THE FLIGHT OF INSECTS 309 



fold of membrane in front of the arm. We have seen that the wing 

 of a Bat largely consists of a greatly developed webbed hand, of 

 which all the digits, except the thumb, are much elongated. But 

 the wing of a Pterosaur appears to have been entirely evolved from 

 a lateral parachuting fold, the front end of this being supported 

 by the arm, and also by the little finger, which was of extraordinary 

 length. The species figured possessed very large eyes, and was 

 no doubt nocturnal or dusk-loving. 



The skeleton of Pterosaurs was specialized in relation to flight. 

 There were, for example, extensive air-cavities in the bones, and 

 the sternum possessed a strong keel, correlated with well-developed 

 breast-muscles. In many species the tail was very short, and 

 sometimes the jaws were toothless, being in that case no doubt 

 covered by horny sheaths, as in the beak of a bird. The smallest 

 of these creatures were no larger than sparrows, but some of the 

 latest evolved forms, the toothless Pterosaurs (species >tPteranodon\ 

 which flourished during the Chalk period, attained gigantic dimen- 

 sions, their spread of wing being sometimes as much as 25 feet. 



The Flying Reptiles probably became extinct as a result of 

 competition with the more highly organized Birds and Bats. Their 

 organs of flight were perhaps less efficient than in those groups, 

 but it is not likely that this was the only or even the chief cause of 

 their failure in the struggle for existence. This must probably be 

 sought in less perfect circulatory and breathing organs, but it is 

 impossible to verify this conjecture, as our knowledge of the 

 group is almost entirely limited to what can be deduced from the 

 structure of the skeleton. 



THE FLIGHT OF INSECTS (INSECTA) 



Insects alone among Backboneless animals have evolved organs 

 of flight, and to this must probably be ascribed their extraordinary 

 abundance, both as regards number of species and number of indi- 

 viduals. In both respects they far surpass all other kinds of land 

 animal taken together. 



Structure of Insects in Relation to Flight. The body of an 

 insect is divided into three regions, i.e. head, thorax, and abdomen. 

 The thorax consists of three successive rings, each of which bears 

 a pair of legs, while in typical cases there are two pairs of wings, 

 attached to the second and third ring respectively. These struc- 



