\\ATOMY AND PHYSIOLOGY 37 



Thorax. The thorax, or middle region, comprises the three segments 

 next behind the head, which are termed, respectively, pro-, meso- and 

 metathorax. In aculeate Hymenoptera, however, the thoracic mass in- 

 cludes also the first abdominal segment, then known as the propodeum, or 

 median segment. Each of the three thoracic segments bears a pair of 

 legs in almost all adult insects, but only the meso- and metathorax may 

 bear wings. 



The differentiation of the thorax as a distinct region is an incidental 

 result of the development of the organs of locomotion, particularly the 

 wings. Thus in legless (apodous) larvae the thoracic and abdominal seg- 

 ments are alike; when legs are present, but no wings, the thoracic seg- 

 ments are somewhat enlarged ; and when wings occur, the size of a wing- 

 bearing segment depends on the volume of the wing muscles, which in 

 turn is proportionate to the size of the wings. When wings are absent 

 (as in Thysanura and Collembola) or the two 

 pairs equal in area (as in Termitidae, Odonata, 

 Trichoptera and most Lepidoptera) the meso- 

 and metathorax are equal. If the fore wings 

 exceed the hind ones (Ephemeridae, Hymenop- 

 tera) the mesothorax is proportionately larger 

 than the metathorax; as also in Diptera, where 



no hind wings occur. If the fore wings are 



FIG. 56. Diagram of the 



small (Coleoptera) or almost absent (Stylopidae) principal sclerites of a thoracic 

 the mesothorax is correspondingly smaller than 



the metathorax. The prothorax, which never parapteron; ps, postscutellum; 



s, scutum; si, scutellum; st, 



bears wings, may be enlarged dorsally to torm sternum. After COMSTOCK. 

 a protective shield, as in Orthoptera, Hemip- 



tera and Coleoptera; or, on the contrary, may be greatly reduced, as in 

 Ephemerida, Odonata, Lepidoptera and Hymenoptera. In the primitive 

 Apterygota the prothorax may become reduced (many Collembola) or 

 slightly enlarged (Lepisma). 



The dorsal wall of a thoracic segment is termed the notum, or tergum; 

 the ventral wall, the sternum; and each lateral wall, a pleuron; the re- 

 striction of these terms to particular segments of the thorax being indi- 

 cated by the prefixes pro-, meso- or meta-. These parts are usually divided 

 by sutures into distinct pieces, or sclerites, as represented diagrammati- 

 .cally in Fig. 56. Thus the tergum of a wing-bearing segment is regarded 

 as being composed of four sclerites (tergites, Fig. 57), namely and in order, 

 prcescutum, scutum, scutellum and postscutellum. The scutum and 

 scutellum are commonly evident, but the two other sclerites are usually 



