1918. 



NATURAL SCIENCES OF PHILADELPHIA. 



79 



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Fig. 2. Diasnim of tracheation of the hind-gut of larva of Argia moesta 

 pidrida, reconstructed from sections. To avoid confusion the trachese are 

 represented as lying farther to right and to left from the gut and from each 

 other than is actually the case (compare Fig. 1). Of the tracheae which supply 

 the gut walls, those which run on the dorsal surface or enter the dorsal longi- 

 tudinal fold are shown in solid lines; those which run on the ventral .surface 

 or enter the lateral folds are shown in broken lines. The tracheae forming 

 the ventral anastomosis are shown by alternating dots and dashes. The Rornan 

 numerals on the left side indicate the anterior ends of the respective abdominal 

 segments, the Arabic numerals on the right the beginning of each row of 

 sections on slide 3 of the series; each row comprises about 60 sections. Abbrevi- 

 ations as in Fig. 1, and as given in the text. 



