318 



"threshold" usually assumed. It is perhaps least successful in the spring 

 of the year, when there is greatest need of reliable prediction in fixing 

 spraying schedules for control of the codling moth. 



The older "degree-day" method (Simpson '03) fails to give depend- 

 able results. Glenn's method is not sufficiently accurate to enable one 

 to evaluate the effects of factors other than temperature, and hence it is 

 most likely to fail in unusual seasons. A new method is needed, 

 therefore, which will take into account the ei¥ects of all variations 

 of all these factors in units of time shorter than the day. The method 

 herein described aims to meet this need by using the amount of de- 

 velopment accomplished in one hour as the basis of calculation.. This 

 amount is a small fraction of the total development which makes up 

 the stage in the life-cycle of the insect. The new unit is called the 

 developmental unit and is to be defined with reference to the total 

 development of which it is a part. It is not a "degree-hour," for it is not 

 a measure of external conditions, but a measure of the response of the 

 organism to those conditions. This response, moreover, is modified by 

 other factors besides temperature ; and so the developmental unit, taking 

 into account all the phenomena affecting the process, is to be thought of 

 properly as the effect of a "phenomena-degree-hour." (See pheno-hour, 

 p. 332.) 



Definition of the unit of development. The developmental unit, to 

 be more specific, is the effect of one degree of mean medial variable tem- 

 perature, operating for one hour in conjunction with mean medial 

 variable humidity and with the air movement, light intensity, and other 

 conditions normal to the habitat of the organism. In the case of insects 

 and many other organisms whose development cannot be measured 

 directly, this effect is best calculated in terms of the total time required 

 to accomplish the stage of the life-cycle under consideration ; for this 

 time is shortened in direct proportion to rise of temperature within media] 

 limits, so that the difference between the time required at a certain degree 

 and the time required at another degree may be taken as a measure of 

 the difference in amounts of development accomplished at those two 

 temperatures. The developmental unit, for any stage in the life-cycle of 

 the insect, is, therefore, defined as the difference in amount of develop- 

 ment produced in one hour by a difference of one degree of mean medial 

 variable temperature (other conditions being average), as shown by the 

 difference in time required to complete the stage. 



Developmental totals. The pupal stage of the ccdling moth, for 

 example, which was considered by Glenn ('22) as requiring an average 

 of 265 "degree-days" for complete development under normal conditions 

 in Illinois, is here considered as normally consisting of 6,480 developmental 

 units (hour units), this new total being the result of calculations based 

 on data covering ten years of observation and experimentation, including 

 Glenn's original data. The "degree-day" total took no adequate account 



