IX] THE RESPIRATORY SYSTEM 191 



developed from the appendix dorsalis, while the lateral caudal gills 

 are placed latero-ventrally, one on either side of the anus, and 

 are developed from the cerci. In a few cases (some species of 

 Argia, and some Hawaiian species of Agrion) the gills are much 

 reduced, and somewhat resemble the appendages of Anisoptera. 



It is not possible yet to determine definitely the actual value 

 of these organs for respiration. Each is connected with its base 

 (i.e. tergite or sternite of segment 11) by a definite breaking-joint, 

 so that it is easily cast off if seized by an enemy. The larvae 

 frequently lose one or more of their gills, and continue to live 

 without them until the next ecdysis, when they are reproduced 

 in a shorter form. It seems quite clear that the larvae do possess 

 an auxiliary means of breathing in water, viz. by means of rectal 

 respiration. There is, however, no development of true rectal 

 tracheal gills of the kind seen in the Anisoptera, and hence the 

 rectum must be a much less efficient organ of respiration than 

 in that suborder. 



In the young larva, the gills are filamentous and hairy. They 

 soon acquire a triquetral form (i.e. triangular in cross-section). 

 This triquetral form is retained in a few larvae, with slight modifi- 

 cations, throughout larval life. In most cases, however, it either 

 tends to swell up into a saccus, or to become flattened into a lamella. 

 The histology of all these forms is essentially similar ; their external 

 forms, and the distribution of their tracheae, vary very greatly. 



Further, the gill may be from the first either a simple complete 

 whole (simple gill) or it may have an extra distal joint (constricted 

 gill). Gills developed along the latter lines shew a very definite 

 constriction into two parts, in all except the most highly developed 

 forms. These tend to obliterate the constriction, which may 

 become reduced to a node (nodate form), i.e. simply the remains 

 of the original joint on one side of the lamella ; or the node may 

 be lost, and its original position only marked by a spine, or by 

 a difference in the consistency of the basal and distal portions 

 of the gill. Also, it is possible for the whole distal part to be 

 lost, so that an originally constricted gill becomes secondarily 

 a simple gill. Such cases can only be determined by careful 

 ontogenetic studies. 



