22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



ogous sternal apodeme in any of the Apterygota. While, therefore, 

 the general structure of the thoracic sterna has been developed in 

 connection with the specialization of the thorax as the locomotor cen- 

 ter of the insect body, the special structure of the thoracic sterna of 

 winged insects has evolved within the pterygote group, probably as 

 an adaptation to an indirect function in connection with the wings. 

 Though Crampton (1926) distinguishes a basisternum and a furci- 

 sternum in some of the Thysanura, the structures separating the 

 sternal regions so named, such as the Y-shaped ridges of Japyx 

 (fig. 6 B, y) are not to be homologized with the sternal apophyses of 

 the Pterygota. 



THE PLEURON 



The pleuron of a thoracic segment offers a more difficult problem 

 in morphology than does either the tergum or the sternum, and the 

 question of the origin of its sclerites has been the subject of much 

 speculation and discussion. Our prevalent ideas concerning the struc- 

 ture of the pleuron have been derived largely from a study of the 

 pterygote thorax, but, since the pleurites as they occur in a winged 

 insect are certainly not primary elements in the wall of any primitive 

 segment, we see, undoubtedly, in the thoracic pleuron of winged 

 insects a highly specialized structure. A study of the apterygote 

 thorax, therefore, might give more valuable suggestions concerning 

 the basic structure of the pterygote pleuron than are to be had from 

 a knowledge of the pterygote pleuron itself, since the pleural structure 

 of the Apterygota should be less removed from the common ancestral 

 structure than is that of the Pterygota. 



The largest number of pleural sclerites is found in the Protura. In 

 the mesothorax and metathorax of Eosentomon, as described by Prell 

 (1913), there are nine principal plates on each side of the segment 

 between the tergum and the sternum (fig. 8). Four of these {a, h, c, 

 d) are more dorsal than the others and constitute a series of tcrgo- 

 pleurites, according to Prell's interpretation. Four others {e, f, g, h) 

 form a ventral series believed by Prell to represent the true pleural 

 plates of other insects. Since Crampton (1914) has named the 

 corresponding area in Pterygota the enpleuron, the series of plates 

 having an analogous position in Eosentomon and other Apterygota 

 may conveniently be termed the eupleural sclerites. The ninth plate 

 in the general pleural area of Eosentomon (Tn) is a semicircular 

 chitinization over the dorsal half of the coxa (Cx), and this sclerite 

 Prell calls the trochantin, since its anterior part is clearly the homo- 

 logue of the usual trochantinal sclerite of pterygote insects. It is 



