240 THE UNIVERSITY SCIENCE BULLETIN. 



matter. Now, these bugs, we will assume, have just been taken 

 in nature. Their digestive tracts probably contain food ma- 

 terial. A few are examined to determine this point. These 

 are two factors at this beginning point to be considered. If 

 the bugs have been feeding recently, the material in the diges- 

 tive tract might be confused with food taken in from the food 

 culture. Therefore the bugs must be divided into two lots. 

 One placed in a sample of the given culture at once to serve 

 as a check on normal (not starved) feeding, and the results 

 observed. The other lot is then allowed to remain in the 

 clear water for a time sufficient to clear much of the digestive 

 tract. For this purpose fresh glass aquaria, with a few peb- 

 bles placed on the bottom to which the bugs may cling, serve 

 admirably. This lot, then, may be placed in another jar of 

 the culture and allowed to remain for times varying from 

 thirty minutes, for food selection, to weeks, for determining 

 maintenance ability of the culture. In making the examina- 

 tion the bugs are removed from the culture to distilled water 

 for a few minutes. Petrie dishes do very well for this, be- 

 cause they are shallow. By means of the forceps a bug is 

 caught by one of its swimming legs and placed in a drop of 

 water on a slide. After removing its head one dissecting 

 needle is run through the thorax to hold it in position on its 

 venter, and the other needle dissects out the digestive tract 

 entire. The carcass of the bug is then removed, fresh, normal 

 salt solution passed over the digestive tract, and notes made 

 as to the general position of the contents, whether in some 

 part of the stomach or in the intestine. Usually there will 

 be two masses of material. The dissecting needle pricks the 

 stomach wall to liberate the material, which promptly speads 

 upon the slide. The other mass is removed in the same way, 

 then the cover slip is added and both of these smears studied 

 under compound. Since the food is likely to be deep green 

 in color, due to the plant chlorophyll, or at least to contain 

 unicellular plant cells, or even bits of Spirogyra or Zygnema 

 still containing plastids, it is convincing to make permanent 

 mounts. To do this the smear is fixed in 8 per cent formalin 

 to which has been added enough copper acetate crystal to give 

 it a pale greenish tinge. This preserves the green color. The 

 mount is then made in glycerine jelly in the usual way and 

 sealed with asphaltum. Mounts made in this way can be used 



