Page:Popular Science Monthly Volume 39.djvu/239

Rh same way. Insects of complete metamorphosis act very differently: the larva becomes stationary and ceases to take food; the skin becomes like parchment, and slight movements of the abdominal rings, when they are touched, are the only signs of life. Yet, from the beginning of the transformation, on the seemingly half-dead being, appear distinctly, although masked by a thick veil, the antennæ, wings, and legs. These organs are symmetrically folded along the body, and the motionless nympha under the veil presents the forms of the perfect insect. It remains sometimes fastened to a wall or hanging from a branch by one of the silky threads which the larva has woven; in other cases it is enveloped in a firm shell or surrounded by a silken cocoon. Nymphæ thus protected remain in the open air, while others, less well guarded, find shelter from bad weather and the direct force of the sun often by being buried deep in the ground.

The insect secures its release from its nymphal envelope at the expense of violent muscular efforts. Sometimes the envelope splits in the back and gives passage in succession to the thorax, the head, the legs, and the wings; sometimes the hinged lid or operculum which closes the shell gives way under the pushing of the prisoner. At the moment of emerging, the young animal is not able to fly; its wings are crumpled and soft, but the circulation soon becomes more active, the wrinkles vanish, and the wings acquire sufficient consistency to permit the new being to take flight.

As we have already seen, the body of the adult insect may be divided into three principal and distinct parts—the head, the thorax, and the abdomen. The head (Fig. 6) bears the mouth, antennæ, and eyes; the three pieces that follow are. the three united rings that constitute the thorax. After the thorax comes the abdomen, the rings of which are movable and capable of gliding upon one another.

The head may be divided into several regions, which it is important to define well. They are, in fact, of great assistance in the description and determination of species. They are four in number: the front, including the space between the eyes; the vertex, or upper part of the head, behind the eyes; the cheeks, below and in front of the eyes; and the epistoma, sometimes called