G. S. Graham-Smith 
455 
which the pupae pass the winter reaches a certain point, which differs 
for each species. 
The “critical point” for C. erythrocephala and for F. manicata and 
F. scalaris seems to be about 48-50° F., for Ophyra about 58° F. and for 
Muscina about 63° F. 
Flies emerge from pupae in warm situations earlier than from those 
in shaded situations, but under natural conditions the difference in the 
time of emerging is probably not more than seven to ten days. 
It will be noticed in Table I that after the mean temperature reaches 
48° F. various periods elapse before the flies emerge. It seems probable 
that' a temperature some degrees lower than the “critical point” is 
needed to produce the changes leading up to the “nymph” stage (see 
Hewitt, 1914, footnote p. 105). It will be noticed further that when 
once flies have commenced to appear they continue to emerge in spite 
of falls in the daily mean temperature. 
In the case of C. erythrocephala and other flies isolated specimens 
emerge at various times in the winter. For instance “ walkers ” were seen 
in the outdoor cage on Jan. 21, 26, 28, 29 and 30 when the maximum 
temperatures on the ground were 51°, 48°, 49°, 47° and 43° F. On 
other occasions during the winter, especially on warm days, the numbers 
present had increased though the developing stages were not noticed. 
It seems likely that in these cases the “nymph” stage had been 
reached in the autumn or sometime early in the winter, possibly owing 
to the pupae being in favoured situations, and that the flies were ready 
to emerge whenever the temperature approximated to the “critical 
point.” It should be noted, however, that on many warm days no 
flies emerged. 
The multiplication of flies. 
Howard (1911) (p. 37), speaking of the “possibilities in the way of 
numbers” of flies, makes the following statement. “The number of 
generations has a direct bearing upon the number of flies, not only at 
different periods during the summer, but also in the early autumn, 
since there is, barring accidents, a constant and definite and enormous 
increase. Of course some summers are warmer than others and some 
are moister than others, and upon these two factors, taken in connection 
with that of available places for breeding, the number of flies must 
depend. 
“Take, for example, the possibilities in Washington, and let-us 
estimate—on the basis of the survival of all eggs and all individual 
