THE MASTERY OF THE AIR 237 



they seem to be sense-organs, but their mean- 

 ing 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 painfully 

 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 



