The Potato Beetle in a Desert 347 



" In both tropical and temperate latitudes, the germ-cells do not develop nor 

 reproduction take place until the conditions of temperature and moisture are 



favorable Likevrise, in the northern United States and Canada, decemli- 



neata may emerge from the ground in April, but the germ-cells do not begin to 

 grow until the coming of the warm moist days in May or possibly June." 



Kammer's (1907) experiments show that Salamandra (maculosa and atra) 

 can be induced in varying ways to lay their eggs, depending in part upon the 

 moisture relation. Jacobs (1909) states for the rotifer Philodina rosela: 



" The period of maximum egg-production had been preceded by a period of 

 desiccation and furthermore, that each desiccation for any length of time has 

 been followed by an increase in the reproductive activity." 



Hennings (1907), working on the bark beetle Tomicus typographus Linn., 

 finds that the amount of water present acted as a regulatory factor for such 

 activities. The results of these investigators show that egg-production may be 

 modified through changed water-relations. 



At Tucson Station A, comparative study of the environmental records indi- 

 cated that when the atmosphere had a high water-content, and when the 

 evaporation rates were low, then egg-laying took place most frequently. This 

 was self-evident, for during the breeding of four generations of the stock at this 

 locality egg-laying occurred during the following periods : July 7 to 19, August 

 4 to 22, 1911 ; June 4 to 15, and July 15 to 29, 1912, which were exactly coinci- 

 dent with the maximum rainy periods at this station. At Tucson Station B the 

 results were similar to those of Station A, since the periods of oviposition were 

 as follows : July 17 to 23, September 3 to 7, 1911 ; June 8 to 20 and July 24 to 

 August 12, 1912, which coincided with high humidities and low rates of 

 evaporation. At the Chicago station, however, the evaporation rates were lower 

 and the egg-laying was controlled by other factors. For the desert complex 

 these results indicated that the optimum for egg-laying was reached when the 

 organism was subjected to a moist medium. 



Since these conclusions were only probable, it was advisable that they be sub- 

 stantiated by further experiment. Therefore, it was necessary to produce 

 artificial differences in the moisture-content of the medium, and observe its 

 effect upon beetles during hibernation and after emergence. 



For the purpose of these experiments, 120 beetles (Tucson A, g. II) were 

 removed from hibernation at 8 p. m. June 19 and were placed in ground-glass 

 stoppered weighing-bottles, so that no moisture could enter. They were removed 

 immediately to the constant-temperature room of the Desert Laboratory, where 

 reactions were tested and all responded positively to light and negatively to 

 gravity. At the same time a sample of the soil surrounding the beetles was 

 taken and found to contain 11.3 per cent of water by weight. The animals were 

 now divided into two lots of 60 adults each (30 of each sex), which were found 

 to weigh 10.007 grams and 9.845 grams, respectively. The first batch was sub- 

 jected to differences in soil-moisture and its effect upon egg-production observed. 

 In the second case the effect of changing evaporation-rates upon oviposition was 

 also observed. 



