Insect thorax : Its areas, structure, function and modification in different orders of insect, sutures of tergum, sternum and pleuron and pterothorax

Insect thorax : Its areas, structure, function and modification in different orders of insect, sutures of tergum, sternum and pleuron and pterothorax

1. ASSIGNMENT ON Insect thorax : Its areas, structure, function and modification in different orders of insect, sutures of tergum , sternum and pleuron and pterothorax   1

2. EVOLUTION OF THE THORAX The thorax must have been evolved very early in the phylogenetic history of the Hexapoda as a locomotor section of the body through the specialization of its appendages for more active progression. In the Apterygota as well as the Pterygota the thorax is distinctly differentiated from the abdomen in the structure of its segments, showing that the inception of the thorax as a body region long antedated the acquisition of wings. The thoracic region of the embryo is well developed as the leg-bearing part of the body at a stage when the gnathal segments ( Gn ) are yet distinct and show no evidence of their future union with the procephalic lobes ( Prc ). Concurrent with the specialization of the thoracic appendages as organs of locomotion, the abdominal appendages were lost, and the future gnathal appendages assumed functions accessory to feeding. When the gnathal segments were then finally combined with the protocephalon to become a part of the definitive head, the hexapods appeared in their modern three-part form.   2

3. AREAS AND GENERAL STRUCTURE OF THE THORAX The thorax of an adult insect is in general easily recognized, since it is the section of the trunk bearing the legs, and the wings when wings are present. Ordinarily the thorax consists of the three body segments following the head, but in most of the Hymenoptera the fourth segment is so intimately associated with the third that it virtually becomes a thoracic rather than an abdominal segment. On the other hand, the thorax is often distinctly divided between its first two segments into a prothoracic part and a meso-metathoracic part. The second part, composed of the wing-bearing segments more or less closely united with each other, may be termed the pterothorax . The insect thorax is divided into three parts: a) Prothorax (pro=first) b) Mesothorax ( meso =middle) and c) Metathorax (meta=last) 3

4. The thorax is a box like structure and is composed of a number of sclerites which play an important role in insect locomotion. Each segment consists of hardened plates or sclerites . a) Dorsal sclerites - notum b) Lateral sclerites - pleuron c) Ventral sclerites – sternum Each of the three thoracic segments contain on pair of legs arise from pleura. Wings articulated between notal and pleural regions. Lateral view of thoracic segments Ventral view of thorax 4

5. Modification in different orders 1) Diplura : The 3 thoracic segments are clearly separated, the prothorax being the smallest ( Carpentier and Barlet , 1951; Barlet and Carpentier , 1962). The sterna and sometimes also the terga are subdivided by transverse sutures but there is no general agreement on the homologies of the sclerites so delimited. The pleural sclerites are reduced but indications of the primitive anapleural and coxopleural arcs have been claimed to occur in some genera. Manton (1972), however, considers that the thoracic sclerites of the Diplura evolved independently of those in the three other Apterygote orders and in the Pterygotes and that it is misleading to seek homologies between the various groups. 2) Collembola (Spring-tails) : The thorax of the more generalized forms consists of three very similar segments but in the Entomobryoidea the prothorax is greatly reduced, and its tergum is fused with that of the mesothorax (Manton, 1972). In the Symphypleona the thorax becomes intimately fused with the abdomen and its segmentation is largely obsolete.   5

6. 3) Odonata ( DRAGONFLIES) : The prothorax, though greatly reduce, remains a distinct segment, but the meso - and meta thorax are intimately fused together and modified to meet the requirements of the legs and wings. 4) Orthoptera (Grasshopper, Crickets, Locust) : The thoracic structures are most fully developed in alate species and are best known in the Acrididae (Snodgrass, 1929). The prothorax is large and its notum is extended laterally so as to conceal a great deal of the small propleuron (Ander, 1939) while the meso - and metathorax are closely associated to form the pterothorax . Pterothoracic nota are usually divided into prescutum , scutum and scutellum , while the intersegmental sclerites are represented by an acrotergite ( precosta ) on the mesothorax and both acrotergite and postnotum on the metathorax . The pterothoracic pleura are clearly divided into episternum and epimeron . Ventrally, the broad sternum of each pterothoracic segment is made up largely of the basi sternum; widely separated apophyseal pits are present and sternellar regions may usually be discerned. Spinasterna occur only on the pro- and mesothorax . 6 Lateral view of thorax of Agrion virgo ai ' 1St abdominal segment; c, coxa; ca, do r sal carinaem , epimeron ; es , episternum ( es , and eS3 are divided into an- and kat -episternum); p, pronotum ; s, spiracle; I, trochanter (double); w, wing.

