452 Observations on Habits and Parasites of Common Flies 
but on cold, wet and sunless days “ walkers ” and grey flies may be found 
in tlie afternoon. In the late summer flies sometimes emerge in the 
evening, and in the autumn “walkers,” which emerge after frosty 
nights, often take all day to develop into coloured flies. 
Conditions influencing emergence. 
Observations and experiments made in the spring months of 1915 
and 1916 indicate that temperature is the factor of greatest importance 
in causing flies to emerge from winter pupae. 
Some data for estimating the necessary temperatures are available 
from the records of the temperatures in the outside cage and in the 
“ tins ” during the spring of 1915. In the “ tins ” the daily maximum and 
minimum temperatures were recorded, and in the cage the maximum 
and minimum temperatures on the ground, in a situation shaded from 
the afternoon sun, and the temperature at a depth of two inches below 
the surface by means of a thermometer three feet long, with a protected 
stem, originally made for recording the temperatures in clothing during 
sterilization. The earth temperatures close to the cage at depths of 
one and two feet in exposed and shady situations were also recorded. 
Chart 1 shows in the upper part the mean daily temperature in “ Tin I ” 
in the shade and the numbers of flies of different species which emerged. 
Most of the pupae were lying loosely in and on the earth at the bottom, 
and were subjected to the influence of this temperature. It will be seen 
that the majority of the blow-flies emerged when the mean temperature 
reached approximately 48° F. and remained at or above that level, and 
that F. scalaris emerged slightly earlier. II. dentipes emerged a little later, 
possibly influenced by a decided rise of temperature of short duration 
between May 6 and 9, and M. stabulans did not emerge till the mean 
temperature reached 63° F. 
The lower portion of the chart shows the emergence of flies in the 
cage and records the daily mean temperatures on the surface and at 
a depth of two inches, and the mid-day readings of the one foot earth 
temperatures in the sun and in the shade. It will be noticed that 
these readings are usually separated by several degrees. Most of 
the pupae were buried at a depth of one to two inches and were 
therefore subjected to the variations in temperature occurring at that 
level. In the cage the majority of blow-flies emerged when the mean 
two-inch temperature reached 48° F. and remained above that level, 
about a week after those in the “ Tin I.” F. manicata also appeared 
