The Potato Beetle in a Desert 355 



reported, by the present writer (1911, 1912), and these results have been 

 substantiated by Shelford (1914 a, I) and his students, Weese (1917), Hamil- 

 ton (1917), and Chenoweth (1917). The writer's experiments upon the potato 

 beetle and other desert animals (1911) showed that "the fundamental activi- 

 ties of this beetle, as well as those of many desert organisms, are directly con- 

 ditioned by their water-content or water-balance. The water-content of the 

 beetle is determined by the evaporating capacity of the air, the leaf-moisture 

 content of its food plant, soil-moisture, and temperature. Variation in any one 

 of these factors may influence not only hibernation, but other habits and 

 reactions." This work was carried on during the next year (1912), in which I 

 stated regarding the behavior of desert animals that " the proportion of water 

 held in the body, or the water-balance, is correlated with various activities, and 

 the lowering of this balance, or surplus, inhibits several functions or processes, 

 and is also followed by reversed response to various external agencies which may 

 exert a stimulatory action." 



Shelford. (1913) records that certain spiders, ground-beetles, wasps, milli- 

 peds, frogs, and salamanders react in consequence of evaporation, and that a 

 short exposure to evaporation conditions increases sensibility to it. Aside from 

 this experimental data, Shelford and Deere (1913) established laboratory 

 methods for determining the reactions of the above animals to evaporation 

 gradients. My experiments differ from Shelford's in being made under natural 

 conditions out-of-doors, while his studies were undertaken in the laboratory. 



Hamilton (1917) studied certain soil insects, in the full-grown larval and 

 adult state, of the family Carabidae, and his results tended to show that an 

 increase in the rate of air-flow did not effect the larvae as much as did an 

 increase in temperature or a decrease in relative humidity; the adults, more- 

 over, offered greater resistance to evaporation and temperature. The experi- 

 ments upon the horned lizard by Weese (1917) demonstrated a clear-cut reac- 

 tion to the substratum temperature gradient, while the evaporation gradient 

 was not the limiting factor. On the other hand, Chenoweth (1917) concludes 

 that the evaporating power of the air is the best index of environmental con- 

 ditions affecting the white-footed woodland mouse, as well as other land 

 mammals, and that the mice reacted to evaporation whether it was produced 

 by movement, dryness, or heat. 



EXPERIMENTS UPON EVAPORATION. TRANSPIRATION. AND BEHAVIOR. 



Previous experiments upon L. decemlineata show that tropic activities for 

 light and gravity can be reversed through desiccation, and furthermore, that 

 normal reactions are restored if the beetles were surrounded by a moist medium. 

 On the other hand, it seemed important in this connection to perform certain 

 experiments, in order to determine if these insects in nature react to losses of 

 water, which might be produced through desiccation by means of the evaporating 

 power of the air immediately surrounding them. Therefore, it seemed advisable 

 to devise certain tests which would show the daily march of evaporation and 

 transpiration when compared with their behavior. 



The first experiment was made to determine the relation between the daily 

 progress of evaporation and transpiration rates of L. decemlineata when exposed 

 at three different strata, which were produced by an association of potato plants 

 that completely filled the bottom of a cage, 6 feet square by 4 feet high, and 



