CHARACTERS, LIFE-HISTORY, AND METAMORPHOSES OF THE COCCIDIDÆ.
The first principal character separating the Coccididæ from the rest of
the Homoptera, and distinguishable without microscopic examination, is
the absence of wings in the females at all stages of their existence.
The second principal character is the absence of any apparatus for
feeding and digesting in the males.
From these two characters it follows that the females can only extend
their operations by, at the best, crawling from plant to plant, or
by being carried about by birds or other agency; also that the males
cannot enjoy more than a very short existence, their work being
entirely confined to impregnating the females. Hence, in any endeavours
to destroy these insects, the males may be disregarded, and the females
only attended to.
Other distinguishing characters, chiefly microscopic, are--
1. The presence of only one joint in the tarsus or fourth joint of the
leg, in both males and females (Plate i., Figs. 6 and 7, _ta_);
2. The presence of only a single claw terminating the leg in both males
and females (Plate i., Figs. 6 and 7);
3. The presence of only two wings, with two halteres, in the full-grown
males (Plate xxi.);
4. The presence of two or more eyes or ocular tubercles, in addition to
the ordinary pair of eyes, in the full-grown males (Plate i., Fig. 14;
Plate viii., Fig. 1, _k_, _m_).
I. The Female Insect.
In general outward appearance the female insects present very variable
forms. They may be either naked, or covered over with some kind of a
shield, which may be fibrous, or waxy, or cottony, or they may have
simply a thin powdery meal scattered over them. The covered insects
are, of course, stationary, although in some cases, before reaching
their full development, they move about, carrying their houses with
them. The naked insects may be either stationary or active.
They attach themselves either to the bark or stem of a plant or to the
leaves. In the latter case it is rare to find them on the upper side;
but, on turning over a leaf, the under-surface is frequently found
covered thickly with them.
In many cases they exude, in the form of minute globules, a whitish,
thick, gummy secretion, answering probably to the "honeydew" of the
Aphididæ. This secretion drops from them on to the plant, and from it
grows a black fungus, which soon gives an unsightly appearance to the
plant. This fungus or "smut" is an almost invariable indication that a
plant is attacked by insects,[C] and may, indeed, give a useful warning
to tree-growers. It is not, however, produced in appreciable quantities
by all species.
[C] Not necessarily a Coccid insect: the fungus may also grow on the
honeydew of Aphis; but it is easy to recognize the difference between
these insects. In every case there is some insect at work where the
fungus is.
The manner of feeding upon the plant is the same as in all the families
of Homoptera--namely, by means of a protruding rostrum, beak, or trunk,
situated on the under-side of the insect. As there is not, in the
female Coccididæ, any well-defined division between the head and the
rest of the body, this rostrum is seen, on turning over the insect, in
the form, usually, of a minute conical projection between, or nearly
between, the first pair of legs, if the legs are present, or a little
within the circumference, if the legs are absent (Plate i., Fig. 5).
An ordinary lens will generally show, springing from the point of the
conical rostrum, three or four longish, very fine, curling bristles.
These bristles are, in fact, hollow tubes, and the insect, inserting
them into the leaf or bark of the plant, sucks through them its food.
It is thus plain that, with often great numbers of scale-insects
sucking at it--pumping, as it were, its life-blood through their
rostra--a plant must of necessity suffer greatly.
Birds do not, as a rule, seem to care much about eating the Coccididæ,
whose work is thus little interfered with by them. The "white-eye"
(Zosterops) or "blight-bird" has been seen feeding on scale-insects;
but its visits are few and far between, and its assistance to the
gardener in this respect not great. The Coccididæ are, however, much
subject to attacks from Hymenopterous parasites, of which some account
will be found in a subsequent chapter (Chap. IV.).
