196 THE RESPIRATORY SYSTEM [CH. 



is lost, except for a slight basal thickening, edged by a short row 

 of minute spines, the gill may be termed denodate. The very 

 narrow, sharply pointed denodate gill of Argiocnemis is clearly 

 a step onwards in reduction from the type seen in Ischnura. The 

 gills of Pyrrhosoma, Agrion mercuriale, and possibly Platycnemis 

 appear to be denodate. 



(vi) The form of gill found in Mecistogaster (Pseudostigmatinae) 

 is very peculiar [31]. The basal portion is stalk-like, the distal 

 foliate and pointed. Possibly the pointed gill of Platycnemis is 

 related to this form. 



(vii) The gills in the Argiini appear to be simple, and either 

 triquetral or saccoid. They seem to be of a more archaic type 

 than those mentioned above, and suggest that the larvae are 

 very sluggish creatures. Some species of Argia have very reduced 

 caudal gills, the anal appendages differing little from the form 

 seen in the Anisoptera. 



Types (i) to (v) appear to form a phylogenetic series. A more 

 complete study of these organs might help us towards a natural 

 classification of the Agrioninae. They suggest, at any rate, that 

 the tribal divisions at present adopted are not natural, but simply 

 aggregations of a number of convergent forms. 



Arrangement of the Main Tracheae. In the typical median 

 lamella (fig. 85) there are two main 

 tracheae or stems running close 

 together beneath the mid-rib, which 

 is more or less unsymmetrically 

 placed. In the lateral lamellae, 

 only a single trunk trachea (the 

 lower anal branch of the dorsal 



trunk) enters the base of the gill. Fig. 85. Arrangement of main gill 

 T ,r A j.1.- j- -j i tracheae in an Aqrionid larva, 



In the Agnomnae this divides into seen from left side> DT left> 



two almost at Once. In the median DT' right dorsal trunk ; II lateral, 

 . , Im median caudal gill; 9, 10 ab- 



gill, the base is broader transversely, dominal segments. Original. 



and receives both upper anal branches 



of the dorsal trunk, which run close together under the mid-rib. 

 Thus each lateral gill is only connected with the dorsal trunk of its 

 own side, whereas the median gill is connected equally with both. 

 The bifurcations of the dorsal trunks occur in the ninth segment. 



