ORDER DIPTKRA 



121 



the p . and the triangnlai 



shield-shaped 



this segment are attached the v I third 



(metathorax) is but partly formed, and cannol be seen from 

 It supports the balanci • h). There are 



broad coverh - under the base of the tru< 



called tin' 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 movewith 

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

 Its wings 330 timi iond, 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- 

 d the Ik's hum. While flying the b reia- 



e h 



Fio. l-n. -Hind body >r abdomen of ly. a, do antral e, 



if abdominal si 



tively low; when the wings are held so at their 



vibrating, the tone is higher, and a high< 11 when 



the fly is held .-I as to prevent all mo1 rnal 



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



produced by the spiracles or breathing-ho 



Tims, the humming sound is not produced by the i 



of the wings alone. The hind body (Fig. L41) 



somewhat conical in shape, and there art only 



four segments when seen from above or b< »w; but in the 



living fly three more may be detected, wh thrust 



