lO 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



any tergum usually forms a transverse fold, or posterior reduplica- 

 tion (fig, 3 B, Rd), that of the protergum sometimes widely over- 

 lapping the mesothorax. 



The sterna of the thoracic segments of insects never have the 

 structure typical of segmental plates, such as that of the more simple 

 thoracic terga, or that of the abdominal sterna of insects, or the 

 sterna of the leg-bearing segments of the Chilopoda. The thoracic 

 sterna have been modified, apparently since an early stage in the evo- 

 lution of insects, through an alteration in the segmental relations of 



Pc ac 



Fig. 3. — Ground-plan of a wingless thoracic segment, showing the usual or 

 typical structure in pterygote insects. 



A, external lateral view, left side ; B, internal view of right side. Ac, ante- 

 costa; ac, antecostal suture; Acs, precoxal bridge; a, articulation of trochantin 

 with coxa ; Bs, basisternum ; CxP, pleural coxal process ; Epm, epimeron ; Eps. 

 episternum ; Fs, furcisternum ; Pc, precosta ; Pcx, postcoxal bridge ; PI A, pleural 

 arm, or apophysis ; PIR, pleural ridge ; PIS, pleural suture ; PS, poststernum 

 S, sternum; SA, sternal arm, or apophysis; .fa, external root of sternal 

 apophysis; Spn, spina; T, tergum; Tn. trochantin. 



the ventral muscles. In the thorax, these muscles are attached not 

 to anterior ridges of the sternal plates, but to processes arising from 

 the posterior parts of the sterna or from intersternal folds or chiti- 

 nizations of the integument. 



It is difficult to select, among the numerous variations in form of 

 the ventral thoracic plates, a structure that may be regarded as 

 " typical " of a thoracic sternum. The most constant sternal land- 

 mark consists of a pair of entosternal arms arising either indepen- 

 dently from the region betw^een the bases of the coxae, where their 

 roots are marked externally by two pits in the cuticula, or jointly 

 from a common median base. Since the second condition is the more 



