The Potato Beetle in a Desert 



359 



Figure 1 shows the plotted results for the data of this experiment. The broad, 

 heavy lines give the results for stratum A, the broken line, the results at stratum 

 B, and the narrow line those for stratum C. The abscissae represent the 2-hour 

 time intervals, and each unit along the ordinate represents for the evaporation 

 curves, 2.5 c. c. ; for the transpiration curves in terms of entire weight, 3 per 

 cent; for the transpiration curves in terms of dry weight, 13.5 per cent; for the 

 relative humidity curves, 5 per cent; and for the temperature curves 2.5° C. 

 These graphs are interesting in that they show a close agreement between the 

 evaporation-rate and transpiration curve for each stratum. Since the humidity 

 and temperature curves coincide closely for the two upper strata, and the 

 evaporation and transpiration curves for these strata vary in a similar direction, 

 it appears that the rate of loss of water from the animals when exposed to the 

 atmosphere agrees closely with the evaporation rates; i. e., the transpiration 

 curves of these organisms, as Livingston (1906) found with plants, are largely 

 controlled by the evaporating power of the air. 



Table 6. — Summary of rate of evaporation in each stratum combined with loss of 



water from the beetle. 



Figure 2 shows the water-loss in percentage, the evaporation-rates, and the 

 air-temperatures, all reduced to unity. The broad, unbroken line represents 

 the rate of water-loss in each case ; the narrow, unbroken line, the evaporation 

 rate for each stratum; and the broken line, the air-temperatures for each 

 stratum. In making comparisons broadly, the air-temperatures agree in being 

 represented by nearly straight lines, so that they were negligible ; but the 

 evaporation curves and curves of water-loss differ for each stratum, yet are 

 similar when compared. At stratum A, the evaporation curve rises more 

 rapidly and higher than the curve of water-loss, and the drop in the evaporation 

 curve is faster than in the curve of water-loss. At stratum B, the curves of 

 evaporation and of water-loss parallel each other until 6 p. m. when the evapora- 

 tion curve drops more suddenly. At stratum C the increased air-movement is 

 an added factor in the environmental complex, so that both the water-loss and 

 evaporation rates rise much higher, although the temperature curves remain 

 the same. The curve of water-loss at stratum C rises more rapidly and higher 

 than the evaporation curves. The latter drops sooner than the curve for 

 evaporation. This appears to be due to the fact that the beetles are more 

 sensitive to the environmental fluctuations than the porous-cup atmometer, so 

 that difference might account for the lagging effect. 



These differences in the rate of evaporation in the three strata and the water- 

 losses are given in Table 6. This table shows that approximately 1 c. c. loss 

 from the cup is equal to 1 per cent loss of water from the beetles exposed. 



