210 THE SEVEN FOLLIES OF SCIENCE 



The dragon fly lives entirely on flies, mosquitoes, and 

 other insects which it captures on the wing, and when a 

 room is so fortunate as to have a dragon fly for a visitor, 

 all mosquitoes and flies are quickly removed. And yet, 

 notwithstanding this well-known fact, let a dragon fly 

 appear in an assembly of young people (or old ones either, 

 for that matter) and there will be an intense commotion 

 and every young man in the party will be put on his mettle 

 in an effort to kill the terrible beast. 



So well known to naturalists are the good offices of the 

 dragon fly that some years ago an effort was made to propa- 

 gate them as an enemy of the mosquito. It was found, 

 however, that while the dragon flies were active destroyers 

 of the mosquito they retired early in the evening, while the 

 late evening and night is just the time when the mosqui- 

 toes are most active. In addition to this the larvae of the 

 dragon fly are very destructive to small fish, and these are 

 well known to be the most efficient destroyers of the larvae 

 of the mosquito. A dozen small fish will clean out all 

 the mosquito larvae in a small pool of water, and what is 

 more, they will keep it clear of these pests. And as the 

 larvae of the mosquito are almost always bred in stagnant 

 pools, this is the most effective mode of getting rid of them. 



The larvae of the larger species of dragon fly are fierce, 

 carnivorous creatures of which the common name is "the 

 water-devil." They spare nothing that comes within their 

 reach and that they can overcome not even weaker 

 individuals of their own species. But the mature insect 

 is a harmless and indeed a beneficent creature and it never 

 stings, for it has no sting. 



