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studies and experiments begun in 1917 and carried on continuously 

 throughout the whole of each year, to and including 1923, in laboratories 

 of the University of Illinois, equipped for the purpose in part by the 

 Natural History Survey. While this paper is, therefore, necessarily 

 technical and aimed especially at an improvement of apparatus and 

 methods of climatological research, the application of conclusions to prac- 

 tical ends has been kept steadily in view, and a summary rehearsal of their 

 uses to horticulture and of the methods of their application follows. 



The weather in its relation to the codling moth is made up of several 

 variable elements, each largely or completely independent of the others in 

 its variation, and all of unequal effect on the life history of the insect and 

 of unequal effect also upon the insect in the different stages of its develop- 

 ment. The most powerful of these variable elements are temperature and 

 humidity, but light, rainfall, air-movement, and rate of evaporation (the 

 rate at which moist objects give up their moisture to the air), are too 

 important to be ignored. By their various degrees and combinations these 

 several elements make up a great number of kinds and gradations of 

 weather v^'hose effects upon insect life can be ascertained only by an 

 experimental variation of each element separately and of various combina- 

 tions of them taken together. Since they are not measurable by any 

 single scale of magnitudes applicable to all of them — differences in heat, 

 light, and humidity, for example, being expressed in different terms — 

 there is no way in which their combined efficacies can be expressed in a 

 single series of numbers except by a study of their joint effects ujxjn 

 the behavior, activities, and rates of development of the insect under 

 examination. 



The most convenient and practically the most important method of 

 such a study is to experiment with artificial variations of these weather 

 elements upon the rate of progress of an insect through its successive 

 developmental stages, and upon the percentage of those in each of the 

 earlier stages which survive to pass on into the next stage. The varying 

 significance to the codling moth of different combinations of various 

 degrees of temperature, humidity, illumination, etc., acting conjointly, 

 may be stated in terms of the average time required under each combina- 

 tion for the hatching of the egg, the growth and pupation of the larva, or 

 the transformation of the pupa to the adult insect ; and the numbers thus 

 obtained may be so tabulated that one knowing the meteorological data 

 of a season up to a certain date may learn by reference to the table just 

 where the insect is in the course of its development at that date, and then 

 calculate the approximate date at which this stage of development will be 

 completed and the insect will pass into the next stage. 



As only two series of meteorological data can be carried on the same 

 table, it has been found most convenient to construct a table of rates of 

 development based on data of the two most potent elements, temperature 

 and humidity, and to apply to the figures of this table any corrections 

 which may be called for by facts concerning the other elements. Such 

 tables and corrective data have been prepared for the codling moth, and 



