330 



its color quality have an influence upon the well-being of the codling 

 moth in some of its stages. Light intensity in combination with temjiera- 

 ture practically controls egg-laying of the moths (Isely and Acker- 

 man, '23). 



Methods of Measurement of Factors. 



Combinations of dififerent temperatures and the dilTerent humidities 

 which accompany them must be considered because of the important 

 effects of their correlated action upon rate of development. Since they 

 vary from hour to hour, and since there is no certainty as to what 

 humidity will accompany a given temperature, it is necessary either to 

 take readings at close intervals or to use averages over short periods, with 

 the periods or intervals agreeing for the two factors. 



Records of average temperature and average humidity for each hour 

 of the day are most desirable for careful experimental or observational 

 work, but under ordinary conditions readings at two-hour intervals are 

 sufficiently accurate for estimating the amount of progress of life-history 

 stages. Either of these methods of reading may be applied to hygrother- 

 mograph tracings such as are shown in Fig. 1. The first three columns 

 of Table II (p. 321) show the readings for the solid-line tracings of 

 Fig. 1. 



Daily or monthly means of rainfall, cloudiness, and percent or hours 

 of sunshine may be taken from Weather Bureau records. These are 

 required for showing the effects of autumn and winter rainfall and are 

 considered in connection with mean monthly temperatures. 



Rate of evaporation has been measured as cubic centimeters of water 

 lost per day from the Livingston porous-cup atmomeler. 



No accurate measurements of the quality and intensity of light have 

 as yet been made. In the experiments herein described, the dift'used light 

 of the experimental cages has been compared with total darkness, and 

 the eft'ect of the light of ordinary electric bulbs passed through red, blue, 

 and green glasses has been determined. While evidence has been obtained 

 showing that these factors have effects, it is not yet possible to apply the 

 results to weather conditions because of the lack of accurate measurement 

 both in the experiments and in nature and because of the impossibility of 

 making tenable comparisons.* 



Definitions of Terms. 

 In order to define terms with which to express the effects of all these 

 phenomena of weather and climate upon the rate of development of an 

 organism, we must regard certain conditions as standard and compare 

 all changes in the rate of its development with its behavior under the 

 standard. Obviously, the conditions normal to the habitat of the species 

 should be taken as standard,* and the most important factor in those 

 conditions should be considered first. We may begin, therefore, with tem- 

 perature, using the range of temperature within which the codling moth 



* Experiments with photo-electric cells given promise of some aid in the 

 approach to the problem of the effects of varying light. 



• The ideal standard is described in PART THREE, p. 359. 



