A few other observers noted similar
phenomena, but nothing more was done till Mr. Poulton's elaborate series
of experiments with the larvae of several of our common butterflies were
the means of clearing up several important points. He showed that the
action of the coloured light did not affect the pupa itself but the
larva, and that only for a limited period of time. After a caterpillar
has done feeding it wanders about seeking a suitable place to undergo
its transformation. When this is found it rests quietly for a day or
two, spinning the web from which it is to suspend itself; and it is
during this period of quiescence, and perhaps also the first hour or two
after its suspension, that the action of the surrounding coloured
surfaces determines, to a considerable extent, the colour of the pupa.
By the application of various surrounding colours during this period,
Mr. Poulton was able to modify the colour of the pupa of the common
tortoise-shell butterfly from nearly black to pale, or to a brilliant
golden; and that of Pieris rapae from dusky through pinkish to pale
green.
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