186 



MR. R. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



(3) The gradual development of the duplex system of gills. 

 (Text-fig. 20.) 



Passing now to the third instar, we notice the following advances made in 

 the gill-basket o£ a transparent larva examined soon after ecdysis : — 



All the gill-folds show deeper undulations, and the tracheal loops are more 

 abundant. 



The efferent trachese are considerably stouter. 



There is thus very little advance on the stage seen in the second instar. 

 When, however, we come to the fourth instar (fcext-fig. 20, B), we find a 

 very interesting stage in the development. Confining our studies for the 



Text-fig. 20. — Diagrammatic representation of dorsal portion of gill-basket, viewed from 

 above, to show the development of the Duplex .System from a primitive Simplex 

 System. Taken from camera-lucida drawing.s immediately after ecdysis. A. From 

 Anax papuetisis, Burm., 2nd Instar ; primitive Simplex Main Fold. B. From Anax 

 papnensis, Burm., 4th Instar ; development of cross-folds. C. From ^schna brevistyla^ 

 Ramb., 7th Instar ; abortion of tracheal supply to main fold, and growth of cross-folds 

 to form foliate gills of the Duplex System (7th Instar oi Anax the same, with papillas). 



c/, cross-fold ; es, secondary efferent tra.chea ; fol, folia : mf, main fold ; mf , ditto, aborted j 



11, 12, 1, positions on clock-face. 



present to the dorsal gill-fold, we notice the following very interesting- 

 points : — 



The undulations of the main gill-fold are deep and very regular. Each 

 secondary efferent trachea gives off its branch tracheae to the fold along the 

 crest of a wave, i. e. to that portion of the fold which is convex to it. From 

 each secondary efferent trachea, close to the main fold, there rises up 



