. Animal biology. Zoology; Biology. 284 METAZOAN PHYLA In many cases the larva bears a considerable degree of resemblance to the mature insect, being hatched, however, with a relatively large head and small thorax and abdomen and frequently with no more than rudiments of wings. As successive molts occur the proportions of the body gradually change and the wings increase in size. The stages which follow the successive molts are called instars. The last molt transforms the larva into an adult in which the regions of the body have acquired the adult size and proportions and wings have become of f
. Animal biology. Zoology; Biology. 284 METAZOAN PHYLA In many cases the larva bears a considerable degree of resemblance to the mature insect, being hatched, however, with a relatively large head and small thorax and abdomen and frequently with no more than rudiments of wings. As successive molts occur the proportions of the body gradually change and the wings increase in size. The stages which follow the successive molts are called instars. The last molt transforms the larva into an adult in which the regions of the body have acquired the adult size and proportions and wings have become of full size and func- tional. Thus in this type of insects there is no stage corresponding to the pupa, and the metamorphosis is termed incomplete. Incomplete metamor- phosis (Fig. 190) occurs, generally speaking, in the lower insects, and those which have it are called Hemimetabola (hem i me tab' o la; G., hemi, half, and metabole, change) or Heterometabola (het er o me tab' o la; G., heteros, different, and metabole, change), the latter because metamorphosis is varied in character in different types. Usually the larvae of insects with incomplete metamorphosis are termed nymphs, and sometimes those of such of these as are aquatic, Jiaiads. There is a more pronounced change when a naiad, which carries on aquatic respiration, becomes an air-breathing adult than when the nymph of a terrestrial form changes into the adult insect. A third group of insects, according to development, is Holometabola (ho lo me- tab' o la; G., holos, whole, and metabole, change). These pass through a complete metamorphosis, which includes both a larval and a pupal stage (Fig, 191). The larva of a butterfly or a moth is called a caterpillar; that of a beetle, a grub; that of a fly, a maggot; and those of other groups have still other names. During the larval period organs may develop which are peculiar to that stage in the life history of the animal. At the close of the larval period a pupa, or chrysalis,
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