234 THE HAUNTS OF LIFE 



times in a minute, but many an insect, such as 

 a humble-bee, vibrates its wings 200 times in 

 a second. In most cases the hum or buzz is 

 simply due to the rapidity with which the 

 wings strike the air, and there is no structure, 

 visible to the naked eye, in the animal king- 

 dom that moves so rapidly as an insect's wing. 

 When the wings are large, as in dragon-flies 

 and big butterflies, the number of strokes in a 

 second is small. There is a fossil dragon-fly 

 whose wings taken together have a span of 2 

 feet from one side to another, but there is 

 nothing like this to-day. 



Insects vary greatly in their power of flight. 

 Many of the two-winged insects cannot fly more 

 than a few hundred yards, and can hardly steer 

 themselves at all, but are borne along by the 

 wind. This is true, for instance, of the mos- 

 quitoes, the bite of which in some countries 

 often causes malarial fever. It is true also of 

 our common house-fly, which may cause dis- 

 ease such as typhoid fever, by walking on our 

 food with dirty feet for it revels in decaying 

 matter, and may come straight from a refuse- 

 heap to our jam-dishes and milk-jugs, carrying 

 with it disease-germs which find there highly 

 favourable conditions for multiplying rapidly. 



