34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



PLEURITES IN LEG-BEARING SEGMENTS 



The method of origin of the pleurites of insects has been an in- 

 teresting subject for speculation to students of the phylogeny of 

 arthropods. The theory put forth that they represent the residual 

 chitinous elements of a subcoxal segment of the leg present in ances- 

 tors of insects has been recently supported and reviewed by Snodgrass 

 (1927). 



Although most attention has been focused in the past on the pleural 

 region in the study of more primitive types of crustaceans and in the 

 study of millipeds, it would appear that the evidence found either 

 for or against this theory in pauropods and symphylids would be 

 even of more importance since these arthropods are more nearly 

 related to insects. 



In Pauropus (pi. 8, fig. 25) it is observed that the pleural region 

 is entirely membranous, there being no pleural plates of any kind. 

 The tergal plates in this genus are curved down laterally but a 

 short distance, leaving a large unchitinized area. The need of plates 

 in this region is met in a way by long dorsal apodemes, one of which 

 passes upward in the body wall from each basal leg segment. 



In Symphyla similar conditions are found but in this class the 

 coxae are supported and hinged to a subcoxal plate. In Symphylella 

 (pi. 12, fig. 36) the pleural regions are bare and membranous as in 

 Pauropus. In Hanseniella (pi. 11, fig. 35) the subcoxal plate is not 

 differentiated' laterally, so that the pleural region becomes well chiti- 

 nized ventrally and anteriorly but passes gradually to a membranous 

 state at the posterior margin of the body segment. In Scutigerella 

 (pi. II, fig. 34) a definite pleural plate is developed. Dorsal to this 

 plate the body wall is membranous, ventrally it is chitinized. There 

 is little evidence to show how this plate originated or what it 

 represents. 



The pleural region of proturans has been investigated by a number 

 of entomologists. The so-called subcoxal plates have already been 

 discussed (page 34). In addition to these plates others have been 

 described (Prell, 1913; Crampton, 1926). The writer must confess 

 that he has never found such additional plates. Slides cleared in 

 potassium hydroxide and mounted either in balsam or glycerine show 

 only the two crescentic sclerites represented in plate 12, figures 37 

 and 39. The same is true of specimens mounted in glycerine jelly, 

 of specimens mounted in the glycerine + chloral hydrate + gum 

 arabic + water mixture. This latter mixture is one of the very best 

 of media for observing such structures. It has been extensively used 



