Gr. S, GrRAHAM-SMITH 
465 
age and partly to wet weather. The autumn mortality is more notice¬ 
able than the summer mortality because, owing to the fall in the ground 
temperature, few flies are emerging to replace the dead. 
From the observations made in 1915, the following tentative con¬ 
clusions seem permissible in regard to the natural summer generations of 
blow-flies; that the normal duration of life is about 30 days as judged 
from the observations on the first and second generations; that under 
natural conditions many of the newly emerged flies die within a few 
days; that a considerable mortality often follows wet, cold and windy 
days; that oppressive sultry weather is productive of the greatest 
mortality, and that flies are very susceptible to very hot sunshine. 
Flies bred under favourable conditions indoors can be kept alive 
for some weeks. 
Bloiv-flies in winter. 
From Dec. 1, 1915 to the end of February, 1916, blow-flies were 
present in small numbers in the open-air cage. Occasional deaths 
occurred and solitary individuals emerged from time to time. It is 
unnecessary to follow the weather conditions throughout this period, 
or to state the numbers of flies present daily, but their resistance to 
adverse climatic conditions may be illustrated by the history of a few 
individuals during February. 
On February 3 four flies, the survivors of seven which emerged 
between January 26 and 30, were present in the cage, and all were still 
alive on February 13. During this period the minimum temperature 
on the grass was below freezing point on nine occasions, and once fell 
as low as 18° F., and 0-85 inch of rain fell. On Feb. 13 three other flies 
emerged, and were seen in their early stages before their wings had 
expanded. On the same day a severe gale began, which lasted till 
Feb. 17, and sufficient snow fell on the night of Feb. 14 to make a 
considerable rent in the roof of the cage. Before this could be mended 
four out of the seven flies had disappeared. The remaining three flies 
were observed daily and lived till Feb. 28th, surviving the worst weather 
of the winter. During the night of Feb. 24 7| inches of snow fell, the 
heaviest fall in the district for several years, and lesser quantities fell 
on three other days together with 2-48 inches of rain. On Feb. 29 one 
of the flies died, on March 7 another died, and the third on March 11. 
These flies had very little shelter, but took advantage of the little 
that could be found by occasionally altering their positions from one 
side of the cage to the other, and survived some of the worst weather 
experienced here for some years. 
