REACTIONS OF THE HORNED LIZARD. 



degrees above this. Unfortunately, in the apparatus available a 

 higher air blast temperature than 43 could not be obtained. 



The reaction to the temperature of the substratum, indepen- 

 dent of the temperature and rate of movement of the air above 

 and consequently independent of the evaporating power is the 

 most definite and well defined response noted in the series, much 

 more so than that in the air temperature and humidity experi- 

 ments and must, therefore, represent an important factor in the 

 daily and seasonal life of the animals. It is probable that this, 

 and not the evaporating power of the air furnishes the most 

 important direct stimulus determining the distribution and daily 

 movements of the species. The surface of the soil in the Rio 

 Grande valley region is cooler than that in the mesa habitat on 

 account of its greater moisture content, and that in the mountains 

 is cooler for this reason and also on account of the climatic vari- 

 ations due to higher altitude. The rise of temperature during 

 the day stimulates the animal to burrow as a means of escaping 

 the excessive heat. At least the surface temperature is of very 

 great importance, as shown by the exceedingly consistent and 

 regular behavior of the animals when this feature of their envir- 

 onment is varied. Unfortunately, but a single specimen of 

 P. douglasii was available at the time the preceding experiments 

 were conducted, and this one was taken at the same place as 

 those of the more abundant species, i. e., on the mesa. It would 

 be of great profit to repeat this experiment with individuals of 

 various species taken in entirely different habitats. 



III. CONCLUSIONS. 



1. The optimum evaporation rate of air for Phrynosoma mo- 

 destum lies between 1.5 and 3 c.c. per hour, as measured by a 

 standard atmometer. This is approximately the average eva- 

 poration rate of outdoor air in the natural habitat of the species, 

 in sunlight, with a light breeze and a temperature of 30. The 

 reaction of the animals to a humidity gradient is not definite 

 unless the gradient be a steep one. Daily variation in the habitat 

 of the species is rather large. 



2. The reaction to an air temperature gradient is more definite 

 than the preceding. As temperature varies, the evaporating 



