THE GILL-CHAMBER OF DRAGONFLY NYMPHS 333 



From this peculiar form, which would seem to be the climax 

 of the series comprising most of the Aeschninae, we can turn to 

 those more typical of the Gomphinae and allied in rectal struc- 

 ture to Anax. Four forms are here to be discussed. 



In Hagenius brevistylis (figs. 17 and 18) the tracheation is 

 intermediate between the few large dividing branches of Cor- 

 dulegaster and the perfectly even series of branches of Aeschna. 

 Each of the primary branches divides less often than in the 

 forms thus far discussed, since each row has only twelve to four- 

 teen gills. The gill bases are arranged as in the Aeschninae. 

 Figure 7 shows the musculature of this species, which is identical 

 with that of all other anisopteran recta. The anal canal is more 

 marked than in the Aeschninae and Cordulegaster, and, as in 

 all Gomphinae, is a definite tube supplied by a branch from each 

 postdorsal trachea. 



Within the rectum is a set of gills much like those of Anax. 

 The single rows are not so definitely paired, and I was unable 

 to find any remnant of the longitudinal fold connecting the gill 

 bases. Each gill base, set at the same angle as in Anax, is 

 oblong in shape, with the longer edge as the base. The ends 

 are slightly diminished, and the gill base is triangular in cross 

 section, with a sharp edge. On this edge are eleven or twelve 

 villi. The villi are twice as long as in Anax, and all of the 

 tracheal loops are in their tips; the villi are covered with setules 

 (fig. 18). In the base of each gill is a mass of fat, and an elon- 

 gate cushion occupies the greater part of each side of the base. 

 The villi are on the crest of the base. The tracheation of each 

 gill is from one primary or secondary branch, as in Cordule- 

 gaster; the same arrangement of two branches entering a gill is 

 found here as in other nymphs. 



Ophiogomphus carolus (figs. 19 to 21) show more typical 

 Gomphine features in its rectum. The tracheation follows the 

 aeschnine pattern, and is not to be distinguished from that of 

 Gomphus (fig. 16). The gill bases are larger than in any form 

 hitherto described, and are no longer in alternation but sub- 

 opposite in adjacent rows. They are oval in outline and form 

 twelve evenly spaced single rows, set at the angle of the aesch. 