7. 5) Dictyoptera (Cockroaches, Mantids ) The prothorax bears dorsally a large pronotum , shield-like and usually over-lapping the head in cockroaches, usually elongate and not covering the head in Mantids . The meso - and metaterga are similar, each subdivided into acrotergite , prescutum , scutum and scutellum , no postnota being present. The pleural region is normal, that of the prothorax not being concealed by lateral expansions of the pronotum (cf. Orthoptera ). The sternal region of the Blattaria has undergone extensive desclerotization , especially in the ptero -thorax where the basi sternum is reduced to a small pair of plates and the sternellum is even more reduced. In the Mantids , the sterna are more fully sclerotized and are dominated by the large basisterna (Adam and Lepointe , 1948; La Greca and Raucci , 1949). In all Dictyoptera there are well-developed furcal arms and a spina is present in the pro- and mesothorax . 6) Isoptera (Termites, White Ants) In the thorax (Fuller, 1924) the terga are well developed. The pronotum shows many taxonomically important variations in shape: it may be flat and shield-like, heart-shaped, laterally lobed or saddle-shaped. The meso - and metanota are sub equal and less variable .   7

8. On the ventral side the prosternum is reduced, the mesosternum large and broad (mainly basisternum ) and the metasternum somewhat smaller and V-shaped. The pleura comprise approximately equal episterna and epimera separated by a more or less straight pleural sulcus; a trochantin is also present. 7) Hemiptera (Bugs) The morphology of the Hemipteran thorax has not been extensively investigated, but a comparison of a number of genera has been made by Taylor (1918) while Larsen (1945, 1950) has studied many Heteroptera . Among Heteroptera the pronotum is tolerably uniform in its characters; it is always large, rarely marked off into separate sclerites , and forms the greater part of the thorax when viewed from above. The mesonotum frequently exhibits a fivefold division, thus presenting the maximum number of sclerites . Of these the most prominent is the scutellum , which is always well developed; in certain Pentatomoidea it extends posteriorly to cover the wings entirely, and imparts to the insect an apterous appearance. 8

9. 8) Coleoptera (Beetles) The prothorax is the largest of the thoracic segments and is usually freely movable, the latter feature being a marked characteristic of the order. The pronotum is composed of a single sclerite and is entirely visible from above. The pleuron is frequently undivided into sclerites , and the notopleural sulcus between that region and the pronotum , on either side, is absent in the Polyphaga : in the latter case a single cuticular shield covers the whole of the dorsal and lateral regions. The significance of the coxal cavities in the classification of the Adephaga is discussed by Bell (1967). The meso - and metathorax are fused together: the former segment is considerably reduced while the latter, on the contrary, is largely developed, except in species in which the wings are absent or non-functional. The tergum of both segments is divisible into prescutum , scutum and scutellum . In the metathorax , a furca is invaginated from the sternum. Hydroplzilus , dorsal aspect of thorax with the segments dis-articulated All anterior wing process; A"posterior do.; AC, axillary cord;P , pronotum ; PN, postnotum;PS , prescutum ; S, scutum ; S"scutellum . Partly after Snod -grass, Proc. U.S. nat. MilS ., 36 9

10. 9) Diptera (Flies) The thorax is characterized by the great development of its second segment which carries the wings, and the correlated reduction of the segments in front and behind it (Young, 1921). The two latter regions are little more than anterior and posterior bands, whose active function is the support of the fore and hind legs. The pronotum in Tipulids is represented by a band-like scutum and scutellum , but is still more reduced in the higher Diptera . The mesonotum forms the greater part of the dorsal aspect of the thorax and is clearly divisible into prescutum , scutum and scutellum : the postnotum of this segment is well developed in Tipulidae , Culicidae and other Nematocera , but is subdivided among the higher Diptera . The boundary between the pre- scutum and scutum is known as the transverse suture and, although complete and V-shaped in Tipulids , it is generally incomplete in the middle line in other Diptera . On the mesonotum are certain more or less prominent lateral swellings on either side, which are recognized by systematists , and known as calli or callosities. 10

11. Thorax of A, Lucila caesar XII; B, Compsilura concinnata , dorsal view x 1 I A, alula; AS, antisquama ; C, calyptra; H, hypopleuron ; HC, humeral callus; HL, haltere ; M, mesopleuron ; M" metapleuron ; P, pteropleuron ; PC, pre-alar callus; ·PIC, postalar callus; S, sternopleuron ; TS, transverse suture; W, wing-base. 11

12. Sutures of Tergum, Sternum, P leuron and Pterothorax Transverse structure of an insect Thorax Tergum : Thoracic terga consisted primitively of three simple segmental plates (the nota) between which lay small intersegmental sclerites (Snodgrass, 1927). In all known insects, however, a secondary segmentation has become established whereby the inter-segmental sclerites have become closely associated with the notum in front or with the one behind. In the latter case the intersegmental sclerite forms a narrow band at the front of the notum and is known as the acrotergite . In the former case, the intersegmental sclerite becomes known as the postnotum and is sometimes a conspicuous plate. 12