The effects of the Coccididæ are not confined altogether to damage to
plants: there are some species producing materials useful to man. For
example, _Coccus cacti_ produces cochineal; _Carteria lacca_ produces
shellac; _Ericerus pé-la_ is used by the Chinese for candles: and
others might be mentioned. But, so far, no New Zealand species appears
to be of any commercial use. _Dactylopius alpinus_ makes a rather rich
red dye in alcohol; _Cælostoma zealandicum_ constructs thick, waxy
coverings, which might possibly be turned to some account; but even
these are probably not worth much.
_Groups._
The groups into which the Coccididæ are, in this work, divided are as
follow:--
1. Female insects constructing for themselves shields
composed partly of secretion, partly of the pellicles
discarded from earlier stages; abdomen not cleft; legs
lost at full growth.
Diaspidinæ.
2. Female insects naked, or covered with shields of
secretion, either waxy, horny, cottony, or felted;
abdomen in all stages cleft; legs either lost or
retained at full growth.
Lecanidinæ.
3. Female insects naked, or covered with shields of waxy
secretion; abdomen of larva ending in prominent
processes, abdomen of adult cleft; legs either lost or
retained at full growth.
Hemicoccidinæ.
4. Female insects naked, or covered with secretion either
waxy, cottony, or felted; abdomen in all stages ending
in prominent processes; legs either lost or retained
at full growth.
Coccidinæ.
_Life-history._
The life-history of the insects in the above groups is as follows:--
All of them pass through four stages of existence: 1, the egg; 2, the
young larva; 3, the second stage of life, or "pupa;" 4, the adult, or
full-grown insect.
1. _The egg._ This is, in all cases, of regularly-oval form, the colour
varying from white to yellow or red (see Plate i., Fig. 1). It may be
produced in great numbers, and in some cases several times in a year.
As a general rule, the female ejects the eggs from her body; but there
are some species, notably in the group _Lecanidinæ_, where the eggs are
hatched within the body, the insect being thus, in a manner, viviparous.
2. _The young larva_ (Plate i., Fig. 2). This is of precisely the same
form both for the male and the female--or, rather, perhaps it should be
said that no definite character has yet been discovered to show which
are male and which are female larvæ. Neglecting slight variations of
form, the larva is very minute--seldom more than about 1/30 in. long,
often as small as 1/100 in.--oval, flattish, possessing a rostrum and
accompanying bristles (setæ), six legs, and two antennæ: and in all
species it is fairly active, travelling as soon as hatched over the
plant in search of food.
3. _The second stage._ Here the first distinction is noticeable between
the male and the female in most cases; but this distinction usually
depends not so much upon the form of the insect as upon the character
of the covering it makes for itself. Confining ourselves at present to
the female, there are differences now noticeable between the groups.
In the _Diaspidinæ_ the insect begins by slipping out of the skin of
the larva; but it does not cast it aside altogether: it makes use of
the old skin as part of its covering. Adding to it a small portion
of fibrous secretion--produced by organs called "spinnerets," which
will be noticed presently--it attaches itself to the plant by its
rostrum and setæ, and lies, inert and stationary, under a little shield
composed half of its old skin and half of secretion. As it also, in
entering this stage, loses its legs altogether, it must remain in the
position it has chosen for the rest of its life. In the _Lecanidinæ_
and in the _Coccidinæ_ the skin of the larva is thrown away altogether,
and the female in her second stage takes up a new position, in which
she may be either naked or covered with a thin coat of secretion,
active or stationary, retaining her legs in most cases, or losing
them in some instances. In all the groups there is almost always some
approach to the form of the full-grown insect noticeable in this second
stage.
