Changes in the Life-cycle 313 
and the process appears to be reaching its limit, the individuals 
may be rejuvenated by altering the composition of the solution 
in which they live. They will then once more begin to divide 
actively and start a new cycle or a new phase of the old cycle. 
After a time under a constant environment their activity may 
again begin to fall off, when a new change may rejuvenate them 
once more. The precise effects produced are not known; for 
it has been found that a change that affects rejuvenescence at 
one time will not do so at another, and a different solution must 
be used. The conclusion seems near at hand that conjugation 
between individuals that have lived under different environments 
may effect the same result in the same way as a change in the 
environment itself. The problem may be, however, more com- 
plicated than this. 
Influence of Food on the Life-cycle of Lepidoptera 
The caterpillars of moths and butterflies undergo a series of 
moults as they grow larger, and during this time they consume 
a very large amount of food. The length of the larval life 
varies enormously. Thus according to Pictet the caterpillar 
of Argynnis paphia takes 15 to 20 days from egg to chrysalis. 
The caterpillars of the Vanessas take three to four weeks; 
Ocneria dispar and Saturnia pavonia two and a half to three 
months; Lasiocampa quercifolia eleven months (of which five 
are in hibernation); Cossus cossus holds the record with a 
larval life of two to three years. 
The pupa or chrysalid stages also have various periods. 
Certain Vanessas take 12 to 18 days, and Ocneria dispar, Lasio- 
campa quercifolia, and Cossus cossus take 18 to 28 days. Satur- 
nia pavonia and S. pyri that become inclosed in the spring 
generally pass two winters as chrysalids or pupe. The length of 
life of these stages is, however, subject to great variation. ‘The 
amount of food obtainable is an important element in the result. 
Lasiocampa quercus hatches from the egg in the summer. 
The larva remains active till about the first of November, hiber- 
nates in the caterpillar stage until the end of April, when it 