13. These transpositions of the intersegmental sclerites of the pterothorax differ in different insects. Thus, in many Apterygota , the Blattids , the Isoptera and many immature forms the meso -and meta thorax each possesses an acrotergite but no postnotum . In most other orders a postnotum is present in both meso - and metathorax , but in the Orthoptera and Coleoptera the meta thorax has acquired two intersegmental sclerites , having both acrotergite and postnotum while the meso -thorax has accordingly no postnotum . The notum is typically divided into three sclerites , the prescutum , the scutum and the scutellum . At the sides of the pronotum in many Lepidoptera are lobe-like structures known as patagia . 13

14. 2) Sternum: The ventral surface of the prothorax is usually composed of 2 distinct sclerites : the eusternum (which is sometimes just called the sternum, and is the combined basisternum and sternellum ) and the acrosternite (from the mesothorax ). The acrosternite is sometimes called the spinasternum or intersternum . In the thorax, the acrosternite may be an independent sclerite or it may migrate anteriorly to become a part of the ventral complex of the metamere preceding the primary suture (there is no prothoracic acrosternite ). The acrosternite may bear a median spine-like apodeme , the intersternal apodeme (or sometimes called the spina ). There are differing degrees of fusion between sternal plates in the thorax such that 4 different conditions may occur: 1) all elements separate - prothoracic eusternum , first spinasternum , mesothoracic eusternum , second spinasternum , metathoracic eusternum 2) eusternum of mesothorax and second spinasternum fuse, the rest remaining separate; 3) eusternum of prothorax and first spinasternum also fuse so that there are now 3 main elements - compound prosternum , compound mesosternum , and metathoracic sternum; and 4) complete fusion of meso - and meta-thoracic elements to form a pterothoracic plate. 14

15. Cross-section of a thoracic segment showing the pleural ridges and sternal apophyses (after Snodgrass, 1935). Arising from the eusternum are a pair of apophyses , the so-called sternal apophyses . The origins of these on the sternum are marked externally by pits joined by a sulcus so that the eusternum is divided into a basisternum and sternellum , while in higher insects the two apophyses arise together in the midline and only separate internally, forming a Y-shaped furca Distally the sternal apophyses are associated with the inner ends of the pleural apophyses , usually being connected to them by short muscles. This adds rigidity to the thorax, while variation in the degree of contraction of the muscles makes this rigidity variable and controllable. The sternal apophyses also serve for the attachment of the bulk of the ventral longitudinal muscles, although a few fibers retain their primitive intersegmental connections with the spinasterna . 15

16. 3) Pleuron : The pleural regions are membranous in many larval insects, but typically become sclerotized in the adult. There are probably three pleural sclerites , one ventral and two dorsal, which may originally have been derived from the coxa . The ventral sclerite , or sternopleurite , articulates with the coxa and becomes fused with the sternum so as to become an integral part of it. The dorsal sclerites – anapleurite and coxopleurite – are present as separate sclerites in Apterygota and in the prothorax of larval Plecoptera In other insects they are fused to form the pleuron , but the coxopleurite , which articulates with the coxa , remains partially separate in the lower pterygote orders, forming the trochantin and making a second, more ventral articulation with the coxa . Above the coxa the pleuron develops a nearly vertical strengthening ridge, the pleural ridge, marked by the pleural sulcus externally. This divides the pleuron into an anterior episternum and a posterior epimeron . The pleural ridge is particularly well developed in the wing-bearing segments, where it continues dorsally into the pleural wing process which articulates with the second axillary sclerite in the wing base 16

17. 4) Pterothorax : The pterothorax is the collective name of the meso- and metathorax, which hold the wings. The overall structure of the entire thoracic region is fundamentally the same as the abdomen, but there are modifications for locomotion, and the degree of modification depends on the flying ability of the insect. A very important modification that the thorax has that the abdomen does not is sclerotization in the pleural region. The pterothroax itself has many extra strengthening ridges, and the conjunctivae is reduced, making the pterothroax a very rigid structure, because the segments cannot telescope. Lateral View of Pterothoracic Segment 17

18. References  Richards,O.W . and Davies,R.G .(1977),IMMS’ GENERAL TEXTBOOK OF ENTOMOLOGY Volume 2 clasification and biology. CHAPMAN AND HALL,London , pp 476-1175 P.J. Gullan and P.S. Cranston 2014 by John Wiley & Sons, Ltd THE INSECTS AN OUTLINE OF ENTOMOLOGY R. E. Snodgrass with a new foreword by George C. Eickwort PRINCIPLES OF INSECT MORPHOLOGY STEPHEN J. SIMPSON ANGELA E. DOUGLAS R. F. CHAPMAN THE INSECT STRUCTURE AND FUNCTION Richards,O.W . and Davies,R.G .(1977),IMMS’ GENERAL TEXTBOOK OF ENTOMOLOGY Volume 1Structure, Physiology and Development CHAPMAN AND HALL,London , pp 476-1175 18

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