IN THE LAKV^ OF ANISOPTERID DRAGONFLIES. 



185 



this latter gill arise partly from the visceral trunk and partly from the 

 latero-ventral branch, from the same stems as the efferents of the ventral 

 gill-fold ; they also are seven or eight in number. 



It will be seen from the above description that, although the form of the 

 gill-folds is very simple, their tracheal supply is very complicated. It may 

 be best understood by ignoring, for the moment, the complications introduced 

 by the branchings of the tracheal trunks at jj and J2, and confining our 

 attention to the part of the gill-basket anterior to these two points. We 

 may then state the position as follows : — 



Each dorsal trunk gives off alternately two sets of primary efferent tracheae, 

 a more dorsal set and a more ventral set. Each trachea of the more dorsal 

 set divides into two branches, one of which becomes a secondary efferent 

 trachea of the dorsal gill-fold, the other a secondary efferent of the latero- 

 dorsal gill-fold. Each trachea of the more ventral set also divides into two 

 branches, one of which becomes a secondary efferent, of the latero-dorsal 

 gill-fold, the other a secondary efferent of the latero-ventral gill-fold. 



Each visceral trunk, on the other hand, gives off onli/ one set of primary 

 efferent tracheae. Each of these trachese divides into two branches, one 

 becoming a secondary efferent of the latero-ventral gill-fold, the other a 

 secondary efferent of the ventral gill-fold. 



In other words, the two dorsal trunks between them supply the whole 

 tracheal system for the dorsal and two latero-dorsal gill-folds, and the upper 

 half of that for each of the latero-ventral gill-folds. The two visceral 

 trachese between them supply the whole tracheal system for the ventral gill- 

 fold and the lower half of that for each of the latero-ventral gill-folds. 



If, now, we admit the trachea Ivb as portion of the visceral system (with 

 which it becomes continuous in later stages), we may say that the dorsal 

 trunks account for the tipjjer tioo-tliirds of the tracheal system of the rectum, 

 the visceral trunks for the lower one-third. The proportions are made up as 

 follows : — 



