EXPERIMENT STATION BULLETINS. 



551 



from the burette into the flask and the latter closed with a rubber 

 stopper. In the meantime the beetle was prepared, weighed and placed 

 in the cage. The precaution of keeping beetles in the open air for one- 

 half hour to an hour before being used in the experiment was found 

 important. Then, the cage (with the insecticide suspended, when it 

 was to be used, as shown in the figure) was lowered into the flask and 

 tlie rubber stopper pushed down tightly. This could be accomplished 

 with but very little interchange of air taking place in the flask, so that 

 solutions of the barium hydrate set away in this manner as checks 



Fig. 2. A simple arrangement for obtaining the CO2 output of insects under tlie influence of certain 



volatile insecticides. 



(containing no insect) could be kept for days without any appreciable 

 precipitate appearing. 



The method of titrating with -^ sulphuric acid has already been 

 described (page 13) and the plan given there has been followed in all 

 similar estimations. 



In one set of experiments several beetles were taken from food at 

 the same time; part of them were used in the air alone, and the rest were 

 used, at the same time, under the same conditions except that they were 

 treated with the insecticide. Grasshoppers were also used in a similar 

 manner. 



The method of recording and figuring the result iof an experiment 

 may be illustrated by a single example. Some of the results are then 

 given in tabulated form. 



