346 Relation" of Wateb to the Behaviok of 



furnish favorable conditions for plant growth as well as pupation and hiberna- 

 tion activities. 



The condition of the experiment required that a certain routine be repeated 

 each time, and some of the most ordinary methods follow. The rates of evapora- 

 tion were obtained with the Livingston atmometers, which were cut down to a 

 cone of 50 mm. in length to avoid an error introduced by having shellacked 

 bases. These were standardized on the rotating machine in the Genetics Labora- 

 tory at Chicago, and showed after standardization a maximum range of 3 per 

 cent from the normal. The dry weights of the insects were obtained as follows : 

 first by killing them in potassium cyanide, and desiccating them at a constant 

 temperature in a vacuum over concentrated sulphuric acid until the dry weights 

 became approximately constant. The soil samples when collected were placed 

 in glass-stoppered weighing-bottles and carried to the laboratory, where they 

 were weighed and dried at a constant temperature of 100° C. The tropic reac- 

 tions were tested in the constant-temperature room (18° to 20° C.) and the 

 beetles were exposed in wire-netting tubes (30 cm. long and 5 cm. in diameter) . 

 All geotropic reactions were tested in the dark, and if the animals crawled to 

 the top of the tube when held in a vertical position they were considered positive, 

 and if they moved to the bottom of the tube, negative. The phototropic reac- 

 tions were tested with an ordinary 32 c. p. electric lamp in a constant-tempera- 

 ture room. If the organisms crawled toward the direct rays of light when the 

 tube was in a horizontal plane they were recorded as positive and if they moved 

 away from the source of light as negative. 



ROLE OF WATER IN THE REPRODUCTIVE ACTIVITY. 



A striking fact that one observes in desert biology is that a remarkable degree 

 of coincidence is shown between the rainy season and the reproductive period 

 of the animals native to such a region. Corresponding periods of inactivity for 

 such organisms occur during the dry season. In studying this problem, Tower 

 (1906) finds this is true for most of the species of Leptinotarsa distributed 

 over the American deserts and similar observations were made by Semper 

 (1881) for several desert forms. 



For many years Tower has introduced chrysomelid beetles into desert com- 

 plexes of Arizona from a wide range of habitats, and the majority of these 

 experiments, which were placed under my care, showed that food, enemies, and 

 the like were not the determining factors in the survival of such organisms, but 

 that in most instances the survival was successful if the proper complex for the 

 reproductive activity was attained. 



Frequent observations at Tucson indicated that the optimum breeding 

 activity of the potato beetle was coincident with the highest water-content of tlie 

 medium, since periods of egg-laying were exactly concurrent with those of rain 

 and with the low rates of evaporation. Therefore, it was important to determine 

 experimentally what relations existed between reproductive behavior and changes 

 of water-content within the medium surrounding these animals. 



The literature of the subject contains much data in regard to the effects of 

 temperature upon the reproductive activity, but almost none upon the relation 

 of water to reproduction. In reference to the genus Leptinotarsa, Tower (1906) 

 states : 



