THE INSECT WORLD. 



6i 



hence the term ' ' mosquito hawk' ' has also been appUed to 

 them. 



It has been suggested that dragon-flies might be artificially- 

 propagated to lessen the mosquito pest ; but this is not feasible, 

 because they fly only during the day, while the mosquito is 

 decidedly more a creature of the night. No dragon-fly volun- 

 tarily remains in woods, except along streams and in open glades, 

 yet here mosquitoes lurk by the million. The proposition looks 

 well at first sight, but is not practical. 



Fig. 32. 



Dragon-fly and its development. — Larva and pupa feeding at i and 3; 2, nymph ready 

 to change ; 4, a pupa skin from which 5, the adult, has emerged. 



The early stages are passed under water, and the larvae are 

 ugly creatures, somewhat resembling the adults, but without 

 wings. They move sluggishly among the aquatic vegetation, or 

 on the mud of the bottom, strongly resembling the latter in 

 color. They do not swim readily, but some are able to propel 

 themselves by drawing into the posterior part of the digestive 

 tract a quantity of water and then expelling it forcibly. This 

 process really answers a double purpose, for this same tract is 

 suppUed with numerous fine tracheal tubes through which oxygen 

 is drawn from the water : two functions, respiration and locomo- 

 tion, being thus curiously combined. Respiration is also carried 

 on by means of leaf-like anal appendages, in which trachea ramify 

 in all directions, forming gill-like structures. As the larvae grow, 

 wing-pads appear, and when these are fully developed the pupal 

 stage has been reached ; the insects continuing active, however, 



