HOUSE-FLY AND OTHER TWO-WINGED INSECTS. 113 



much clianged in form and use. There are also broad 

 scales, under the base of the true wings, called the winglets 

 (alulae). The fly's wing is broad, thin, and transparent, 

 and strengthened especially along the front edge by slen- 

 der, hollow rods called yeins. There are five principal veins, 

 some of which are branched, and they are in some 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 fiy is its voice. There are three differ- 



Fio. 121. — Hind body or abdomen of house-fly. a, dorsal, b, ventral, c, side, 

 view; sp', sp", the four pairs of piracies. (Magiiined.) 



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

 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. 134) is broad and 

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

 segments when observed from above or below ; but in the 

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

 out of the body like the joints of a telescope, and form a 

 sort of egg-layer (ovipositor). Flies have no sting, though 

 certain kinds can bite and stab with their mouth-parts. 

 The legs are long and slender, and composed of the same 

 8 



