i6 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



present is a development of the precosta normally attached to the 

 tergum following, and the i>ost-tergal region in the proturan dorsum 

 does not appear to be a part of the small precosta behind it in either 

 the mesothorax or the metathorax. 



In Leplsma the lateral margins of the thoracic terga are somewhat 

 produced above the bases of the legs, in Machilis they form free 

 lobes reaching down on the sides of the segments and overlapping the 



Fig. 6. — Thorax of Japy.v sp., with base of head and of abdomen. 



A, dorsal view, showing larage precostae (Pc) of mesotergum (Ti), metater- 

 gum (T3), and first abdominal tergum (IT). B, ventral view, showing anterior 

 apotomal folds (i,j, k) of sterna, and sutures (y) of Y-shaped ridges of thoracic 

 sterna. 



leg bases, but there is no reason for believing that these tergal exten- 

 sions in the Thysanura have any phylogenetic relation with the tergal 

 lobes from which the wings of pterygote insects are presumed to 

 have evolved. 



In the Pterygota, the terga of the thorax reach their highest degree 

 of development in the second and third segments, where they present 

 numerous specializations fitting them to their functions of supporting 

 the wings and of giving efhcient attachment to the principal muscles 

 that move the wings. The features of the wing-bearing terga, how- 



