244 



COLLEGE ZOOLOGY 



Pupa 

 (chrysalis) 



End view 



Side vi 



view 



Larva (caterpillar) 

 Figure 143. The life cycle of the imported cabbage butterfly. 



modified so as to adapt insects to their en- 

 vironments. 



Wings 



The mesothorax and metathorax, each, 

 bear a pair of wings in most insects. Certain 

 simple species (Thysanura, Fig. 129) do not 

 possess wings; others (lice and fleas, Fig. 

 160) are adapted to parasitic life and the 

 wings are degenerate. The flies (Diptera) 

 each have a pair of clubbed threads called 

 balancers (halteres) in place of the meta- 

 thoracic wings. Wings enable their owners 

 to fly rapidly from place to place, to escape 

 from enemies, and to find a bountiful food 

 supply. The success of insects in the struggle 

 for existence is in part attributable to the 

 presence of wings. Modifications in wing 

 venation come about by reduction or by 

 addition. In the beetles (Coleoptera, Fig. 

 131) the forewings are sheathlike and are 

 called elytra. The forewings of Orthoptera 

 (grasshoppers, etc.. Fig. 131) are leathery 

 and are known as tegmina. The number of 

 wing beats differs according to the species. 

 Thus yellow swallow-tailed butterflies aver- 



age about 6 beats per second, dragonflies 

 about 30, house flies about 160, honey bees 

 about 400 and the wings of the humming- 

 birds make about 750 beats per second in 

 forward flight. In contrast, man can perform 

 a fast-finger piano trill at about 10 beats per 

 second. 



Legs 



Legs are used for various purposes and are 

 highly modified for special functions. Run- 

 ning insects, such as the ground beetle, 

 possess long, slender legs (Fig. 154); the 

 mantis has its forelegs fitted for grasping 

 (Fig. 154); the hindlegs of the grasshopper 

 are used in leaping (Fig. 131); the forelegs 

 of the mole cricket are modified for digging; 

 and the legs of the water bug are fitted for 

 swimming. Many other types of modifica- 

 tions could be mentioned. 



The legs of the honey bee (Fig. 144) are 

 perhaps as remarkably adapted for a variety 

 of purposes as those of any living insect. 

 Honey bees are easily obtained and studied 

 in the laboratory, and hence are selected 

 here for further description. The prothoracic 