4. _The full-grown insect._ Here there is almost unlimited variety
of form, colour, and habits. The insects may be naked or covered,
active or stationary. In the _Diaspidinæ_ the process just described
is repeated: the female slips out of her second skin, but still keeps
both it and the first over her, adding more fibrous secretion from the
spinnerets; so that, in fact, she lies an inert, legless, slug-like
object, under a covering composed partly of the two skins, partly of
secretion. (See Plate i., Fig. 3: _a_ is the discarded larval skin, _b_
the discarded skin of the second stage, both being used as part of the
shield. In the genus _Aspidiotus_ these skins would be in the centre
instead of at one end.) In the _Lecanidinæ_ (except in one single
genus) and the _Coccidinæ_ the second skin is discarded altogether;
but the insect may either construct a new shield or remain naked, may
be either with or without legs, either active or stationary. Once this
last stage of her existence entered upon, the female prepares for
laying her eggs. In most species the services of a male are needed; in
some, as far as can be made out after investigation of many years, no
males are found. The female, if naked, either hatches her eggs in her
own body or lays them on the plant; if covered, she fills her shield
with the eggs. The naked insects often cover the eggs themselves--e.g.,
_Lecanium hemisphæricum_; or, again, deposit them in an ovisac, a
mass of cottony secretion--e.g., _Pulvinaria camellicola_ or _Icerya
purchasi_.
II. The Male Insect.
It has been remarked above that, as the full-grown males of the
_Coccididæ_ are destitute of any organs for feeding whatsoever, there
is no reason for making systematic attacks on them for economical
purposes. Their function is simply to impregnate the females, and their
life at this stage must necessarily be very brief. It will suffice
in this place to observe that in all cases these males are small,
two-winged flies, their size varying from about 1/40 in. to 1/4 in.
in length; colour usually yellow or red; wings longer than the body,
hyaline (glassy) and often iridescent, and, in repose, lying flat,
partly crossing each other. The antennæ are long, slender, and hairy,
consisting of nine or ten joints. The legs are also slender and hairy,
the tarsus having only one joint, and terminating in a single claw. The
insects are generally very active. Types of antenna, foot, wing and
haltere, and a diagram of the arrangement of the eyes and ocelli, are
given in Plate i., Figs. 7, 12, 13, 14, 15, 17.
The males are thus so small and rapid in their movements that it is
difficult in most cases to find them in a free state. The usual way to
procure them is by hatching them from the pupæ. In their course of life
they pass through four stages, as do the females--viz.: 1, the egg; 2,
the larva; 3, the pupa; 4, the full-grown insect.
1. _The egg_ is, as far as can be made out, precisely the same as that
of the female, though Dr. Signoret believes that in one or two species
there may perhaps be minute differences.
2. _The larva_ is, as stated above, similar to that of the female.
3. _The pupa._ Here the first distinctions between the sexes may be
noted, and these are principally observable in the cocoons or puparia,
rather than in the insect itself--at least to outward appearance.
The male pupa is, in all cases--even in those where the female pupa
is naked--enclosed in some kind of covering. In the _Diaspidinæ_ the
puparium is formed partly of fibrous secretion and partly of discarded
skin; only, as the full-grown male emerges from it as a fly, and does
not remain on the plant, there can be only one such skin--that of the
larva; consequently it is easy to distinguish the male puparia from
the shields of the adult females by the presence of only one discarded
pellicle instead of two. In the _Lecanidinæ_ and the _Coccidinæ_
the male puparia are distinguishable usually by a narrower and more
cylindrical form than those of the females, where these latter are
covered; in the naked species the males are generally in white waxy or
cottony cocoons.
Examination of the pupæ in these coverings will generally show more or
less developed processes on the back and sides, which are so evidently
the rudiments of the future wings that the presence of a male is
not doubtful. In other respects the male pupæ are not always to be
distinguished from the females.
3. _The full-grown male_ has been described above. It is usually
easy to procure specimens, provided the pupæ are obtained. If any of
these, in their coverings, are put into pillboxes with glass tops, or
any place where light reaches them, they will generally produce the
full-grown insect sometimes in a few days, sometimes after several
weeks. The time of year for this seems very variable. Males emerge from
the puparia apparently indifferently (in New Zealand) in summer or
winter.