II FLIES WITH AQUATIC LARV^ 129 



and loses its action upon tissues if exposed to the light. 

 The bottle in use, which should be of small size, must 

 be coated with blackened paper, and kept in a wooden 

 box when not immediately required. A drop of 

 the solution fixes the cells and their nuclei very rapidly. 

 After a minute's exposure to the osmic acid, wash the 

 tissues in a few drops of dilute alcohol, and finally 

 with absolute alcohol. Then the stain may be applied. 

 Try methyl green, picro-carmine and haematoxylin 

 upon as many salivary glands, and learn the best 

 length of exposure. A few minutes will suffice. 

 Until practice has been got, it is well not to attempt 

 permanent preparations. Mount in dilute glycerine, 

 and do not omit to draw everything noteworthy. The 

 preparations will last for several days without further 

 precautions. 1 



In the living larva the heart may be seen busily at 

 work. It is a transparent tube, situated at the hinder 

 end of the body on the dorsal side. Two pairs of 

 valvular inlets admit the blood to the heart ; then the 

 contraction of its muscular walls drives the blood along 

 the narrow dorsal vessel, which passes above the 

 alimentary canal to the head. Here the regular blood- 

 channel ceases ; the blood escapes, and bathes all the 

 organs of the body, ultimately making its way back 

 again to the heart. 



It is evidently a matter of some difficulty for the 

 Chironomus larva to procure a sufficient supply of air. 

 It lives in water, often in deep water, which abounds 

 in decaying organic matter, and can contain but little 



1 The salivary cells and nuclei of the Chironomus larva have 

 been well described by Balbiani (Zool. Anzeigcr, iSSi). 



K 



