l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



through the abdomen and thorax. In the abdomen, except in the first 

 segment, the fibers are attached to antecostal ridges of the sterna, and 

 in the thorax to the anterior margins of the sternal plates, except in 

 the prothorax where they are inserted anteriorly on the head. Here 

 is unquestionably a uniformly primitive condition of the ventral 

 musculature. It is probably of no phylogenetic significance that, an- 

 terior to the fifth abdominal segment, the fibers of these muscles are 

 of the length of two segments and are attached to alternate sterna. 

 The sternal chitinization of each thoracic segment of the Protura, 

 according to Berlese (1910) and Prell (1913), consists of two plates 

 (fig. 8), one lying before the bases of the legs, the other between and 

 behind them. In the mesothorax and metathorax the second sternal 

 plate bears a median apodemal ridge, which in the Eosentomidae is 

 forked anteriorly and has the form of a Y with the arms extending 

 toward the bases of the coxae. 



In Japyx the principal part of the sternal region in each thoracic 

 segment consists of a large quadrate plate conspicuously marked 

 externally by the lines of a Y-shaped ridge on the inner surface 

 (fig. 6 B, y). The arms of the ridge extend outward and forward to 

 the bases of the legs, where each becomes continuous with a basal 

 ridge of the coxa and constitutes a sternal coxal articulation. The 

 anterior part of each sternmii consists of a semimembranous area 

 (fig. 6 B, ^) indistinctly separated from the rest, and bearing four 

 prominent setae in a transverse row. Before this area there are two 

 well-marked sternal folds {i, j) that appear as replicas of it, each 

 bearing likewise four setae similarly placed. A single fold with four 

 setae occurs between the metasternum and the first abdominal sternum 

 {IS). These sternal folds, the stenial " apotomes " of Enderlein 

 (1907), the " intersternites " of Crampton (1926), are usually re- 

 garded as intersegmental, but that they belong to the sternum follow- 

 ing is shown by the fact that the anterior margin of the first one of 

 each thoracic set, as seen in side view (fig. 10, i), coincides with the 

 line of the antecostal suture of the tergum {ac) of the same segment, 

 A striking feature of the sternal structure in the thorax of Japyx is 

 the reversed overlapping of the sternal plates, the posterior edge of 

 each principal plate being covered externally by the anterior fold of 

 the sternum following. The posterior ends of the median ridges of 

 the sternal apodemes project as free processes into the body cavity. 

 There are no continuous bands of longitudinal muscle fibers in the 

 thorax of Japyx, as there are in the Protura. Grassi (1886) says, in 

 both Japyx and Campodca the longitudinal ventral muscles of the 

 thorax are not recognizable with certainty. In this respect these 



