THE MASTERY Or THE AIR 237 
stalked half dumb-bell; they seem to be sense- 
organs, but their meaning is obscure. 
In their flight insects are often truly admir- 
able, but it may be noticed that some fly only 
once in their lifetime, namely, when they are 
starting a new generation, and that some do 
not fly at all. The simplest of all insects, the 
Spring-Tails and Bristle-Tails, seem never to 
have had wings, a state of affairs to be dis- 
tinguished from what is seen in fleas, which 
seem to have lost the wings their ancestors 
had long ago. The flea, as every one is pain- 
fully aware, makes up for its loss of flight by 
its power of taking extraordinary leaps. 
The leaping powers of many insects, such as 
grasshoppers and crickets, suggest the theory 
that insects originally used their wings as 
parachutes in taking skimming leaps along 
the ground or from branch to branch, before 
they were able to use them to strike the air as 
organs of true flight. Just as a creature must 
walk before it can run, so perhaps the winged 
insect had to jump and parachute for ages 
before it could fly, until the muscles of the 
wings grew strong. The fact that the wings 
of insects often contain air-tubes and blood- 
spaces suggests that they originally helped in 
