wintering stage. The vanessid butterflies hibernate as imagos; the
'vapourer' winters in the egg, the magpie as a young ungrown larva, the
'tiger' as a half-size larva; the Agrotis caterpillar feeds through the
winter, growing all the time; the codling caterpillar completes its
growth in the autumn, and winters as a full-size resting larva; lastly,
the 'whites' hibernate in the pupal state. And in every case it is
noteworthy that the form or habit of the wintering stage is well adapted
for enduring cold.

Our native 'whites' afford illustration of another interesting feature
often to be noticed in the life-story of double-brooded Lepidoptera. The
butterflies of the spring brood differ slightly but constantly from
their summer offspring, affording examples of what is called _seasonal
dimorphism_. All three species have whitish wings marked with black
spots, larger and more numerous in the female than in the male. In the
spring butterflies these spots tend towards reduction or replacement by
grey, while in the summer insects they are more strongly defined, and
the ground colour of the wings varies towards yellowish. In the
'Green-veined' white (_Pieris napi_) the characteristic greenish-grey
lines of scaling beneath the wings along the nervures, are much broader
and more strongly marked in the spring than in the summer generation,
whose members are distinguished by systematic entomologists under the
varietal name _napaeae_. The two forms of this insect were discussed by
A. Weismann in his classical work on the Seasonal Dimorphism of
butterflies (1876). He tried the effect of artificially induced cold
conditions on the summer pupae of _Pieris napi_, and by keeping a batch
for three months at the temperature of freezing water, he succeeded in
completely changing every individual of the summer generation into the
winter form. The reverse of this experiment also was attempted by
Weismann. He took a female of _bryoniae_, an alpine and arctic variety