ORDER DIPTERA. 



121 



pieces, the prascutum, scutum, and the triangular or 

 shield-shaped scutellum (Fig. 140, set'}. To the sides of 

 this segment are attached the wings. The third ring 

 (metathorax) is but partly formed, and cannot be seen from 

 above. It supports the balancers (Fig. 140, h). There are 

 also broad covering scales under the base of the true wings, 

 called the tegula. The fly's wing is broad, thin, and trans- 

 parent, and strengthened especially along the front edge by 

 slender, hollow rods called veins. There are -six principal 

 veins, some of which are branched, and they are in most 

 cases connected by a few cross-veins. The wings move with 

 great swiftness. The house-fly, when held captive, moves 

 its wings 330 times a second, and the tip of the wing de- 

 scribes a figure 8 in the air. 



The hum of the fly is its voice. There are three differ- 

 ent tones in the fly's hum. While flying the tone is rela- 



FIQ. 141. Hind body or abdomen of house-fly, a, dorsal, 6, ventral, c, side, 

 view; sp'-sp", the four pairs of abdominal spiracles. Magnified. 



tively low; when the wings are held so as to prevent their 

 vibrating, the tone is higher, and a higher one still when 

 the fly is held so as to prevent all motion of the external 

 parts. The latter tone is the true voice of the fly, and is 

 produced by the spiracles or breathing-holes of the thorax. 

 Thus, the humming sound is not produced by the motions 

 of the wings alone. The hind body (Fig. 141) is broad and 

 somewhat conical in shape, and there are seen to be only 

 four segments when seen from above or below; but in the 

 living fly three more may be detected, which can be thrust 



