cts, the
Orthoptera[5], characterised by firm forewings and biting jaws; in all
of them the change of form during the life-history is comparatively
slight. A great contrast to those insects in the structure of the
mouth-parts is presented by the Hemiptera, an order including the bugs,
pond-skaters, cicads, plant-lice, and scale-insects. These all have an
elongated, grooved labium projecting from the head in form of a beak,
within which work, to and fro, the slender needle-like mandibles and
maxillae by means of which the insect pierces holes through the skin of
a leaf or an animal, and is thus enabled to suck a meal of sap or blood,
according to its mode of life. In many Hemiptera--the various families
of bugs both aquatic and terrestrial, for example--the life-history is
nearly as simple as that of a cockroach. It is the family of the
plant-lice (Aphidae) that affords typical illustrations of that
alternation of generations to which reference has been made.

[5] See outline classification of insects, p. 122.

The yearly cycle of the common Aphids of the apple tree has been lately
worked out in detail by J.B. Smith (1900) and E.D. Sanderson (1902). In
late autumn tiny wingless males and females are found in large numbers
on the withered leaves. The sexes pair together, and the females lay
their relatively large, smooth, hard-coated black eggs on the twigs;
these resistant eggs carry the species safely over the winter. At
springtide, when the leaves begin to sprout from the opening buds the
aphid eggs are hatched, and the young insects after a series of moults,
through which hardly any change of form is apparent, all grow into
wingless 'stem-mothers' much larger than the egg-laying females of the
autumn. The stem-mothers have the power, unusual among animals as a
whole, but not very infrequent in the insects and their allies, of
reproducing their kind without having paired[6] with a male. Eggs
capable of parthenogenetic development, produced in large numbers in the
ovaries of these females, give rise to young which, developing within
the body of the mother, are born in an active state. Successive broods
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 bea