In the Dragon-fly, for example (fig. 147), when the cutis had become expanded to its mature larva condition, it secreted from its surface the external epidermic crust which gives form to the larva (b): this outward integument remains, of course, unchanged when once formed, and retains the same appearance during the whole period of the existence of the insect in its larva state; but underneath this cuticle, and consequently concealed from observation, the growth of the living dermis still goes on, and important organs begin to appear, which had no existence when the last larva-investment was secreted. The wings have sprouted as it were from the shoulders, and already have attained to a certain growth; the whole integument of the larva becomes useless, and a new one is wanted; the process already described is repeated, - the old cuticle becomes detached from the surface of the body, and the cutis begins to secrete for itself a new covering moulded upon its own shape. The newly-formed wings, therefore, and other newly-developed processes of the dermis, secrete horny coverings for themselves in the same manner as other parts of the surface of the body; and thus, when the insect leaves its old skin, and once more escapes from confinement, it presents to view the wing-cases which distinguish the pupa.

(924). Whatever may be the form of the pupa, its covering is secreted in a similar way; it is the living and vascular skin which, though concealed, continually grows more perfect in its parts; and the cases secreted by it at distant intervals correspond in shape with the different phases of its development.

Metamorphosis of the Dragon fly. A, B, eseape of the imago from its pupa case; C, expansion of the, as yet, undeveloped wings.

Fig. 181. Metamorphosis of the Dragon-fly. A, B, eseape of the imago from its pupa-case; C, expansion of the, as yet, undeveloped wings.

(925). After having attained the pupa state, the last steps of the process are completed, and the dermic system becomes fully developed in all its parts. The oral apparatus attains its perfect condition; the wonderfully-elaborate structure of the eyes is completed; the antennse assume their full development; the legs, enclosed in those of the pupa, attain their mature form; and the wings, which have been continually growing, although concealed in the wing-cases of the pupa, acquire their ultimate size: the perfect insect is ready for liberation, and, enclosed in its last covering, creeps out of the water in which it has so long resided, to enter upon a new state of existence. Fixing itself upon some plant in the neighbourhood of its birthplace, the imprisoned Dragon-fly splits its pupa-case along the back (fig. 181, a), and slowly extricates its head and body; it then draws its wings from their coverings, and its legs from those of the pupa as from cast-off boots; and at length (fig. 181, b), getting its body from its now useless covering, it becomes entirely free. The wings, before soft and crumpled, slowly expand (fig. 181, c); the nervures harden; the extended membranes dry; and in a short time the winged tyrant of the insect world (fig. 146) commences his aerial career.

(926). A strong argument in favour of the above views concerning the production of successive skins from the dermis is derived from the phenomena attending the cure of wounds in insects. If a perfect insect be wounded, the wound is never healed at all; and if a larva or pupa is similarly injured, the wound remains uncicatrized until the next moult, when the newly-formed integument is found to exhibit no traces of the injury. The secreted and extra-vascular cuticle cannot cicatrize; but the living and vascular dermis is not only able to repair injuries inflicted upon itself, but, in secreting the next investment, to obliterate all indications of their occurrence.

(927). The changes above described are produced by the progressive development of the dermic or tegumentary system, the parts of which, as we have already seen, becoming strengthened and consolidated by degrees, ultimately acquire that density of structure which the external skeleton of the insect exhibits in its perfect or imago state. But while this extraordinary metamorphosis is going on externally, other changes not less important are in progress in the interior of the body. The size of the alimentary canal, and the shape, proportionate dimensions, and general arrangement of the different parts composing it are secretly and imperceptibly undergoing variations in accordance with the altered necessities of the animal. We have already seen a conspicuous example of this in Lepidopterous insects, § 916; and in other orders equally striking instances might easily be selected. One of the most remarkable is met with in many Hymenoptera, as, for example, in Bees (Apis), Wasps (Vespa), and Ant-lions (Formica leo), as well as in most of the Ichneumonidce. In all these genera, the larva being concealed in a close cell during its development, under circumstances which would render the evacuation of excrementitious matter an obvious inconvenience, both the larva and pupa (fig. 176) are entirely without either intestinal canal or anal orifice, - what little excrement is produced by the digestion of the highly nutritive substances wherewith these larva) are fed being collected in a blind cavity or caecum placed behind the stomach, until the accomplishment of the last change - at which period, the insect, liberated from its confinement, becomes provided with a pervious intestine, and able to get rid of feculent matter.

(928). The fat-mass (§ 919), which at the close of the larva state has reached its maximum of development, is gradually absorbed during the concealment of the insect in its pupa-case, its nutritive portions being no doubt appropriated to the nourishment of the pupa; so that in the mature insect the fatty material has almost entirely disappeared, nothing being left in its place but the dense cellular web in which the fat had been deposited.