258 ENTOMOLOGY FOR MEDICAL OFFICERS 



and is the only insect that is known to moult after reaching 

 sexual maturity. 



The eggs are numerous, and are shed into water. The 

 larva (Fig. 117) has long, filamentous or feathery anal cerci, 

 and soon develops numerous tracheal gills on the sides of 

 some of the abdominal segments; these gills are commonly 

 broad leaves, but are sometimes feathery, and are sometimes 

 protected by a special cover. The larvae are found in water 

 of all kinds ; some swim actively by means of their long 

 feathery caudal filaments, others adhere to stones, and others 

 burrow in the mud. Some of the free-swimming forms are 



FiQ. 117.— Larva of a May-fly. 



carnivorous, and have been proved by experimental observa- 

 tion to be destructive to mosquito-larvse. 



May-flies, at certain seasons and in the evening, occur in 

 myriads together. An illuminated house-boat in a tropical 

 delta is the place to learn what a swarm of may-flies can be 

 like : they will fill the boat if the lights are not put out. 



Order Odonata : Dragon-flies. 

 (Lat. odonatus = with buskins of felt.) 



Dragon-flies are of interest for us, since both as adults and 

 in the aquatic larval stage they are highly voracious of other 

 insects, and the aquatic larvae of one large group of dragon- 

 flies— the ^^r/<7«/;«^ (Fig. 118)— are among the most active 

 enemies of mosquito-larvae. 



The order includes two families of insects having an 



