INSECTS 



they could glide through the air from the branches of one 

 tree to another as well as can a modern flying squirrel by 

 means of the folds of skin stretched along the sides of its 

 body between the fore and the hind legs. If such lobes 

 then became flexible at their bases, it required only a 

 slight adjustment of the muscles already present in the 

 body to give them motion in an up-and-down direction; 

 and the wings of modern insects, in most cases, are still 

 moved by a very simple mechanism which has involved 

 the acquisition of few extra muscles. 



It appears, however, that three pairs of fully-developed 

 wings would be too many for mechanical efficiency. In 

 the later evolution of insects, therefore, the prothoracic 

 lobes were never developed beyond the glider stage, and 

 in all modern insects this first pair of lobes has been lost. 

 Furthermore, it was subsequently found that swift flight 

 is best attained with a single pair of wings; and nearly 

 all the more perfected insects of the present time have 

 the hind pair of wings reduced in size and locked to the 

 front pair to insure unity of action. The flies have 

 carried this evolution toward a two-winged condition so 

 far that they have practically achieved the goal, for with 

 them the hind wings are so greatly reduced that they 

 no longer have the form or function of organs of flight, and 

 these insects, named the Diptera, or two-winged insects, 

 fly with one highly specialized and efficient pair of wings 

 (Fig. 167). 



The Paleodictyoptera became extinct by the end of the 

 Carboniferous period, and their disappearance gives 

 added support to the idea that they were the last sur- 

 vivors of an earlier type of insect. But they were by 

 no means the primitive ancestors of insects, for, in the 

 possession of wings alone, they show that they must have 

 undergone a long evolution while wings were in the course 

 of development; but of this stage in the history of insects 

 we know nothing. The rocks, so far as has yet been 

 revealed, contain no records of insect life below the upper 



[92] 



