The prevailing opinion that the house fly lives through the winter as an
adult, hiding in cracks and crevices of buildings, etc., appears to be
erroneous. Under outdoor conditions house flies are killed during the
first really cold nights, that is, when the temperature falls to about
15° or 10° F. In rooms and similar places protected from winds and
partially heated during the winter flies have been kept alive in cages
for long periods, but they never lived through the entire winter. In
longevity experiments one record of 70 days and another of 91 days was
obtained. No uncaged house flies were found during three seasons'
observations in unheated and only partially heated attics, stables,
unused rooms, etc., where favorable temperature conditions prevailed.
The common occurrence in such places of the cluster fly and a few other
species, which may be easily mistaken for the house fly, is responsible
for the prevailing belief as to the way the house fly overwinters. There
is therefore no reliable evidence whatever that adult house flies
emerging during October and November pass the winter and are able to
deposit their eggs the following spring, although they may continue
active in heated buildings until nearly the end of January. On the other
hand, there is evidence that house flies pass the winter as larvæ and
pupæ, and that they sometimes breed continuously throughout the winter.
In experiments at both Dallas, Tex., and Bethesda, Md., house flies have
been found emerging during April from heavily infested manure heaps
which had been set out and covered with cages during the preceding
autumn. In the Southern States, during warm periods in midwinter, house
flies may emerge and become somewhat troublesome; they frequently lay
eggs on warm days.
The second way in which the house fly may pass the winter is by
continuous breeding. House flies congregate in heated rooms with the
approach of the winter season. If no food or breeding materials are
present they eventually die. However, where they have access to both
food and suitable substances for egg laying they will continue breeding
just as they do outdoors during the summer. Even in very cold climates
there are undoubtedly many places, especially in cities, where house
flies would have opportunity to pass the winter in this manner.