62 



CAUDAL GILLS OF ZYGOPTERID LARViE, 



of Ischnura heterosticta is easily the most suitable. If placed in 

 a small dish or watch-glass, this larva will persist in resting 

 with its gills in a vertical plane (their natural position). To 

 overcome this difficulty, I found it best to remove the larva 

 altogether from water, and place it sideways upon a glass-slide. 

 A cover-slip was then let down upon the gills, and water from a 

 pipette was run into the wedge-shaped space between the slide 

 and the cover-slip. Thus the gills were enclosed in water, while 

 the rest of the larva was in air. In such a position, the larva 

 will live for some hours, and seldom makes any attempt to move. 



Text-fig. 4. 

 Diagram of the blood-circulation in the caudal gills and last three abdo- 

 minal segments of Ischnura heterosticta Burm. (Middle portions of 

 gills cut away). The seven currents shown are as follows : — 1, dorsal 

 circulation in seg. 8; 2, ditto in seg. 9; 3, circulation in median gill 

 {mg); 4, ventral circulation in seg. 8; 5, ditto in seg. 9; 6, ditto in 

 seg. 10, with extension to left cercoid (c); 7, circulation in left lateral 

 gill {ly). a, anus; hj, breaking-joint; c, cercoid; c/", confluence of 

 blood-canals in gill; rf/t, dorsal blood-canal; h, heart; ng, ganglion of 

 seg. ^; ost, ostia; fh, ventral blood-canal; 8-10, abdominal segments. 



The circulation of blood in the gills is very remarkable. I 

 had naturally expected to find the blood entering the gills by 

 the ventral canal, and passing out of them dorsally. But I 



