6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



the anterior phragma of the mesotergum (iPh) run continuously 

 through the prothorax and the neck (Cv) to be inserted on the post- 

 occipital ridge of the head (PoR). This ridge, as the writer has else- 

 where contended (1928), is evidently the intersegmental fold between 

 the first and second maxillary segments. The neck, therefore, must be 

 derived from the posterior part of the second maxillary, or labial, 

 segment and from the anterior part of the prothorax, there being no 

 satisfactory evidence of the existence of a separate neck segment. 

 If so, the first postcephalic intersegmental line, or that between the 

 labial and prothoracic segments (fig. 4, ilsg), must be contained in the 

 membranized cervical region, where the protergal costa is lost. By the 

 suppression of the primary intersegmental line between the head and 



VMcl 



Fig. 3. — Diagram of the body segmentation of an insect, and the primitive 

 relation of the longitudinal muscles to the definitive segmental plates of the body 

 and to the head ; show^ing the reversed overlapping of the sterna betvi^een the 

 thoracic and abdominal regions. 



Cv, cervix ; DMcl, dorsal longitudinal muscles ; H, head ; IS, first abdominal 

 sternum; IT, first abdominal tergum ; Ppt, periproct, or terminal segment; 

 5"i, S2, Sz, thoracic sterna ; Ti, T2, Tz, thoracic terga ; VMcl, ventral longitudinal 

 muscles ; XI, eleventh abdominal segment. 



the thorax, giving continuity to the muscle fibers of two segments, 

 the head acquires a much greater freedom of motion than it could 

 have if it were attached to the body by an ordinary intersegmental 

 membranous ring. 



The loss of the protergal antecosta deprives the prothorax of the 

 possibility of being a wing-moving segment, and there is nothing to 

 suggest that the prothorax ever possessed movable organs of flight. 

 The reduction of the primitive gnathal region of the body and its con- 

 densation into the head capsule, accompanying the transfer of the 

 gnathal appendages to the head, shifted the center of gravity pos- 

 teriorly in the insect's body, and the paranotal lobes of the second and 

 third thoracic segments were developed into movable wings, leaving 

 the prothorax as a free segment between the head and the pterothorax. 



The most conspicuous modifications of the thoracic terga occur in 

 the mesothorax and the metathorax of winged insects, where clearly 



