10 PRACTICAL ZOOLOGY 



organs are the wings and legs. Some insects, like the bedbug and 

 the flea, are without wings and depend entirely upon their legs for 

 purposes of locomotion ; whereas others, as the butterflies, make 

 flight their chief method of progression. The grasshopper uses 

 both wings and legs, but the latter are more effective during the 

 ordinary activities of daily life. Both wings and legs are at- 

 tached to the middle of the three principal parts of the body ; 

 a part called the thorax (Fig. i). The four wings are arranged 

 in two pairs fastened by movable joints at the sides near the 

 upper surface. The front pair are rather leathery in structure, 

 serving as a protection for the thin, membranous back wings, 

 which are folded beneath them when at rest. The thin wing 

 membranes are strengthened by minute tubes which are defi- 

 nitely arranged in every wing and are similar in number and 

 position in the individuals of every kind of insect but different 

 in the different kinds. 



FLYING. The movements of the wings during flight are quite 

 interesting. The front edge of the wing is firm, whereas the mem- 

 brane as a whole inclines upward when the wing is lowered, and 

 downward when the wing is raised. This results in resistance 

 from behind, which propels the insect forward. The wings on 

 opposite sides of the body move up and down together, and the 

 faster they vibrate the more rapidly the insect progresses. The 

 house fly makes 330 strokes per second, the dragon fly 28, and 

 the cabbage butterfly 9. 



LEGS. The six legs of the grasshopper are arranged in three 

 pairs ; one pair is attached to each of the three parts or seg- 

 ments which make up the thorax. Other insects are likewise 

 provided with three pairs of legs. The legs are used by insects 

 chiefly for locomotion, but also for many other purposes, and an 

 examination of their structure will often enable us to determine 

 their functions (Fig. 2) . Some are long and slender and fitted for 

 running (Fig. 2, b) ; others are flattened out and bordered with 

 bristles, making them effective swimming organs (e) ; some are 

 short and shovel-shaped for digging in the earth (d) ; a few enable 



