330 STEPHEN G. RICH 



In all three of these forms the bases of the gills are set at the 

 same angle as in Cordulegaster. They point 45° cephalad from 

 a transverse line. In the next form they lie 10° nearer a trans- 

 verse position, but this is not noticeable from the outside of the 

 rectum. 



Anax Junius (figs. 12 to 15) possesses a rectum that is exactly 

 like that of Aeschna or the other two forms of this family, so far 

 as external appearance goes. But internally, in place of the 

 buttress folds and main folds, there are double rows of semi- 

 circular or semilunar gills, each with nine to twelve villi on it; 

 these villi are arranged in two rows, one each side of the crest 

 of the gill. The villi vary in length, but are never longer than the 

 gill is high. Each villus has two or three small setules on its 

 rounded tip. The loops of the tracheae are in the tips of these 

 villi as well as in the edge of the gill proper. Figures 12, 13 

 and 14 show these structures better than much description. 

 Figure 15 is a contour map designed to show the formation of 

 each double row. The longitudinal fold persists as a rounded 

 protuberance, filled with fat, connecting the bases of the gills. 

 I failed to find any tracheae in it. 



In Anax Junius there are usually seventeen or eighteen gills — 

 for such the buttress folds have now become — in each single row. 

 This is the lowest number found in the Aeschninae: Aeschna 

 has twenty-five, Boyeria and Basiaeschna have about twenty- 

 four each, Anax seventeen to nineteen. In one European species 

 of Anax, as shown by Suckow ('28), there are sixteen to a row. 



The structure of each gill corresponds to that- of the buttress 

 folds of other Aeschninae. There is fat in the base, through 

 which the tracheae pass as they branch, and two epithelial 

 cushions, not oblique but transverse in position, are on the base 

 of each gill. 



This genus has great historical interest, as it contains the 

 'Aeschna grandis' of Cuvier, Suckow, Dufour, Roster, and others. 

 Except for detail differences in the placing of setules on the 

 villi, variation in the number of gills to a row up to twenty-four, 

 and variation in the number of villi to a gill, their figures and 

 descriptions aeree closely with what I found in A. Junius. 



