THE LIFE-STORIES OF SOME SUCKING INSECTS
We may now turn our attention to some examples of the remarkable
alternation of winged and wingless generations in the yearly life-cycle
of the same species, mentioned at the end of the last chapter.
Cockroaches and grasshoppers belong to an order of insects, 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 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.
[Illustration: Fig. 6. Apple Aphid (_Aphis pomi_), virgin females, _a_,
wingless; _b_, winged. Magnified 20 times.]
A family of Hemiptera, related to the Aphidae and equally obnoxious to
the gardener, is that of the Coccidae or scale-insects. These furnish an
excellent illustration of features noticeable in certain insect
life-histories. In the first place, the newly-hatched young differs
markedly from the parent in the details of its structure. A young coccid
(fig. 7 _c_) is flattened oval in shape, has well-developed feelers
(fig. 7 _d_) and legs, and runs actively about, usually on the leaves or
bark of trees and shrubs, through which it pierces with its long jaws,
so that it may suck sap from the soft tissues beneath. After a time it
fixes itself by means of these jaws and the characteristic scale or
protective covering, composed partly of a waxy secretion and partly of
dried excrement, begins to grow over its body. The female loses legs and
feelers, and never acquires wings, becoming little more than a sluggish
egg-bag (fig. 7 _e_). The male on the other hand passes into a second
larval stage in which there are no functional legs, but rudiments of
legs and of wings are present on the epidermis beneath the cuticle, as
shown by B.O. Schmidt for Aspidiotus (1885). The penultimate instar of
this sex in which the wing-rudiments are visible externally lies
passively beneath the scale, its behaviour resembling that of a
butterfly pupa. The adult winged male (fig. 7 _a_) leads a short, but
active life.
[Illustration: Fig. 7. Mussel Scale-insect (_Mytilaspis pomorum_). _a_,
male; _b_, foot of male; _c_, larva, ventral view; _d_, feeler of larva;
_e_, female, ventral view. After Howard, _Yearbook U.S. Dept. Agric._
1904. Magnified, _a, c, e_ x 20; _b, d_ x 120.]
Another family allied to the Aphidae is that of the Cicads, hardly
represented in our fauna but abundant in many of the warmer regions of
the earth. Here also the young insect differs widely from its parent in
form, living underground and being provided with strong fore-legs for
digging in the soil. After a long subterranean existence, usually
extending over several years, the insect attains the penultimate stage
of its life-story, during which it rests passively within an earthen
cell, awaiting the final moult, which will usher in its winged and
perfect state.
In the life-histories of cicads and coccids, then, there are some
features which recall those of the caterpillar's transformation into the
butterfly. The newly-hatched insect is externally so unlike its parent
that it may be styled a larva. The penultimate instar is quiescent and
does not feed. But while the caterpillar shows throughout its life no
outward trace of wings, external wing-rudiments are evident in the young
stages of the cicad. In the male coccid we find a late larval stage with
hidden wing-rudiments, the importance of which, for comparison with the
caterpillar, will be appreciated later.