128 NATURAL HISTORY OF AQUATIC INSECTS CH. 



An inexperienced worker with the microscope, look- 

 ing for an interesting subject, which will neither tax 

 his knowledge nor his practical skill too heavily, will 

 find the salivary gland of Chironomus exactly suited 

 to his wants. For it is an object of considerable bio- 

 logical interest, and yet it can be studied without 

 mechanical appliances or special manipulative skill. 

 A few words of explanation will enable a mere novice 

 to make for himself most instructive preparations. 



Lay a live larva on a clean glass slip, and cut off its 

 head with a scalpel. The transparent salivary glands 

 at once begin to flow out, together with the abundant 

 red fluid, which is the blood of the larva. By simply 

 laying a cover-glass on the glands, and examining 

 them with a quarter-inch objective, the nuclei can be 

 seen and studied. But the details of their structure 

 cannot be made out unless the tissue is stained. A 

 staining fluid applied to the yet living cells will 

 not act. (See p. 38.) It is therefore desirable to 

 kill the cells, and to do it rapidly without allowing 

 disorganisation of the nuclear structure to take place. 

 The reagents which accomplish this for us are called 

 fixing agents. Heat, such as that of boiling water, 

 will suffice in many cases. Momentary immersion in 

 strong alcohol is another means. The preparation 

 may be irrigated on the slide by a few drops of 

 absolute alcohol, which are afterwards to be drained off 

 and replaced by the staining fluid. Most microscopic 

 workers prefer osmic acid. A weak solution is em- 

 ployed, say \ per cent. Precautions, especially with 

 respect to the action of light, are requisite in the case 

 of osmic acid. It becomes reduced, and turns black, 



