PHYSIOLOGICAL  STUDIES.                                    15

IV. PHYSIOLOGICAL STUDIES.

EFFECT OF HUMIDITY ON PUP^.

Elwyn (1917) has studied the effects of differences in humidity on
pupal mortality and on the duration of the pupal stage in Drosophila
melanogaster. He found that the duration of this stage was the same
at humidities of 100 per cent, 64 per cent, and 0 per cent; but the
mortality was greater at lower humidities: 2.5 per cent at 100 per
cent humidity, 12.3 per cent at 64 per cent humidity, 52.9 per cent
at 0 per cent humidity. The mortality was also greater, at the lower
humidities, among pupse isolated very soon after pupation than among
those a few hours older. This was presumably because the pupa-case
did not harden enough to interfere seriously with evaporation until a
few hours after pupation.

EFFECT OF TEMPERATURE ON LENGTH OF LIFE.

Loeb and Northrop (1917) found that temperature affects the dura¬
tion of the larval and pupal stages and the length of life of the adult.
Their later experiments were carried out in bacteriologically sterile
cultures (see below). They concluded that the duration of each of
these stages was halved (or decreased even more than that amount)
by each rise of 10° C. in temperature. This temperature coefficient
is stated to be similar to that observed for many life processes and
for chemical reactions. Loeb and Northrop also found that, at each
temperature studied, the duration of the larval period is about 1.4
times as long as that of the pupal stage, and about 0.2 as long as the
life of the imago. That is to say, different temperatures affect the
different stages proportionately.

NUTRITION.

Henneberg (1902) first pointed out that Drosophila larvse probably
live chiefly on yeast cells. This has been confirmed by Delcourt and
Guyenot (1910, 1911), Guyenot (1917), Loeb and Northrop (1916),
and Baumberger (1917, 1919). The recent workers have all used
bacteriologically sterile cultures. Delcourt and Guyenot obtained
such cultures by the "dilution method" of repeatedly placing females
on fresh sterile media for short periods. They ultimately obtained
cultures in this manner that were sterile by bacteriological tests. Loeb
and Northrop sterilized eggs by washing them with a solution of
corrosive sublimate. Baumberger sterilized puparia by washing them
with 85 per cent alcohol. The investigations of these workers show
that the larvse of D. melanogaster feed principally on yeast, but can
utilize dead yeast as effectively as live yeast. Certain kinds of bacteria
may be substituted for yeast, and a few larvse can be brought to
pupation on sterile banana without either yeast or bacteria.    Powdered