604 PHOTOPERIODISM IN INVERTEBRATES 



group was placed. The cabinet was maintained on a long day, and after 

 the appropriate number of hours of illumination for each group had 

 passed, a lightproof cover was placed over the box containing that 

 group. In the evening, in total darkness, all covers were removed so 

 that the photoperiod for all groups began when the lights later turned 

 on automatically. 



Photoperiods in temperature experiments were given as described 

 for critical day length experiments, with three temperature levels 

 employed. Intermediate temperature levels were those obtained in the 

 photoperiod cabinets described above. Low-temperature conditions 

 were provided by walk-in cold rooms, and those for high temperature 

 by a small inside room where temperatures were normally elevated 

 and rather constant. Occasionally an infrared lamp was turned on in 

 this high-temperature room as a supplementary heating unit; it was 

 placed on the floor and directed against a wall opposite the animals. 

 Results at high temperature were the same whether or not the infrared 

 lamp was employed, and it is therefore assumed that its use did not 

 add an important variable to the experiment. Temperatures for all 

 levels were recorded daily from thermometers whose bulbs were im- 

 mersed in a volume of water approximating that in each culture dish. 



Light-intensity experiments were conducted in a large lightproof 

 room located in a frame annex to the main laboratory building. A 

 steam heater and circulating fan controlled by thermostat maintained 

 a mean temperature of approximately 20°C. Temperatures remained 

 fairly constant, but extremes of IS.S'^C and 24 °C were recorded by 

 a monitoring thermograph. The effectiveness of experimental low 

 intensities could be determined by extending a noninductive photo- 

 period of 12 hr to an inductive photoperiod of 13Vi hr. Mirrors, 

 placed on tables at varying distances from a 15-watt clear incandescent 

 lamp, were angled so as to reflect light from the lamp onto the table 

 top. Mounted on the lower side of the mirror frame was a 15-watt 

 fluorescent lamp which provided light for the basic 12-hr photoperiod 

 (40 ft-c intensity). Four stations were employed: A, B, C, and D. 

 Cultures at station B were located so that the distance from the 

 incandescent lamp to the culture dishes (via the mirror surface) was 

 twice that from the lamp to dishes at station A. The same relation 

 held for stations C and B and also for D and C. The experimental 



