These pass through no metamorphosis, and also, in the mature condition, are destitute of wings. The young of these insects (Aptera) on escaping from the ovum resemble their parents in all respects except in size; and though they may change their skins frequently, they undergo no alteration before reaching the perfect condition, except that they grow larger.
In the insects belonging to this section there is a metamorphosis consisting of three stages. The young on escaping from the ovum is termed the "larva;" when it reaches its second stage it is called the " pupa," or " nymph;" and in its third stage, as a perfect insect, it is called the "imago." In the Hemimetabola, the " larva," though of course much smaller than the adult, or " imago," differs from it in little else except in the absence of wings. It is active and locomotive, and is generally very like the adult in external appearance. The "pupa," again, is a little larger than the larva, but really differs from it in nothing else than in the fact that the rudiments of wings have now appeared, in the form of lobes enclosed in cases. The " pupa " is still active and locomotive, and the term " nymph " is usually applied to it. The pupa is converted into the perfect insect, or " imago," by the liberation of the wings, no other change being requisite for this purpose. From the comparatively small amount of difference between these three stages, and from the active condition of the pupa, this kind of metamorphosis is said to be " incomplete."
In some members of this section, however - such as the Dragon-flies - the larva and pupa are aquatic, whereas the imago leads an aerial life. In these cases (fig. 175) there is necessarily a considerable difference between the larva and the adult; but the larva and pupa are closely alike, and the latter is active.
These - comprising the Butterflies, Moths, Beetles, etc. - pass through three stages which differ greatly from one another in appearance, the metamorphosis, therefore, being said to be "complete." In these insects (fig. 179) the "larva" is vermiform, segmented, and usually provided with locomotive feet, which do not correspond with those of the adult, though these latter are usually present as well (fig. 179). In some cases the larva is destitute of legs, or is "apodal." The larva is also provided with masticatory organs, and usually eats voraciously. In this stage of the metamorphosis the larvae constitute what are usually called "caterpillars" and "grubs." Having remained in this condition for a longer or shorter length of time, and having undergone repeated changes of skin, or " moults," necessitated by its rapid growth, the larva passes into the second stage, and becomes a "pupa." The insect is now perfectly quiescent, unless touched or otherwise irritated; is incapable of changing its place; and is often attached to some foreign object. This constitutes what - in the case of the Lepidoptera - is generally known as the "chrysalis," or "aurelia" (fig. 179). The body of the pupa is usually covered by a chitinous pellicle, which closely invests the animal. In some cases (e.g., in many Dipterous insects) no traces of the future insect can be detected in the pupa by external inspection ; but in the Lepidoptera the thorax and abdomen are distinctly recognisable in the pupae; whilst in others (e.g., Hymenoptera) the parts of the pupa are merely covered by a membrane, and are quite distinct. In some cases the pupa is further protected within the dried skin of the larva; and in other cases the larva - immediately before entering upon the pupa-stage - spins, by means of special organs for the purpose, a protective case, which surrounds the chrysalis, and is termed the "cocoon."
Fig. 179. - Metamorphosis of the Magpie-moth (Phalaena grossulariata).
Having remained for a variable time in the quiescent pupa-stage, and having undergone the necessary development, the insect now frees itself from the envelope which obscured it, and appears as the perfect adult, or "imago," characterised by the possession of wings.