THE GILL-CHAMBER OF DRAGONFLY NYMPHS 337 



A. The Brayichial Chamber of Platheinis lydia (figs. 24 to 27) 



Scott ('05), studying the general tracheation of this nyniph, 

 described the whole respiratory system, but did not go into 

 certain details of the rectum as far as necessary for this study. 



Opening this short, stubby nymph discloses in the abdomen a 

 brush of tracheae much closer together than those of Cordule- 

 gaster. The branches from the dorsal and ventral trunks alike 

 are crowded close together and the trunks taper abruptly at the 

 point where the branches arise. Eleven branches are given off 

 from the trunk to the dorsal row\s of gills, and a similar set to 

 those situated laterally. Caudad of this bunch of branches, 

 each dorsal trunk is free of branches for a space, and then gives 

 off a brach, larger than the others, which passes to seven or eight 

 gills at the caudal end of each of the four dorsalmost single rows. 

 The postdorsal is quite as in the Aeschnidae. The ventral trunk 

 has a similar bare place caudal to a bunch of branches, and then 

 gives ofLa close group of five or six branches and the postventral 

 trachea. 



One feature of the rectum is very different from the aeschnid 

 structure. The double rows of gill bases have apparently shifted 

 in position. Three are to the right, three to the left of the 

 median line. The branches from one side of the dorsal trunk 

 do not supply halves of adjacent rows or share in supplying a 

 median row. The branches from each side of the dorsal trunk 

 and those from each ventral trunk pass to a particular double row 

 of gill bases. Most of the branches divide once before they enter 

 the bases, and the halves of a base are supplied from different 

 branches. Figure 23, a teased preparation from an allied genus, 

 shows this last feature. Figures 24 and 25 show the ventral 

 and dorsal tracheation and the arrangement of gill bases. Thus 

 the libellulid double rows are in no way homologous with those 

 of the Aeschnidae; the single rows are homologous throughout 

 both families. 



The gill bases stand at an angle of about 40° from transverse, 

 pointing caudad rather than cephalad, owing to the position of 

 the double rows and the gills in them. The muscles of the rec- 



