. Animal biology. Zoology; Biology. 280 METAZOAN PHYLA the case of insects with complete metamorphosis, while they are being expanded after emergence from the pupa, tracheae extend into the veins. Around the tracheae are spaces which are extensions of the hemocoel and which convey blood. In growing wings there are also living tissues between the two sheets of surface cuticula. When the wing becomes mature and fully expanded, however, these tissues cease to be living; respiration and circulation in the wing stop; and the append- age becomes a hard, dry structure, which is moved as a whole by mu
. Animal biology. Zoology; Biology. 280 METAZOAN PHYLA the case of insects with complete metamorphosis, while they are being expanded after emergence from the pupa, tracheae extend into the veins. Around the tracheae are spaces which are extensions of the hemocoel and which convey blood. In growing wings there are also living tissues between the two sheets of surface cuticula. When the wing becomes mature and fully expanded, however, these tissues cease to be living; respiration and circulation in the wing stop; and the append- age becomes a hard, dry structure, which is moved as a whole by muscles within the body. There are a great many modifica- tions of wings shown in several of the accompanying figures. Sometimes they are soft and membranous, at other times heavily chitinized and very rigid. Examples of the latter type are the anterior wings of the beetles, known as elytra, and of the locusts, known as tegmina; in both cases these wings serve as protecting sheaths for the folded posterior pair of membranous flight wings when the latter are not in use. In the flies the hind wings are reduced to minute threadlike rods tipped with knobs and known as halteres, or balancers (Fig. 185). Fig. 185.—A crane fly, showing the Insects generally possess tracheae, modified hind wings or halteres. Male, u xi,- ± -u u- i. j. adult. (From Sanderson, -Insect Pests," ^^ brcathmg tubcS, whlch OpCU to after Weed, by permission.) About natu- the OUtside by laterally placed "^^' spiracles (Fig. 181). Spiracles are elliptical openings each guarded by two flaps, which may be closed to prevent the entrance of dust. The number of spiracles varies in different types, though there is only one pair to a metamere. The number of metameres which may have them is 11, including the pro- thorax, the mesothorax, the metathorax, and the first 8 metameres in the abdomen. They are always lacking, however, in one or more of these metameres, and the maximum number present is 10 pairs. The a
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