Several of the insects mentioned in this survey, like the last-named codling moth, are occasionally double-brooded.  As an example of the many Lepidoptera, which in our islands have normally two complete life-cycles in the year, we may take the very familiar White butterflies (Pieris) of which three species are common everywhere.  The appearance of the first brood of these butterflies on the wing in late April or May is hailed as a sign of advanced spring-time.  They pair and lay their eggs on cabbages and other plants, and the green hairy caterpillars feed in June and July, after which the spotted pupae may be found on fences and walls, attached by the silken tail-pad and supported by the waist-girdle.  In August and September butterflies of the second brood have emerged from these and are on the wing; their offspring are the autumn caterpillars which feed in some seasons as late as November, doing often serious damage to the late cruciferous crops before they pupate.  The pupae may be seen during the winter months, waiting for the spring sunshine to call out the butterflies whose structures are being formed beneath the hard cuticle.

Reviewing the small selection of life-stories of various Lepidoptera just sketched, we notice an interesting and suggestive variety in the 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