ANATOMY AND PHYSIOLOGY 47 
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 
H 
: Fic. 56. 
in legless (apodous) larve the ay 
thoracic and abdominal segments es sl ps 
ven eee 
are alike; when legs are present, but ; 
no wings, the thoracic segments are 
somewhat enlarged; and when pre] 
wings occur, the size of a wing- 
bearing segment depends on the vol- 
ume of the wing muscles, which in 
turn 1s proportionate to the size of 
the wings. When wings are absent  epimeron; es, episternum; p, 
: = i prescutum; pr, parapteron; ps, 
(as in Thysanura and Collembola ) postscutellum; S, scutum; sl, 
or the two pairs equal in area (as scutellum; st, sternum.—After 
; a : CoMSTOCK. 
in Termitide, Odonata, Trichoptera 
and most Lepidoptera) the meso- and metathorax are equal. If 
the fore wings exceed the hind ones (Ephemeridz, Hymenop- 
tera) the mesothorax is proportionately larger than the meta- 
Diagram of the principal scle- 
rites of a thoracic segment. em, 
thorax; as also in Diptera, where no hind wings occur. If 
the fore wings are small (Coleoptera) or almost absent (Sty- 
lopide) 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 notwm, 
or tergum, the ventral wall, the sternim; and each lateral wall, 
a plewron; 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- 
