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. 
