384 DAVID E. FINK. 



the surface of the soil in the breeding house or in the field (Fig. 

 i). In all cages it was impossible for beetles to escape or for 

 others to find entrance from the exterior. Access for the worker 

 was by a convenient opening on the top or side of the cage. 



The water content was obtained by first weighing and slitting 

 the animals and afterwards placing them in an oven at a temper- 

 ature between 90-95 C., until a constant weight was obtained. 

 The percentage of water was computed on the basis of dry weight. 



Although the extraction of fat from the body of an organism 

 with solvents cannot be achieved quantitatively without change 

 in the nature of the compounds in which the fatty acids are 

 present, this was not considered an objection in these experiments 

 as the result desired was the total fat content in whatever form 

 it might occur. The tissues, however, had to be dried and 

 powdered. To prevent partial oxidation of the more unstable 

 unsaturated acids, the drying took place at room temperature, 

 or in an oven at a temperature not exceeding 30 C. The 

 extracting solvent used extensively was a mixture of equal 

 proportions of ether and alcohol, which was warmed and renewed 

 several times by decantation. In addition, the method described 

 by Voltz (28), and modified to suit the material under investiga- 

 tion was also used. Calculations were made in percentage based 

 on dry weight of the organism. 



Metabolism determinations (respiratory exchange) were ob- 

 tained by methods essentially similar to those described by the 

 author (12) in a previous publication. Other methods used will 

 be described in the text under appropriate headings. 



EXPERIMENTS AND DISCUSSION. 

 Food Experiments. 



The effect of different food plants on potato beetles of the first 

 generation emerging from pupation was studied. Altogether, 

 during June, 1923, and 1924, about 800 beetles were divided 

 into four main groups and fed on various food plants as follows: 



Group i. The foliage of potato, tomato, eggplant and peppers. 



Group 2. The tuber3 or fruit of the above plants. 



Group 3. The foliage of potato for six days, followed by sliced 



