of these wingless virgin females (fig. 6 _a_) appear through the spring
and summer months, and as the rate of their development is rapid, often
the whole life-story is completed within a week. The aphid population
increases very fast. Later a generation appears in which the thoracic
segments of the nymphs are seen to bear wing-rudiments like those of the
young cockroach, and a host of winged females (fig. 6_b_) are produced;
these have the power of migrating to other plants. We understand that
wings are not necessary to the earlier broods whose members have plenty
of room and food on their native shoots, but that when the population
becomes crowded, a winged brood capable of emigration is advantageous to
the race.

[6] Such virgin reproduction is termed 'parthenogenesis.'

Many generations of virgin female aphids, some wingless, others winged
when adult, succeed each other through the summer months. At the close
of the year the latest brood of these bring forth young, which develop
into males and egg-laying females; thus the yearly cycle is completed.
Variations in points of detail may be noticed in different species of
aphids. The autumn males and egg-laying females are, for example,
frequently winged, and the same species may have constantly recurring
generations of different forms adapted for different food-plants, or for
different regions of the same food-plant. But taking a general view of
the life-story of aphids for comparison with the life-story of other
insects, three points are especially noteworthy. Virgin reproduction
recurs regularly, parthenogenetic broods being succeeded by a single
sexual brood. A winged parent brings forth young which remain always
wingless, and wingless adults produce young which acquire wings. The
wings are developed, as in the cockroach, from outward and visible
wing-rudiments.