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 abdomens of insects never bear true legs but may possess bristle- 

 like or springing appendages as in some primitive insects (Fig. 189) and in 

 aquatic forms have tracheal gills along the sides or at the posterior 

 end of the body. These gills may be threadUke or leaflike or may be 

 much branched. In them is a network of tracheae between the cavities 

 of which and the water there is an interchange of gases. 



The terminal metameres of the abdomen are often greatly modified, 

 being reduced in size and forming parts which enter into various types 



