THE CIRCULATION. 28 



which open beneath by a very fine fissure and run from the margin 

 to the centre of the lobes, and then open into the mouth. This 

 is a provision highly necessary to the fly, as it feeds upon half- 

 rotten pulpy substances, which would otherwise entirely fill 

 its long tubular mouth and completely stop it up. A shuttle- 

 shaped chitinous organ supports the parts of the mouth, 

 and is enclosed within the proboscis. This acts as a pump, 

 drawing the fluid aliment from the mouth of the insect, and 

 injecting it into the sucking stomach or crop.* 



The crop is a large reservoir for food, and is situated in the'ab- 

 domen, it is capable of holding a sufficient supply of nourishment 

 for several days. No solid food is taken by the fly, as it lives 

 entirely upon fluids or such substances as are dissolved by its 

 copious saliva, which is secreted by a pair of large tubular salivary 

 glands. The aliment is gradually regurgitated from the crop into 

 the proventriculus and chyle stomach,f which extends the whole 

 length of the thorax, and which is lined with cylinder epithe- 

 lium undistinguishable from that in the highest vertebrates. 



The chyle stomach passes insensibly into the intestines, which 

 are about one inch in length. A pair of large bile tubes open 

 into them near their inferior extremity, and form a tubular liver 

 of considerable magnitude by continually dividing, and end in 

 blind extremities; they are very tortuous, and are filled with liver 

 cells, containing fat globules and pigment. 



Section VII. The Circulation* 



The circulation of the blood in insects is earned on with- 

 out the aid of blood-vessels. The circulating fluid is contained 



* Sucken-Magen, Weismann, f Chyhis-Magen, Ibid. 



