THE GILL-CHAMBER OF DRAGONFLY NYMPHS 325 



are called 'epithelial cushions' by Sadones, and the name will 

 fit all genera, although originally devised for Libellula, in which 

 these cushions are very much specialized. Their histology is 

 figured and described by Sadones: they consist of elongate cells, 

 fibrillar in structure, beneath the usual thin cuticle. I have in- 

 dicated also how far up into the fold the fatty tissue reaches. 

 In the main fold the fatty tissue is entirely in the very base of 

 the fold. The cavity within the fold and the spaces among the 

 the fat-cells communicate with the haemocoele, as Sadones ('95) 

 has shown. 



Most important is the tracheation of the folds. Each gill 

 base — the base of each buttress fold — receives two tracheal 

 branches, one at each end. The one at the inner end passes 

 up at the point where the buttress fold joins the main fold. The 

 buttress folds are arranged alternately on each side of the main 

 fold which thus receives alternate tracheae from each side. The 

 tracheae which pass up into this point of junction of folds divide 

 into two parts, one of which ramifies out into the main fold, the 

 other of which does likewise in the buttress fold. The other 

 tracheal branch which reaches the gill base branches out into the 

 periphery of the buttress fold. As all these branches approach 

 the edges of the folds, they continue to divide into fine tracheoles, 

 with which the edges are filled. The edges of the folds are thus 

 the parts functional in respiration. 



These fine tracheoles are actually imbedded in the epithelium 

 of the fold, as Sadones has shown for two other genera. Con- 

 trary to Mackloskie ('83), they do not end blindly, but anasto- 

 mose and form actual loops. Each ultimate tracheole bends 

 around and loops back into another branch, either from the same 

 tracheal branch or from another. The fringe of loops is continu- 

 ous along the edge of each fold, irrespective of whence the loop 

 arises. This is indicated in figure 5 and shown in more detail 

 in figure 6. 



The edge of the folds appear serrulate on first view, but turns 

 out under higher magnification to be set with chitinized thicken- 

 ings (fig. 6). The main fold is of uniform height, and slightly 

 waved or plaited (fig. 4). There is a sudden depression at the 



JOURNAL OF MORPHOLOGY, VOL. 31, NO. 2 



