346 STEPHEN G. RICH 



and three on the right and three on the left wall in the Libellu- 

 lidae. In either case they are evenly spaced around the rec- 

 tum. The single rows are all homologous, but the grouping of 

 them in double rows is different in the two families. 



2. The rectum receives numerous branches from the dorsal 

 and ventral tracheal trunks: those from the dorsal trunks supply 

 the dorsal two-thirds of the rectum; those from the ventral 

 trunks, the ventral third only. 



3. In the Aeschnidae there is an anal canal extending caudad 

 of the rectum. 



4. Each half of one of the six longitudinal rows of respira- 

 tory structures consists of a series of twelve to thirty gills or 

 folds, which may bear villi distally. In the base of each gill or 

 fold is a mass of fat, and on the surface of it one or more thick- 

 ened epithelial cushions; the distal end of the gill or fold or 

 villus upon this is filled with loops of tracheae and is the part 

 functional in respiration. Each gill or fold receives two tracheal 

 branches. 



5. In the Aeschninae, Cordulegaster, and Lanthus there is 

 also a tracheae longitudinal fold extending between the closely 

 adjacent halves of each row of respiratory structures. 



6. In the Gomphinae (except Lanthus) and in Anax, the gills 

 bear villi along the edge, and these are the respiratory structures. 

 The number of gills in these forms is at its lowest. 



7. In the Libellulidae there are no villi and no longitudinal 

 folds, and the longitudinal rows are arranged in pairs in positions 

 intermediate between those they occupy in the Aeschnidae. 

 Each half row consists of about thirty linguiform flattened gills, 

 extending lengthwise of the rectum, or nearly so, and separate 

 from each other. There are no epithelial cushions on the 

 cephalic face of the gills, and the fat is restricted to the most 

 caudal angle of the gill. 



8. The tracheae which pass to the rectum are evenly spaced 

 along the trunks in the Aeschnidae, except in Cordulegaster, 

 where there are four large branches which divide at once. In the 

 Libellulidae the branches arise very close together and distinct 

 from the tracheae which continue the trunks caudad. The 



