Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 603 



push which would end in a crash-landing except for the flexiblity and spread 

 of the middle and front legs and the jack-knife bend of the hind ones. As 

 animals walk and run they alternately balance and move their bodies. The bal- 

 ance is a momentary rest on one, two, or three feet, depending on the type, 

 whether human, horse, beetle, or others (Fig. 10.10). The movement, also 

 momentary, is a falling forward of the body or a fall coupled with a pull. As 

 an insect walks it balances by resting on a tripod, the first and last leg of one 

 side, and the middle leg of the other. The balance quickly shifts into movement 



B 



Fig. 30.12. Mouthparts of insects are precision tools, mandibles of two species 

 of grasshoppers that eat different foods. Left, the lubber grasshopper (Brachystola 

 magna) feeds on foliage. Right, another grasshopper {Menuaria macnlipennis) 

 feeds on seeds. (Redrawn from Isely. Courtesy, Brues: Insect Dietary. Cam- 

 bridge, Mass., Harvard University Press, 1946.) 



as the other three legs are swung forward. In this latter trio, the front leg pulls 

 the body, the middle one lifts it, and the hind one pushes. The insect goes for- 

 ward in such a slight zigzag that it seems to be a straight line. 



Wings. Many invertebrates can walk and crawl but only the insects can fly. 

 The wings of birds are highly modified front legs; those of insects have no rela- 

 tion to their legs. The wings of most insects are connected with the body by 

 flexible joints to which the flight muscles are attached. In grasshoppers and 

 other insects that gradually change form, wings are direct outgrowths of the 

 posterior dorsal edges of the meso- and metathorax (Fig. 30.7). While it 

 is developing, the wing pad contains tracheae, nerves, and blood. The ar- 

 rangement of the tracheae usually determines the future pattern of the veins. 

 By the time the wing is mature it is comparable to a flat envelope composed of 

 chitin and the dead remains of cells. Within it the walls of the tracheae are 

 thickened and transformed into solid rods, the veins. Although so much of the 

 wings is chitinous, blood continues for a time to circulate slowly through it 

 outward to the tip and back to the body by another route (Fig. 30.13). 



The patterns of veins (wing venation) are important in showing relation- 

 ships between species. All of them seem to have evolved from one or a few 

 basic ones. The more primitive insects, mayflies, grasshoppers and others 

 have many veins. Specialized insects such as bees have few veins. During the 

 long history of insects the veins have been reduced in number but are better 

 placed and mechanically more efficient. 



