Structure of the Thorax

In the evolution of the typical insectan body plan there have been two phases associated with the development of the thorax as the locomotory center; in the first the walking legs became restricted to the three thoracic segments, and in the second articulated wings were formed on the meso- and metathoracic terga. Accompanying each of these developments were major changes in the basic structure of the secondary segments of the thoracic region. These changes were primarily to strengthen the region for increased muscular power.

In the apterygotes and many juvenile pterygotes the thoracic terga are little different from those of the typical secondary segment described in Section 2. In the adult the terga of the wing-bearing segments are enlarged and much modified (Figure 3.18). Although it may remain a single plate, the tergum (or notum, as it is called in the thoracic segments) is usually divided into the anterior wing-bearing alinotum, and the posterior postnotum. These are firmly supported on the pleural sclerotization by means of the prealar and postalar arms, respectively. The antecostae of the primitive segments become greatly enlarged forming phragmata, to which the large dorsal longitudinal muscles are attached. As wing movement is in part brought about by flexure of the terga (see Chapter 14, Section 3.3.3), which is itself caused by contraction and relaxation of the dorsal longitudinal muscles, it is clear that the connection between the mesonotum and metanotum and between the metanotum and first abdominal tergum must be rigid. The intersegmental membranes are therefore reduced or absent. Additional supporting ridges are developed on the meso- and metanota, the most common of which are the V-shaped scutoscutellar ridge and the transverse (prescutal) ridge (Figure 3.18A). The lateral margins of the alinotum are constructed for articulation of the wing. They possess both anterior and posterior notal processes, to which the first and third axillary sclerites, respectively, are attached. Further details of the wing articulation are given in Section 4.3.2.

The originally membranous pleura have been strengthened to varying degrees by scle-rotization and the formation of internal cuticular ridges. In some apterygotes, for example, two small, crescent-shaped pleural sclerites may be seen above the coxa, though the rest of the pleuron is membranous. In the prothorax of Plecoptera there are likewise two sclerites, but these are much larger than those of apterygotes and occupy more than half the pleural area. In the thoracic segments of all other pterygotes the pleura are fully sclerotized and are

Wing Bearing Segment
FIGURE 3.18. (A) Dorsal view of a generalized alinotum; and (B) lateral view of a typical wing-bearing segment. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

additionally strengthened by the formation of an internal pleural ridge that extends dorsally into the pleural wing process (Figure 3.18B). Articulating with this process is the second axillary sclerite. Each pleural ridge is extended inwardly as a pleural arm (Figure 3.19) that meets and may fuse with similar apophyses from the sternum. The pleural ridge is seen externally as the pleural sulcus (Figure 3.18B) above the coxa. This groove divides the pleuron into an anterior episternum and posterior epimeron. Often these sclerites are divided secondarily into dorsal and ventral areas, the supraepisternum and infraepisternum, and supraepimeron and infraepimeron. Derived from the episternum and epimeron and appearing above them usually as distinct, articulated sclerites are the basalar and subalar, to which important wing muscles are attached.

In the thorax the acrosternite of the typical secondary segment forms an independent intersegmental plate or intersternite. The intersternites, which are found between the pro-and mesothorax and between the meso- and metathorax, are known as spinasterna because each bears an internal spine to which a few ventral muscles are attached. Frequently, the spinasterna fuse with the segmental plate, eusternum, of the preceding segment. The euster-num is a composite structure, comprising the primary sternal plate and the sternopleurite. The eusternum may be divided secondarily into an anterior basisternum and posterior ster-nellum by the sternacostal sulcus (Figure 3.20). The latter is the result of an invagination to form the sternacosta, aridge of cuticle that unites the sternal apophyses (Figure 3.19). In the higher pterygotes these apophyses are borne on a median internal ridge and form a Y-shaped furca (Figure 3.19). As noted earlier, these apophyses combine with the pleural arms to form a rigid internal support. The latter provides attachment for the major longitudinal ventral muscles and certain muscles of the leg.

It must be emphasized that many variations occur from the rather general description of a thoracic segment provided above. In all insects the prothorax is modified by the development

Furca Insect

FIGURE 3.19. Diagrammatic cross-sections of the thorax to show the endoskeleton. (A) Normal condition; and (B) condition when furca present. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

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