THE GILL-CHAMBER OF DRAGONFLY NYMPHS 329 



tween the tracheation of the gill bases of this form and of Cor- 

 dulegaster is that here each base receives branches from dif- 

 ferent primary branches or their divisions, while in Cordule- 

 gaster the pair of branches to each base arises from the divi- 

 sions of only one prhnary branch. 



The tracheation of the ventral side is similar to that of the 

 rectum of Gomphus descriptus (fig. 16). The gill bases are as 

 in figure 8, and the large number is accommodated by each 

 branch dividing once more than in Gomphus. There is the same 

 even spacing of branches as on the dorsal side. The post- 

 ventral is much less prominent than in Cordulegaster, and in 

 the Gomphinae reaches its smallest extent. 



Within the rectum of Aeschna constricta is the same type of 

 longitudinal and buttress folds as in Cordulegaster, but with 

 slight change of form. They are rounded and lack the nicks 

 separating them from the main fold. They are as tall as the 

 main fold, and in some few cases a trifle taller. Near the ends 

 of the rectum they become less prominent. Figure 9 shows a 

 typical portion of the folds, with cushions at the base on both 

 sides, as in Cordulegaster, fat indicated and tracheation shown. 

 There are six folds running longitudinally, and twenty-four more 

 or less buttress folds on each side of each. The folds are in 

 three dorsal and three ventral double rows, exactly as in Cor- 

 dulegaster, and homologous therewith. Ris ('13) found folds 

 like these buttress folds in Aeschna cyanea. 



In Basiaeschna Janata (fig. 10) the buttress folds have taken a 

 new shape. They are narrowed at the base, and the cephalo- 

 distal corner is prolonged into a sharp angle. The cushions are 

 here somewhat twisted around and upwards. Otherwise the 

 structure, in every respect, is as in Aeschna. 



Boyeria vinosa (fig. 11) has a somewhat different form of 

 buttress folds. The fat comes further up into it. The cushions 

 are twisted as in the last form, but the fold itself is rounded along 

 the top edge and bears distally a slightly recurved and down- 

 pointing apfex. It is also more wrinkled as it lies in the rectum 

 than are the forms hitherto described. It is distinctly lower at 

 its inner edge than is the longitudinal fold and shows a slight 

 notch at the point of junction with the longitudinal fold. 



