430 



tellum comprises the area caudad of the spring-vein and cephalad of 

 the first abdominal segment. 



Mcsothoracic Spiracles and Mesostigmal Plates (Figs. 41, 43-45, 

 212-216). — The mesothoracic spiracles of Zygoptera are large and 

 have exceedingly large tracheal trunks connected with them. As in 

 the nymph, the spiracles have migrated dorsad and are located near 

 the lateral angles of the mesoprescutum and beneath the projecting 

 caudal margins of the pronotum. Adjacent to the spiracle on two 

 sides, are two heavy plates, the ventral one of which is highly polished 

 (Fig. 45, mstv), allowing the prothorax to play upon it to a certain 

 extent. The caudal plates (mstg) are usually triangular and assume 

 a variety of forms in different species. Both of these plates belong 

 to the peritreme of the spiracle. The caudal plate has been assumed 

 by Snodgrass ('09) to be homologous with the depressed area in 

 Anisoptera which extends across the dorsum just caudad of the pro- 

 notum. A study of the nymphs of Anisoptera proves conclusively 

 that such is not the case, for in the nymph the depressed area may be 

 observed to develop from the mesepisternum. Another possibility in 

 the derivation of the caudal plates is that they have arisen from the 

 mesoprescutum, and the wide depressed area of Anisoptera may also 

 have had the same origin. This is strongly supported bv the apparent 

 disappearance of all traces of the prescutum in the adults. There is, 

 however, a remnant of the prescutum in the adults of Gomphus where 

 the area occupied by the prescutum lies entirely within the transverse 

 depression and the true stigmal plate is closely applied to the stigma. 

 From this it seems that the depressed area of Anisoptera can not be 

 homologous with the spiracular plates of the Zygoptera, but that it 

 must have developed simply from a depression of the mesepisterna. 



Use has been made of the caudal stigmal plates in the classification, 

 especially in the case of the females, of the genus Argia. Kennedy 

 ('02a) and Calvert (Calvert and Hagen, '02 1103) were the first to call 

 attention to these plates in America, but their use was hinted at as long 

 ago as 1865 by de Selys ('65: 381). In the genus Argia the caudo- 

 mesal angles are the variable parts of the sclerites. There is considerable 

 difference also in the plates of females of the Coenagrionidae, and 

 individuals of this sex may often be separated by the use of this 

 character. In the Lestinae and Agrionidae, the character seems to be 

 without value, which fact makes the members of the genus Lestes, at 

 least, one of the most difficult of all genera of Zygoptera to determine. 



Mesoplenra (Figs. 43, 45). — The mesopleura are closely united 

 to the metapleura in most Zygoptera and the interpleural suture has 

 been lost in many cases. This suture can be traced for its full length 



