ii FLIES WITH AQUATIC LARV^ 127 



it is absorbed, while the insoluble refuse is passed 

 directly into the intestine. A pair of salivary glands 

 is found in the fore part of the body. These are 

 hollow sacs lined by a single layer of cells. Each is 

 furnished with a duct. The two ducts unite in front, 

 and pour the salivary secretion into the mouth. 

 Whether the saliva takes any direct part in the diges- 

 tion of the food I do not know for certain, but it 

 undoubtedly furnishes the material for the silken 

 threads, with which the larva binds together the walls 

 of its burrow. 



The student who is provided with a microscope 

 will find the cells which line the salivary glands an 

 excellent subject for study. The nuclei of these cells 

 are unusually large, and of complicated structure. 

 Each nucleus is a transparent spherical mass, enclos- 

 ing one or two smaller spheres, the nucleoli. Besides 

 the nucleoli, the nucleus contains a long cord, which 

 is wound into an irregular coil. This cord appears to 

 be made up of a number of discs placed end to end 

 like a pile of coins. If there are two nucleoli, each is 

 joined to one end of the cord : if there is only one 

 nucleolus, it receives both ends. The physiological 

 meaning of this intricate structure has not yet been 

 elucidated. It is an interesting but unexplained fact, 

 that nuclei of similar structure, though of very much 

 smaller size, occur in some plants, e.g. in the young 

 seeds of Fritillaria. It may be that the large size and 

 complex structure of the salivary nuclei of Chironomus 

 is connected with the very abundant secretion poured 

 forth by a comparatively small number of cells, but 

 this is mere conjecture. 



