608 CAUDAL GILLS OF ZYGOPTKHID LAKV.E, 



given ill Parti, of this paper (Tliesc; Proceedings, 1917, Vol, xlii.. 

 Part i., pp. 39-44). The method of double-embedding is essential 

 for these tiny sti-uctures, and every detail must be carried out 

 with the greatest care, if one would have a final result of any 

 value at all. The best results were those that, after staining, 

 were de-stained back almost to the lightest possible limit. Such 

 preparations should, of course, be fully studied at once, unless 

 one is willing to risk the chance of their fading before the woi-k 

 is completed. 



In Part iv., the results obtained in Part iii. are considered in 

 conjunction with those obtained from a study of the caudal fila 

 ments of the Perlaria and Plectoptera, the combined evidence 

 from all those sources giving us some valuable phylogenetic 

 results. 



Part iii. Ontogeny of the Caudal Gills. 



Struchire of the Gills in the neivly-hatched larra or Second Instar. 



(Text-figs. 33, 34). 



'Iext-Hg.33 shows the external form of the gills in the newly- 

 hatched larva of Austroagrion cyane Selys, which is typical of 

 all Vertical Lamellar Gill-types. The gills are elongated, slender 

 filaments, clothed with short, stiff hairs externally, for most of 

 their length, but carrying, near the tip, a few longer and more 

 plial>le hairs. The tip itself is bluntly pointed, and carries a 

 long, stiff, bristle-like hair. The gills are quite transparent and 

 witliout pigmentation. The median (dorsal) gill possesses two 

 main loi]gitudinal trnchea?, one derived from each dorsal trunk 

 of the abdomen. The lateral gills, however, each possess only 

 one main longitudinal trachea, derived from the dorsal trunk of 

 its own side. The trachej^ do not reach as far as the tip of the 



gill- 



In transvei-se section, the gills are nearly circular (Text-fig. 34). 

 The cuticle is thin, and is followed, on its inner side, by the 

 ahnost equally thirj hypodermis, which shows only from four to 

 si.x nuclei in any given section. The interior of the gill consists 

 of an iin<iifierentiated blood-space, or portion of the litemoccele. 

 In the median gill, the two main tracheie lie mid-laterally, one 

 on either side, close to the hypodermis. In each lateral gill, the 



