PHYSIOLOGICAL STUDIES. 15 



IV. PHYSIOLOGICAL STUDIES. 



EFFECT OF HUMIDITY ON PUP/E. 



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 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 per cent humidity. The mortality was also greater, at the lower 

 humidities, among pupae 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 larvae probably 

 live chiefly on yeast cells. This has been confirmed by Delcourt and 

 Guy^not (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 larvae of D. melanogaster feed principally on yeast, but can 

 utilize dead yeast as effectively as live j^east. Certain kinds of bacteria 

 may be substituted for yeast, and a few larvae can be brought to 

 pupation on sterile banana without either yeast or bacteria. Powdered 



