CY R. J. TILLYARt). 



611 



of the gill. In Austroagrion cyane, but not in Ischrmra hetero- 

 sticta, this stage is marked by the presence of a distinct pigment- 

 zone on the distal fourth of the gill. 



Sections of the gills at this stage show some important devel- 

 opments internally (Text-fig. 37). Their form, in the basal half, 

 now begins to approach the Triquetro-quadrate Type; while, in 

 the slightly widened and flattened 

 distal half, they have taken on, in 

 cross-section, a distinctly oval form, 

 which is the preliminary step to the 

 differentiation of the rhachis and 

 blade of the true Lamellar Type. 

 The tracheae are now much larger, 

 even the second longitudinal trachea 

 of the lateral gills being easily made 

 out. They project definitely into 

 the hsemocoele, and both lie at the 

 mid-lateral level. But, most im- 

 portant of all, is the division of the hiemocoele into two parts, by 

 an outgrowth of the hypodermis forming a definite internal 

 lamina across its cavity, between the two main tracheae. This is 

 the first differentiation of the dorsal and ventral blood-canals. 



St7'ucture of the Gills in the Fifth Instar. (Text-fig. 38). 

 Externally, there is only a slight increase in the width of the 

 distal half of the gill, so that the difference in shape from that 

 shown in Text-fig 36 is scarcely appreciable. Internally, the 

 differentiation of the hyemocoele has proceeded one step further. 

 The internal lamina has split up to form the inner boundaries of 

 four separate divisions, viz., the dorsal and ventral blood-canals, 

 and two smaller divisions to enclose the two main tracheae 

 respectively. It is important to notice here that, in the median 

 gill, the dorsal blood-canal is smaller than the ventral, while, in 



■'■■ T.S. through the caudal gills of Austroagrion cyane Selys, fourth instar, 

 at about one-third from ba.se; dh, dorsal blood-canal; tr, main trachea; 

 //■', branch-trachea destined to become second main trachea in lateral gills; 

 rh, ventral blood-canal; ( x 370). [N.B. — In this instar, the division of 

 the hiemocoele does not extend much beyond half-way]. 



