to 7 are pre-genital segments, 8 th and 9 th are known as genital segments as they form genital appendages i.e . ovipositor in female and aedeagus or penis in male. Tenth and eleventh segments are known as post-genital segments. Reproductive structures are located on the 9 th segment in males and on the 8 th and 9 th abdominal segments in females. The 11 th segment of insect bears a pair of appendages, the cerci . Cerci are sensory in function . In earwing cerci are forceps like and are useful in defense, prey capturing and unfolding wing.

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