The Potato Beetle in a Desert 349 



Thus it was discovered that the beetles of group A from the moist soil showed 

 a loss of only 0.128 gram, while their reactions were, as before, positive to light 

 and negative to gravity. The beetles of group B, however, from the medium 

 moist soil, showed a much greater loss in weight (0.367 gram), although their 

 responses were unchanged, except in the case of 3 which were positive to gravity. 

 The beetles of group C from the dry soil indicated the greatest decrease in 

 weight (1.123 grams), and showed a reversal in their behavior. 



After this test the beetles were put into separate cages out-of-doors and 

 allowed to breed under natural conditions. A comparison of the rates of 

 evaporation, obtained with Livingston atmometers when placed within these 

 cages, indicated that the environment was uniform. When the activities of 

 these insects were closely observed, the following results were obtained : Those 

 beetles from the wet soil, whose reactions as previously tested, were still positive 

 to light and negative to gravity, moved immediately upward on the potato 

 plants, and began feeding on the uppermost leaves. This indicated that their 

 activities were normal, and on June 30 eggs were laid. On the other hand, 

 those animals from the medium-wet soil also fed on these plants, but no eggs 

 were laid until July 4. This showed that oviposition was postponed 4 days, 

 but that the dry-soil beetles, whose responses were now reversed, were negative 

 to light and positive to gravity. They immediately burrowed into the ground 

 and remained there until the arrival of the summer rains, July 13, when they 

 emerged and laid eggs on July 15. In this case oviposition was delayed 15 days. 

 An analysis of these results follows : 



These experiments showed that differences in soil-moisture produced changes 

 in the water-content of these animals as well as modified their behavior. Since 

 the egg-production was changed, we must conclude that beetles emerging from 

 soils of high moisture-content lay their eggs sooner than those issuing from drv 

 soils. The soil no doubt has played an important role in the economy of desert 

 organisms, which are known to respond accurately to environmental changes 

 such as we have described; otherwise many forms would have ceased to exist 

 where they are now widely distributed in desert regions. 



EXPERIMENTS WITH EVAPORATION RATES. 



The second batch of beetles was used to determine the effect of differences 

 in rates of evaporation upon insects just emerged from hibernation. The 

 apparatus for this experiment consisted of three uniform bell-jars placed over 

 pots of potato plants. Each pot was sunk into adobe soil in the bottom of a 

 vivarium, which was an open inclosure covered with wire netting. One of the 

 bell-jars was provided with 8 atmometers, which, through the evaporation of 

 water-vapor from their surfaces, produced both a high relative humidity and a 

 low rate of evaporation. The second jar was furnished with 2 atmometers, and 

 the evaporation-rate was greater in this jar than in the first ; but the third was 

 kept dry, for, since no water or atmometer whatsoever was used, a high rate of 

 evaporation was secured. The food-plants in this test were kept in a normal 

 healthy condition by the use of automatic soil-watering cups placed in the 

 earth near the bottom of the pots. A few preliminary experiments demon- 

 strated that the dry bell- jar in direct sunshine would become several degrees 

 warmer than the more moist ; consequently a shade was so placed as to give 



