DRAGON-FLIES 423 



abdominal gills, and in this case respiration may, according to 

 Hagen,^ take place in four different manners : (1) by ten pairs of 

 stigmata ; (2) by lateral branchiae well furnished with tracheae ; 

 (3) by caudal branchiae ; (4) by rectal branchiae. It is further 

 said that in this Insect the lateral branchiae persist in the imago. 

 Although the means of respiration of the nymphs have been 

 fairly well ascertained, yet the mode in which the nymph is 

 prepared for the sudden change from the aquatic to aerial life 

 is still obscure, the condition of the stigmata not being 

 thoroughly elucidated. It appears probable, however, that the 

 young nymph has no stigmata ; that these organs appear in the 

 course of its development, being at first quite impervious, but 

 becoming — at any rate in the case of the larger and more im- 

 portant pair — open previous to the final ecdysis. We have 

 mentioned the contradictory opinions of Eeaumur and Dufour, 

 and will now add the views of some modern investigators. 

 Oustalet says ^ that there are two pairs of spiracles in the 

 nymphs ; the first pair is quite visible to the naked eye, and is 

 situate between pro- and meso-notum ; it is in the nymph closed 

 by a membrane. The other pair of spiracles is placed above the 

 posterior pair of legs, is small and completely closed. He does 

 not state what stage of growth was attained by the nymphs he 

 examined. Palm(5n was of opinion that not only thoracic but 

 abdominal spiracles exist in the nymph,^ and that they are com- 

 pletely closed so that no air enters them; he says that the 

 spiracles have tracheae connected with them, that at each moult 

 the part closing the spiracles is shed with some of the tracheal 

 exuviae attached to it. The breathing orifices are therefore for a 

 short time at each ecdysis open, being subsequently again closed 

 by some exudation or secretion. This view of Palmen's has been 

 thought improbable by Hagen and Dewitz, who operated by 

 placing nymphs in alcohol or warm water and observing the 

 escape of bubbles from the spots where the supposed breathing 

 orifices are situate. Both these observers found much difference 

 in the results obtained in the cases of young and of old nymphs. 

 Hagen concludes that the first pair of thoracic spiracles are 

 functionally active, and that abdominal stigmata exist though 



1 OR. Soc. ent. Belgique, xxiii. 1880, p. Ixvii. 



2 Ann. Sci. Nat. (5) xi. Zool. 1869, p. 377. 



3 Zur Morphologic des Tracheensijstems, Helsingfors, 1877, p. 38. 



