26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



The pleural pattern of Lepisma is surprisingly similar to that of 

 Lithobius (fig. 9), and is one also that conforms with the pleural 

 pattern in Eosentomon (fig. 8), except that the eupleural series of 

 plates is represented by a single sclerite in Lepisma (fig. 11, r). It is 

 not implied, however, that Lithobius and Lepisma have any immediate 

 relationship, or that either is descended from Eosentomon, but that 

 through the disintegration of a primitive chitinization in the pleural 

 region of a common ancestor, there have resulted the pleural patterns 

 of Eosentomon, Lithobius and Lepisma. 



The pleuron of Machilis has little resemblance to that of the other 

 Apterygota. It consists of a single, small, triangular plate (fig. 12, t) 



Fig. 12. — Left mesothoracic leg and pleural sclerite of Machilis, anterior view. 



Cx, coxa ; Sty, stylus ; t, pleural sclerite, Tar, tarsus ; u, apodeme of pleural 

 sclerite. 



closely attached to the base of the coxa, but extending dorsally to the 

 base of the lateral fold of the tergum. Many writers, probably follow- 

 ing Hansen (1893), designate this sclerite the trochantin (or sub- 

 coxa), and its close connection with the coxa would suggest its tro- 

 chantinal nature. Its basal angles are continued into a fold that sur- 

 rounds the base of the coxa; its triangular lateral surface is marked 

 by a vertical groove in which a deep invagination forms a long, 

 slender, internal arm {11) to which is attached a muscle from the 

 tergum. Crampton (1926) assumes that the plate belongs to the 

 eupleural series, and that its areas before and behind its external 

 suture are equivalent to the episternum and epimeron of pterygote 

 insects. It is suggested by Prell that the median plate in the eupleural 

 series of Eosentomon is likewise the common fundament of the 



