FLIGHT AND SWIMMING 43 



theless, the supreme exemplars of flight — dragon- 

 flies and hawk-moths — are amongst four-winged 

 insects, though even in these the hind wings tend to 

 become smaller than the front ones. In most moths a 

 hook links front and hind wings together, and the 

 absence of this hook serves to distinguish butterflies 

 from them. 



In beetles the fore-wings are converted into shards 

 or covers, held stiffly in flight, whilst the movement 

 itself is entirely due to the hind-wings, which are 

 curiously bent near the tip, so that we do not wonder 

 that few shard-bearers are good fliers. 



In bees and wasps the fore-wings are strongly 

 developed and securely hooked on the front edge of the 

 other pair, which in flies become reduced to balancers. 

 They take no part in flight, but are converted into 

 highly sensitive, nervous organs. 



Movements of vertebrates : I. The adaptations oj 

 tfsh. — In vertebrates, the problem of movement in 

 water, on land, and in air has been solved time after 

 time. There are walking and flying fish, swimming 

 and darting lizards, gliding and swimming snakes ; and 

 winged reptiles, though now extinct, were once abun- 

 dant. Birds fly both in water and air, whilst bats, 

 amongst mammals, are almost as exclusively animals 

 of flight as whales and seals are animals of the water. 



In the lowest fish, as in worms and insect larva:, the 

 body is essentially a muscular tube, capable of being 

 thrown into wriggling, eel-like movements, by which 

 pressure is brought to bear on the water at each 



