100 A. O. WEESE. 



Phrynosoma modestiim is not, essentially, a desert and heat- 

 loving species, although it seems more tolerant of desert condi- 

 tions than P. douglasii. It appears in the greatest abundance in 

 the early summer months, and during the early autumnal rainy 

 season when the aerial temperature does not exceed 32 C., and 

 when the temperature of the substratum in the sunlight is not 

 over 38. During these periods the animals actively move about 

 all day, remaining during the night in protected nooks under 

 bushes or in the burrows of other small animals, or occasionally 

 buried in the loose soil. When the daily maximum temperature 

 becomes greater the period of activity begins earlier in the 

 morning and is terminated toward noon, when the temperature 

 rises. During the heated part of the day the lizard is at rest, 

 nearly, if not quite, covered by the loose soil or sand. The 

 method by which this position is reached is very characteristic. 

 The snout is directed downward and moved from side to side, 

 the body extremely flattened, while the legs take part in a rapid 

 horizontally clawing movement. The net result of this series of 

 motions is to cover the animal with the loose soil, the depth vary- 

 ing according to the character of the soil, the individual, etc- 

 When the temperature is high, sunlight is avoided and burrowing 

 is more likely to take place in the shade of vegetation, a bank of 

 soil or a rock. Other lizards have been observed to burrow in like 

 manner as a means of escape from enemies. 



2. Color and Color Changes. 



The color of the animal generally resembles rather closely the 

 color of the substratum, but this is not always the case. In 

 general, individuals observed after a rain are darker in color (the 

 soil is also darker) and very much lighter on a hot dry day. Ex- 

 periments have shown, in the case of P. douglasii, that a rise in 

 temperature, darkness or an increase in the evaporating power of 

 the air causes a centripetal migration of the melanophoric pig- 

 ment, while the opposite conditions induce a corresponding 

 movement. Within the limits of the changes occurring in the 

 habitat, the variation in the evaporating power of the air is the 

 most potent of the above factors of color change. No direct 

 connection between the color of the animal and that of the 

 substratum has been confirmed experimentally. 



