NO. II LEGS OF PRIMITIVE ARTHROPODS EWING 35 



by Berlese for various minute arthropods, and by Walch for mite 

 larvae to excellent advantage, also by several other workers. Speci- 

 mens stained with acid fuchsin after proper caustic and other treat- 

 ment show the same conditions. 



The pleural regions of the thorax in Protura are not bare and 

 membranous as in Pauropns and certain symphylids. but are almost 

 completely covered by the downward continuation of the tergal plates 

 (pi. 12, fig. 37). These may actually reach to the sternites. 



In the Collembola (pi. lo, fig. 31) the pleural regions of the thorax 

 are very similar to those of Protura. Pleural plates appear to be 

 absent. Crampton (1926) has figured several thoracic pleural plates 

 for coUembolans. The writer has not verified his results, but in 

 Isoioma finds only the subcoxal plates. The pleural region of the 

 prothorax in Isotoma is membranous, but that of the mesothorax 

 and metathorax is covered by the downward continuations of the 

 large tergal plates, which actually reach to the subcoxae of the legs. 



THE ORIGIN OF INSECT PLEURITES 



Snodgrass (1927) has presented an excellent exposition of the 

 possible method of the origin of the thoracic pleurites from the sub- 

 coxae of the legs. Those desiring a summary of this theory are re- 

 ferred to his paper. The evidence presented in the writer's investi- 

 gations in general strongly supports the theory of subcoxal origin. 

 However, the writer is inclined to hold the view that the first process 

 toward the production of the eupleural sclerite and the trochantin — 

 fundamental elements found in many generalized insects — was the 

 formation of a pseudo joint in the primitive subcoxa. A divided, 

 functional and almost complete subcoxal segment is found today in 

 certain coUembolans (pi. 10, fig. 31) and in certain thysanurans 

 (pi. 5, fig. 14). As such subcoxal segments became reduced, they 

 probably lost their cylindrical shape by their inner parts being ob- 

 literated, which would leave them crescentic ; and at the same time 

 the chitinous part of their outer walls became reduced forming 

 crescentic plates. Such plates exist today in Japyx and certain other 

 thysanurans; are found fused in certain springtails (pi. 10, fig. 31) 

 and are particularly characteristic of the Protura (pi. 12, fig. 39). 



Some would evolve the trochantin from a primitive sclerite which 

 had both a dorsal and a ventral condylic articulation with the coxa. 

 In fact Prell (1913) holds that the Protura have such a primitive 

 trochantin. In this contention, however, there may be an error. Ac- 

 cording to the writer's observations the ventral articulation of the 



