August, ’24] GLASER: REARING FLIES FOR EXPERIMENTAL WORK 493 
Fermenting bran and hay, horse manure, and other materials were 
used for a larval medium, but the horse manure was selected as the best 
and most practical. This must always be obtained in a perfectly fresh 
condition, so as to exlude the possibility of ovipositions by wild flies. 
Unless a culture is started with wild adults early in the sprang, the 
probability is that the preoviposrfcion period and fertilization have been 
completed and feeding of the wild flies may be omittted. The females 
soon seek holes or crevices between particles of manure and crawl into 
these if possible. The ovipositor is then extended and periodically 
undergoes contractive movements which expel the eggs. The time of 
the hatching of the larvae varies with the temperature. At 30°C. or 
35°C., the larvae will hatch within twelve hours after oviposition; if 
below 30°C., it may take twenty-four hours, and if below 20°C., hatching 
may be postponed for a few days. According to Hewitt (1) “there are 
three larval stages or instars, the larva moulting twice during the course 
of its development.” This author gives the following most probable 
minimum time of duration for each of the stages from egg to adult: 
egg stage, 8 hours; 1st instar, 1 day; 2nd instar, 1 day; 3d instar, 3 
days; pupal stage, 3 days; making the total minimum period from egg 
to adult 8 days, 8 hours. Under certain experimental conditions Hewitt 
was able to obtain as short a record as 8 days, 4 hours, but he states 
that this is probably exceptional and that “it is unlikely that in the 
hottest weather development would be accomplished in less than nine 
days. These minimum periods, are the result of a large series of experi¬ 
ments in which it was a more common occurrence to have a batch of 
larvae developing in ten, fifteen or twenty days.” The writer was able 
to verify Hewitt’s observations and obtain 13 to 19 days as the time 
required from egg to adult under temperature conditions varying from 
20°C.-35°C. 
The total durations given above, it must be explained, are those 
from egg to adult and not from adult to adult. The preoviposition 
period, or the period between emergence from the pupal case and the full 
development of the adult male and female reproductive organs followed 
by fertilization, is an important phase in the life of these insects. This 
phase in the life history can only be accurately determined by breeding 
flies for a number of generations. 
In 1923 the writer (2) showed that certain types of food are impor¬ 
tant longevity and reproductive factors for adult flies. To keep adult 
house flies alive long enough to obtain the full development of their 
ovaries and testes it is necessary to feed them sugar or assimilable 
