TRACHE.E IN THE NYMPH OF PLATHF.MIS LYDIA. 353 



arrange these families according to branchial complexity we 

 would have the following order : (i) Agrionidae, (2) /Eschnidae, 

 (3) Libellulidae. And this is the order in which they are ar- 

 ranged by systematists. It merely indicates that the tracheal 

 system is correlated in its complexity with the complexity of 

 other parts, and corroborates Lubbock's statement that the 

 tracheal system conforms to the contour of parts. 



As to the phylogenetic origin of the branchial apparatus very 

 little is known. Chun, '75, stated that Leydig had described in 

 Pliryganea grandis a structure which indicates that the rectal 

 branchiae of Libellulidae might be considered as being developed 

 from the rectal glands of other insects. Sharp, '95, after careful 

 examination of Leydig's work failed to find such reference. 

 Chun and others, however, have regarded this as the probable 

 origin of branchiae. Sadones, '95, after a careful examination 

 of the evidence concludes by saying that Chun and the others 

 who hold the above views persist in " perpetuant une notion 

 erronee des anciens anatomistes." 



In the nymph of PlatJieinis lydia the pair of thoracic stigmata 

 between the pro and mesothorax are practically closed. When 

 the nymph is placed in a thick solution of glycerine it invariably 

 comes to the surface and protrudes the posterior end of the ab- 

 domen out of the fluid, the anal valves opening and closing 

 vigorously. The caudal end always protrudes, indicating that 

 the stigmata are not functional as yet. Hagen, '80, on the other 

 hand believes that the thoracic stigmata are functional in the 

 nymph. The best evidence is furnished by Dewitz, '90, who 

 conducted a series of experiments. When the older nymphs of 

 sEscJina were placed in alcohol bubbles escaped from the thoracic 

 stigmata. But in immature nymphs no gases escaped though 

 these were subjected to a much severer test. The transition 

 then from rectal gill breathing of the nymph to the open stigmatal 

 type of the adult is not as sudden as ordinarily supposed, but 

 that as the nymph grows older it gradually changes from gill 

 breathing to the open stigmata of the adult. 



When one recalls that the fine tracheoles to the rectal gills 

 originated ultimately from the dorsal and ventral tracheae and 

 that these are connected posteriorly and anteriorly then it ap- 



