PHYSIOLOGICAL STUDIES. 15 
IV. PHYSIOLOGICAL STUDIES. 
EFFECT OF HUMIDITY ON PUPA. 
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 pup 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 larve probably 
live chiefly on yeast cells. This has been confirmed by Delcourt and 
Guyénot (1910, 1911), Guyénot (1917), Loeb and Northrop (1916), 
and Baumberger (1917, 1919). The recent workers have all used 
bacteriologically sterile cultures. Delcourt and Guyénot 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 larve 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 larve can be brought to 
pupation on sterile banana without either yeast or bacteria. Powdered 
