98 



CAUDAL GILLS OF ZYGOPTERID LARV.*:, 



8. Thf" Denodatf^ Vertical Lamella. 

 (Text-figs.26, 27: Plate ii., figs.17-20). 

 As the final stage in the process of reduction from a two- 

 jointed to a secondarily simple lamella, we have the detwdate 

 form of gill, found in the Australian genera Ayrwcvemh and 

 Ausfrocnemis, and in the New Zealand genus Xanfhocnemis. 

 These are all characterised by complete loss of the node. The 

 original position of the node is no longer indicated, as in the 



subnodate form, by a 



point at which the row® 



of spines along the outer 



edge stops short; for in 



these gills, the spines, if 



present, are developed 



along the whole outei- 



Text-fig. 26. border of the gill, from 



Denodate gills of Au8troc)iewii< .ytleiidida base to tip. The spines 



(Maitin), in natural position, lateral are, however, always small 



view; (x 6). ^j-^^j weakly formed, and 



are more usually replaced by slender hairs. 



In the three Australasian genera in w^hich denodate gills are 

 known to occur, the shape of the gill is long, narrow, and well 

 pointed. The distinction between the thicker basal portion and 

 the thinner distal portion can be seen, but is not as clearly 

 marked as it is in the shorter and broader denodate gills in the 

 larvfe of some Palaearctic species. Ris(27) has called attention 

 to this peculiarity in the gills of Agrion mercurial e Charp., and 

 Pyrrhosoma nymphida Selys, without offering an explanation of 

 it. In Agrion jyueUa Linn., and A. pulcheUiim Vanderl., on the 

 other hand, as Ris' figures well show% the gills are of the true 

 nodate form. 



The three Australasian genera with denodate gills may be 

 separated as follow^s : — 



^Gills pale, transparent, not heavilj^ pigmented, not as long as the 



abdomen of the larva 1. 



Gills heavily pigmented, semi -opaque, quite as long as the abdomen oi 

 the larva... Amtroc-mmi^ Tillyard; (Text-fig.26; PI. ii., figs.18-20). 



