The Potato Beetle in a Desert 379 



tability. The results of Shelford (1914) and of Chenoweth (1917), however, 

 agree most closely with those which are recorded in this paper. Both of these 

 observers conclude that a high rate of evaporation increases sensibility or irri- 

 tability through loss of water, a condition which might account for the altera- 

 tions in the reactions of the potato beetle. 



GENERAL DISCUSSION UPON THE ROLE OF WATER IN 

 LIVING THINGS. 



In order to show why such a large number of reactions in the potato beetle 

 were controlled by its water-relation, it was considered necessary to review only 

 literature which bore directly upon these studies. "It was assuredly not 

 chance," to quote Henderson (1913), " that led Thales to found philosophy and 

 science with the assertion that water is the origin of all things." He also states 

 that the action of water now appears to be a momentous factor in geological 

 evolution, and the physiologist has found that water is invariably the principal 

 constituent of living organisms. Thus, according to this observer, water makes 

 up from 70 to 85 per cent of fishes, about 87 per cent of oysters, 85 per cent of 

 apples, 78 per cent of potatoes, and 95 per cent of the edible portion of lettuce. 

 It is interesting in this connection to add that my results upon the potato beetle 

 show it to contain 80 per cent water. Henderson further says that the organism 

 itself is essentially an aqueous solution in which are spread out colloidal sub- 

 stances of vast complexity, and as a result of these conditions there is hardly a 

 physiological process in which water is not of fundamental importance. Accord- 

 ing to Livingston (1903), it is absolutely essential that every living mass of 

 protoplasm be saturated with water, since vital phenomena occur solely in 

 aqueous solutions. 



Physiologists have long recognized that water is of the greatest importance 

 for normal activity of tissues and that all exchanges of material, all supplies of 

 food, and metabolic processes in general are dependent upon it. Aberhalden 

 and Hall (1908) assert that water is absolutely necessary as a solvent for 

 numerous compounds, for it brings into play various chemical reactions, which 

 take part in building up and breaking down substances without number ; it is 

 also a carrier of nourishment to the body and provides the means for the removal 

 of its waste products. In discussing the physical importance of water, Hammar- 

 sten and Mendel (1911) show that water by its evaporation is an important 

 regulator of temperature. Davenport (1897) states that growth is due chiefly 

 to imbibed water, and Estabrook (1910) also demonstrates that growth in 

 paramoecium is due almost solely to inhibition of water. MacDougal (1912), 

 Lloyd (1905), and others demonstrated that many plants absorb water directly 

 from the air. In respect to this subject, in animals the frog has perhaps 

 received most attention, and according to Hill (1908) frogs take up water 

 through the skin ; they do not drink, for a thirsting frog with its gullet tied 

 increases in weight no less than one with the gullet open. He also states that a 

 frog can be gradually dried to less than 39 per cent of its normal weight with- 

 out fatal results. My own data for the potato beetle show that it can be desic- 

 cated to less than 50 per cent and still live, while CataJpa Janigera will die if 

 reduced by 25 per cent and the June bug if dried less than 15 per cent of its 

 normal weight. 



