i9i3 _I 9 I 4-] The Common House Fly. 1 1 9 



with sacs. Air is driven through all this complicated ramifica- 

 tion of tubes by muscular action in the insect. While resting, 

 only slight movement of the abdominal muscles are to be 

 noticed ; this suffices to induce sufficient oxidation of its tissues 

 while quiescent. On the other hand, the muscles of the wings 

 are relatively of enormous size. The faster the flight, therefore, 

 the more rapid and intense the respiration. The action of the 

 wings resembles a force-pump. Indeed the two actions of 

 muscular contraction and relaxation and of respiration are 

 mutually interdependent. Just think for a moment of the 

 intense muscular action induced in causing the wings to 

 vibrate at a velocity measured by thousands of strokes per 

 minute, and made known to us by the musical note thus 

 produced. Not only this, but the flight may be kept up for 

 hours on end, and you will understand how necessary an active 

 and complete aeration of the tissues must be to enable the fly 

 to sustain itself for so long a time in the air. With all this 

 excessive amount of movement we can hardly agree with the 

 vainglorious fly of iEsop, which, sitting upon the axle-tree of 

 the chariot wheel, said, " What a dust do I raise ! " 



The reproductive system : In the female the two ovaries 

 are so large as almost to fill the abdomen. Each ovary 

 contains about seventy strings of eggs in various stages of 

 development. They open into two ducts which unite and 

 end in the ovipositor. A set of small vesicles in connection 

 with the central oviduct store the spermatozoa received from 

 the male. The long ovipositor allows of the eggs being laid 

 in the crevices of the substance on which the larvae will feed. 



The excretory system, which may correspond to the urinary 

 system, consists of two Malpighian tubes, each very long and 

 convoluted, and intimately bound up with the fat body. These 

 empty into the intestine at the junction of the fore and large 

 intestines. 



The house fly lays a large number of eggs, from 120 to 

 140 at each batch, and four batches may be laid by a single 

 fly, and this with wonderful foresight. She selects a spot 

 which contains suitable pabulum for the future grub. Thus 

 horse-manure heaps form the most suitable breeding-grounds. 

 These eggs are laid in clusters, or in large masses consisting of 

 many thousands of eggs. The eggs are in the shape of white, 



