239 
The variations in the total of effective day-degrees in the cases of 
individuals shown in the above table arises from several causes. Some 
of the extreme variations resulted, no doubt, from inaccuracies in obser- 
ions; but for the most part they resulted from the use of the day as 
e unit, from which cause the recorded period may exceed or fall short 
of the true period by almost a day. Since in midsummer the daily accu- 
‘ulations may reach 27 effective day-degrees, the variations from this 
cause alone may be from 136 to 190. The variations from this source 
are so large that they overshadow all the lesser ones and make it impos- 
sible to estimate the errors which probably arise from them. This source 
of error might have been avoided by using a smaller unit of time. There 
is also a difference in eggs as to the length of time required for the 
cubation period. Moisture, evaporation, and light also may have a 
pene influence on the rate of development. 
_It is probably true that variations arising from individual differences 
are comparatively slight and that the wide variations shown in the tables 
are due to the first two causes above mentioned. The extreme variations 
py therefore be regarded as of little significance and the average of the 
arge number of observations may be relied upon as being approximately 
the true heat factor. 
of” RELATION OF TEMPERATURE TO DEVELOPMENT OF THE LARVA 
The conclusions arrived at are based upon observations made on 
44 larve reared in apples on the tree and 214 larve reared in picked 
apples. It was found that larve reared in picked apples had a shorter 
period relative to the temperature than those reared in apples on the tree. 
_ These data were first tabulated in detail and the periods and mean 
daily temperatures averaged for each of the observations, then for groups 
of larve which began the period on the same day, and lastly for groups 
al ranged according to average mean daily temperatures.’ A summary of 
the results thus obtained from the observations on larve reared in apples 
on the tree is given in Table 3. 
