ISO The Outline of Science 



has been solved four times, and the solution has been different in 

 each case. The four solutions are those offered by insects, extinct 

 Pterodactyls, birds, and bats. Moreover, as has been pointed out, 

 there have been numerous attempts at flight which remain glori- 

 ous failures, notably the flying fishes, which take a great leap and 

 hold their pectoral fins taut; the Flying Tree-Toad, whose 

 webbed fingers and toes form a parachute; the Flying Lizard 

 (Draco volans), which has its skin pushed out on five or six 

 greatly elongated mobile ribs; and various "flying" mammals, 

 e.g. Flying Phalangers and Flying Squirrels, which take great 

 swooping leaps from tree to tree. 



The wings of an insect are hollow flattened sacs which grow 

 out from the upper parts of the sides of the second and third 

 rings of the region called the thorax. They are worked by power- 

 ful muscles, and are supported, like a fan, by ribs of chitin, which 

 may be accompanied by air-tubes, blood-channels, and nerves. 

 The insect's body is lightly built and very perfectly aerated, and 

 the principle of the insect's flight is the extremely rapid striking 

 of the air by means of the lightly built elastic wings. Many an 

 insect has over two hundred strokes of its wings in one second. 

 Hence, in many cases, the familiar hum, comparable on a small 

 scale to that produced by the rapidly revolving blades of an 

 aeroplane's propeller. For a short distance a bee can outfly a 

 pigeon, but few insects can fly far, and they are easily blown 

 away or blown back by the wind. Dragon-flies and bees may be 

 cited as examples of insects that often fly for two or three miles. 

 But this is exceptional, and the usual shortness of insect flight is 

 an important fact for man since it limits the range of insects like 

 house-flies and mosquitoes which are vehicles of typhoid fever 

 and malaria respectively. The most primitive insects (spring- 

 tails and bristle-tails) show ho trace of wings, while fleas and lice 

 have become secondarily wingless. It is interesting to notice that 

 some insects only fly once in their lifetime, namely, in connection 

 with mating. The evolution of the insect's wing remains quite 



