331 



develops most rapidly, rather than a degree arbitrarily assumed as a 

 "starting point" for development. 



This optimum range of temperature can be determined for any stage 

 in the life-cycle only by a series of preliminary experiments performed 

 at intervals of a few degrees throughout the whole range of temperatures 

 under which the insect is known to thrive. The results of constant- 

 temperature and variable-temperature experiments whose means are com- 

 parable, covering this whole range (with variations in humid ty, etc., 

 carefully controlled so as to accompany variations in temperature in a 

 manner closely approximating that characteristic of average weather con- 

 ditions in the optimum climate for the stage) — the results of such experi- 

 ments, when properly correlated, should give the necessary basis for 

 defining standard conditions. Under these standard conditions, the time 

 required to complete the stage may be taken as a basis for comparing the 

 rate of development at any temperature. 



That range of temperature within which the time to complete the 

 stage is shortened in exact proportion to the rise of temperature is 

 designated as the medial range; that is, within the medial range, the 

 increase in the rate of development bears a fixed ratio to the number of 

 degrees which the temperature rises. For the codling-moth larva in an 

 apple, we find that the medial range is approximately from 55° to 75° F., 

 and that for all other stages, including the hibernated larva, the medial 

 range is approximately from 65° to Ro° F. Medial humidities are those 

 which usually accompany these medial temperatures in normal weather. 



Under such standard conditions a given individual may be con- 

 sidered to accomplish a certain amount of development within one 

 hour, this amount being as standard as the conditions which define it. 

 This reaction of the organism to all these environmental phenomena 

 operating for one hour is to be considered as consisting of a certain 

 number of developmental units, each of which is a small part of the 

 total development making up the stage of the life-cycle. As the rate of 

 development in any given case is dependent primarily, though not 

 entirely, upon the number of degrees of temperature above the actual 

 threshold of development (whatever that may be),* the unit of develop- 

 ment may be determined, under standard conditions, from the difference 

 between the rate at one temperature and that at another temperature one 

 degree! higher ; and this unit is to be defined with reference to all these 

 conditioning factors, each factor being expressed in the terms in which 

 it is commonly measitred. 



The developmental unit is, therefore, the difference between the 

 amount of development taking place in one hour at a given degree 

 of mean medial \ariable temperature and the amount of development 

 taking place in one hour at a temperature one degree higher, with 



• The actual threshold is not a fixed temperature but varies with other conditions. 



t The Fahrenheit scale is used in this paper. The Centigrade scale, which is 

 preferable for several reasons, is used in a book on Experimental Animal Ecology, 

 now in course r.t preparation, to be published in 1927 by Williams and Wilkins Com- 

 pany, Baltimore, Md. 



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