ANATOMY AND PHYSIOLOGY 



47 



Diagram of the principal scle- 

 rites of a thoracic segment, em, 

 epimeron; es, episternum; p, 

 prsescutum; pr, parapteron; ps, 

 postscutellum; s, scutum; si, 

 scutellum ; st, sternum. After 

 COMSTOCK. 



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

 thorax may bear wings. 



The differentiation of the thorax as a distinct region is an 

 incidental result of the development of the organs of locomo- 

 tion, particularly the wings. Thus 

 in legless (apodons) larvae the 

 thoracic and abdominal segments 

 are alike ; when legs are present, but 

 no wings, the thoracic segments are 

 somewhat enlarged; and when 

 wings occur, the size of a wing- 

 bearing segment depends on the vol- 

 ume 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 Termitidse, 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 meta- 

 thorax; as also in Diptera, where no hind wings occur. If 

 the fore wings are small (Coleoptera) or almost absent (Sty- 

 lopidae) the mesothorax is correspondingly smaller than the 

 metathorax. The prothorax, which never bears wings, may 

 be enlarged dorsally to form a protective shield, as in Orthop- 

 tera, Hemiptera 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 notuin, 

 or tergum; the ventral wall, the sternum; and each lateral wall, 

 a pleuron; the restriction of these terms to particular segments 

 of the thorax being indicated by the prefixes pro-, meso- or 

 meta-. These parts are usually divided by sutures into dis- 



