The Life-History and Anatomy of Butterflies 



plete tube, which is known as the proboscis, and which, when not 

 in use, is curled up spirally, looking like a watch-spring. At 



Fig. 30. — Cross-section of the sucking-tube of the 

 milkweed butterfly, to show the way in which the halves 

 unite to form a central canal (c): tr, trachea, or air- 

 tubes; n, nerves; in, m$, muscles of one side. (Magnified 

 125 diameters.) (Burgess.) 



the upper end of the proboscis, in the head, is a bulb-like en- 

 largement, in the walls of which are inserted muscles which have 



Fig. 31.— Longitudinal section of the head of the 

 milkweed butterfly : cl, clypeus ; mx, left maxilla, 

 the right being removed; mfl, floor of mouth; ce, 

 oesophagus, or gullet; ov, mouth-valve; sd, salivary 

 duct; dm and_/«j, dorsal and frontal muscles, which 

 open the sac. (Magnified 20 diameters). (Burgess.) 



their origin on the inner wall of the head. When these muscles 

 contract, the bulb-like cavity is enlarged, a vacuum is produced, 



15 



