CHAPTER II.
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.