328 STEPHEN G. RICH 



SURVEY OF THE FORMS OF RECTUM 

 I. FAMILY AESCHNIDAF 



A. Subfamily Cordulegasterinae 



In this subfamily there is only one genus, represented in the 

 Cayuga fauna by the single species Cordulegaster diastatops, 

 described in detail in the preceding section (figs. 1 to 6). 



B. Subfamily Petalurinae 



There is no representative of this subfamily in the Cayuga 

 fauna. I am as yet unable to obtain material for study. 



C. Subfamily Aeschninae 



Of this subfamil}^ we have four genera whose nymphs are com- 

 mon: Aeschna, Basiaeschna, Boyeria, and Anax. The recta of 

 these four genera which I have studied are not distinguishable 

 in an}' external feature. It is necessary to open them to find any 

 points of difference. The rectum possesses an arrangement of 

 gill bases, a tracheation, and a general appearance identical in 

 all four. Therefore, I describe the external appearance of the 

 rectum in but one of them — Aeschna. 



Figure 8 shows the general appearance of the rectum of Aeschna 

 constricta, viewed from the dorsal side. The tracheal branches 

 which leave the dorsal trunks are not grouped or fused at the 

 base as in Cordulegaster, but are evenly spaced or nearly so. 

 Each of these primary branches, which vary in number from ten 

 to thirteen, passes to two or three gill bases of each of two single 

 rows. The most caudal branch before the postdorsal trachea 

 passes to four or five gill bases. The postdorsal is as in Cor- 

 dulegaster. 



There are about twenty to twenty-four gill bases to each 

 single row. They are arranged, as in Cordulegaster, in three 

 dorsal and three ventral double rows, but they are not fused at 

 the center of the row, although in some specimens they are much 

 closer than indicated in the figures. The chief difference be- 



