In all cases coccids secrete a protective covering, which assumes different forms; this fact, together with the chief methods of female development, is utilised for the purpose of this work to arrange the coccids under three main types as follows:-- 1. LESS SPECIALISED FORMS.--Examples are the mealy-bugs and cottony-cushion scale, which belong to the more generalised or least specialised representatives. The protective body covering is in the form of a powdery or mealy secretion; the legs and antennæ are retained throughout life, and the insect remains freely mobile. A typical-form life-cycle may be studied in that of the cottony-cushion scale (Figs. 5 and 6a). During development the female insect passes through three larval stages; each of these stages is, on the whole, similar, except for size and minor structural changes, and the white powdery secretion that covers the reddish body of the adult. 2. INTERMEDIATE FORMS.--An example is the olive scale (Fig. 5). In such forms there is a tendency to specialisation, owing to more or less sedentary habits in later life, and protection is afforded by a thickening and toughening of the cuticle on the upper surface of the body. Unlike the cottony-cushion scale, the female olive scale passes through two larval stages; the minute first stage larva is active and very flat; it soon settles upon a leaf and commences to feed, when it becomes much flatter and a little larger; the second stage differs from the first in size and in the development of a dorsal longitudinal ridge, which eventually forms the cross-bar of the two transverse ridges that are characteristic of the third or adult stage, when the insect swells and assumes the shape of the mature form. After settling in the first larval stage, the insect becomes very sluggish, and does not move, except to migrate, as most do, from the leaves to the twigs, there to take up a permanent position. The legs and antennæ are retained throughout life, but in the adult are functionless, being folded against the body; in some species of intermediate forms the appendages become atrophied during development. In the olive scale, and related forms, the toughened cuticle not only serves as a protection to the insect, but also as a receptacle for the eggs (Fig. 5); as these are laid and increase in numbers, the body of the parent diminishes and is crowded against the dome-shaped cuticle. 3. SPECIALISED FORMS.--The apple mussel-scale (Figs. 5 and 7, Nos. 2 and 6) is a representative of this group, the members of which are markedly specialised, the legs and antennæ of the adult female becoming completely atrophied during development, and the shape of the body profoundly altered; protection is afforded by a scale-like covering not attached to the body. In the mussel-scale development there are two larval stages: the first, like all coccids, has the legs and antennæ well developed and is active.