THE GILL-CHAMBER OF DRAGONFLY NYMPHS 339 



B. Subfamily Macromiinae 



In this subfamily the only species available was Didymops 

 transversa. In all features, save the shape of the gills, this 

 agreed with the structure in Plathemis. Figure 29 shows the 

 gill; it is sharp at the tip, the point being directed caudad. 

 There are no buttons on the gills, but a ridge on the caudal side 

 leads down from a bulbous projection near the tip of the gill. 

 There are thirty gills in each single row. Tracheation, gill place- 

 ment, fat, cushions, are all as in Plathemis. 



C. Subfamily Cordulinae 



The forms in this subfamily also show little difference from 

 Plathemis or from each other. 



In Dorocordulia libera (figs. 32 and 33) the tracheation is 

 almost identical with that of Plathemis, but is somewhat less 

 concentrated ventrally. The gills number thirty to a row. 

 Figure 32, a typical gill; figure 33 is one of the six most caudal 

 gills of each row. The gills at the caudal end are twice as high 

 and wide as those at the cephalic end of the rectum; the sizes 

 grade evenly along the rectum. 



In Tetragoneuria cynosura the external features of the rec- 

 tum are not distinguishable in any way from those of Plathemis. 

 The interior of the rectum is also very similar to that of Plathe- 

 mis, save that the button on the caudal edge of the gill is absent 

 and the gills are slightly truncate at the tip. The most caudal 

 five gills are a little smaller than the others, which are all alike 

 in size. 



The rectum of Helocordulia uhleri agrees remarkably with 

 that of Dorocordulia libera in most respects, the only difference 

 being that all of the gills are of the same shape as the most caudal 

 of Dorocordulia. They grade in size, the most cephalic ones being 

 slightly the smaller. 



Miss Poletajev ('80) figures for a species of Epitheca some 

 gills which do not agree with any cordulids that I have seen. 

 They are shown as obovate plates, fastened at a point. Ris 

 ('13) describes gills in Cordulia aenea identical with those in 

 Platheznis. 



