NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 33 



Oil the base of the proleg, but the principal muscle is a long bundle 

 of fibers arising on the lateral wall of the body and attached distally 

 in the foot pad. The prolegs of the caterpillar serve principally for 

 the support of the long and heavy abdomen and for grasping a stem 

 or twig when climbing. 



Wings: Of all the animals that fly, the insects alone have wings 

 developed independently as organs of flight. The others have con- 

 verted a pair of legs into wings. To be sure, the winged creatures 

 of fiction imitate the insects in having wings specially created for 

 flight, but it is doubtful that any of them could really fly if alive. 



The insect wings grow out in immature stages of the nonmetab- 

 olous orders as small flat lobes from the edges of the back on the 

 mesothorax and metathorax. Some fossil insects have similar 

 lobes on the prothorax, suggesting that the ancestors of the winged 

 insects had three pairs of paranatal lobes. Since at this stage they 

 could not have served for flight, it is postulated that at first the lobes 

 enabled the insects to glide through the air a longer distance than 

 they could jump (in the manner of flying squirrels, etc.). If the 

 second and third lobes then became lengthened and flexible at their 

 bases, they might have been able to flap up and down, and thus sus- 

 tain the insect in the air longer. 



From some such early stage of wing development it seems to have 

 required some evolutionary experimenting to produce an efficient 

 mechanism of flight. The dragonflies have the simplest way of mov- 

 ing the wings. Each wing is pivoted on a process of the pleuron 

 and is moved by antagonistic muscles inserted on the wing base at 

 opposite sides of the fulcrum. Yet the dragonflies even today are 

 among the most efficient of flying insects. The cockroaches, mantids, 

 and termites are weak flyers compared with the dragonflies, and it 

 is not well understood just how they move their wings. They have 

 neither the dragonfly mechanism nor the typical thoracic muscula- 

 ture of the higher insects. The wings in these groups are supported 

 on pleural fulcra, and muscles acting on the wing base before and 

 behind the fulcrum probably effect a depression of the wings, while 

 it is possible that the numerous leg muscles attached on the back 

 sufficiently flatten the notum to raise the wings. Though the thoracic 

 musculature of these insects is well known, no real study has been 

 made of its action on the wings. 



The typical insect wing mechanism is found in the mayflies and 

 in all the higher orders. It provides for the up-and-down wing 

 movements and for a partial rotation of each wing on its long axis. 



