PHYSIOLOGY: WEB AND NORTHROP 
385 
the three stages is approximately constant for all temperatures as far as 
our experiments go. The ratios are given in table 2. 
The values for the ratios at 15°, 20°, and 25° agree very well, while 
at 30° the value for the imago falls off; one of the cultures at 30° gave, 
however, the value of 0.22. 
We may, therefore, say that as far as our present experiments go the 
ratio of the duration of life of the insect to the duration of the larval stage 
is approximately constant for all temperatures and that the same is true for 
the ratio of the larval to the pupa stage. 
If it is permissible to consider the production of a hormone as the 
limiting factor for the duration of the larval stage we face the pos- 
sibility that the production of a hormone (or a poison) may be also the 
TABLE 1 
Duration of the Three Stages oe the Liee oe Drosophila in Days 
STAGES 
10° 
15° 
20° 
25° 
30° 
31.5° 
Larva. . . . 
56.8 ±1.1 
18.0 ±0.2 
8.38 ±0.04 
6.32 ±0.04 
4.62 ±0.04 
4.9 ±0.05 
Pupa. . . . 
Does not 
13.5 ±0.2 
6.1 ±0.1 
4.22 ±0.03 
3.43 ±0.04 
hatch 
Imago 3 . . . 
40.0 ±0.9 
28.5 ±0.2 
13.7 ±0.4 
Total length of life 
54.5 
39.0 
21.7 
TABLE 2 
Ratio of Duration of the Three Stages 
STAGES 
15° 
20° 
25° 
30° 
1.31 ±0.03 
1.37 ±0.03 
1.49 ±0.02 
1.35 ±0.02 
0.21 ±0.2 
0.22 ±0.1 
0.33 ±0.1 
limiting factor for the duration of the imago stage and the duration of 
a normal life in general. 
On the basis of this assumption we can readily understand that by 
inadequate feeding of the larva the duration of the larval stage may be 
prolonged since the inadequate feeding may delay the formation of 
the hormone responsible for metamorphosis. It would be wrong to 
state that the duration of the larval stage is determined by the energy 
value of the food taken up by a normal larva since in the frog at least 
the larval period can apparently be prolonged indefinitely if certain 
hormones are lacking though the larva may grow considerably during 
this period. In the experiments on parthenogenetic frogs one of us 
has twice obtained a tadpole unable to metamorphose in a year or in 
a year and a half. One of these had grown beyond the normal tadpole 
