196 



KNOWLEDGE. 



[August 2, 1897. 



perceptible means of propulsion. Having, apparently, 

 anticipated the latest discovery of naval mechanics, 

 be propels his vessel by means of a jet of water violently 



driven out of the 

 stern ; as the water, 

 however, has to be 

 drawn in again by 

 the same vent (the 

 rectum), his jour- 

 neys by this method 

 are not very long, 

 and are somewhat 

 spasmodic. This 

 drawing in and 

 expelling of water 

 through the rectum 

 is the ordinary 

 method of respira- 

 tion also, which is 

 effected chiefly 

 under water, at 

 least during the 

 earlier larval 

 stages ; but the 

 larva is also able 

 to breathe atmo- 

 spheric ak in a way 

 not yet fully under- 

 stood, I believe, but 



Pl&. 1.— Larva of Dragou FIv. !Ma?k extended. 



which was to be expected in an insect whose later existence 

 was to be spent in air — not in water. It may be noticed 

 that as the time for the final change approaches, the 

 nymph is fond of resting with either the extreme tip 

 of the tail out of water, or else the head and one or both 



within, and forms with the two lateral ones a minute 

 channel as large as a fine needle, which is always open— - 

 even when the spines appear to be quite closed — and this 

 apparently also 

 serves as a spiracle. 

 In several of the 

 figures (('.,'/., Figs. 

 3, 4, and 5) white 

 threads are seen 

 proceeding from 

 the split nymph 

 skin. These are 

 the trachea', which 

 connect with the 

 pair of spiracles 

 already referred to, 

 and are ruptured 

 as the imago comes 

 out. They easily 

 locate the position 

 of the spiracles in 

 the dry nymph 

 skin, if any diffi- 

 culty is experienced 

 in finding them 

 from the outside. 



When full fed 

 the dull brown eyes 

 of the larva become 



Fig. 2. — Larva of the Dragon Fly. 



brilliaut green, being indeed the colour of the perfect eyes 

 within ; the larva then crawls laboriously up some stick 

 or water plant, the body close pressed against the stem, 

 and assisted by the scales and spines of the abdomen— not 

 disdaining the friendly help of a pencil if such be forth- 



Fia. 3. Fig. 4. Fio. 5 



The Metamorphosis of tlic Dragon Fly : " Slowly the body emerges." Photographed from Life. 



shoulders (if they may so be called). Now if a dry nymph 

 skin be examined, two spiracles like horny lips will be 

 found, one on each side, behind the head, placed vertically 

 in the fold between the segments bearing the first and 

 second pair of legs, and these spiracles in the living larva 

 may be noticed open when above water ; and, again, of the 

 three spines closing the rectum, the two lateral ones are 

 aharp, but the third is cut off at the point and grooved 



coming, and trying apparently, with vigorous switchings 

 of the abdomen, the stability of its chosen support. 



After about ten minutes to half an hour of drying, the 

 part immediately behind the eyes and above the wing cases 

 swells, cracks, and opens, and discloses the bright green 

 body of the perfect insect. Slowly the body emerges ; the 

 brown cases of the eyes are thrust farther and farther aside, 

 and the enormous head with its compound eyes protrudes 



