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.