TYPE TB ACHE ATA. 507 



apparatus is termed the " mask." Respiration is carried on 

 by tracheal gills, consisting in Agrion of three leaflike pro- 

 cesses situated at the posterior end of the body, and also by 

 the terminal portion of the intestine, into which water is 

 taken and which is abundantly supplied with tracheae. The 

 water can be forcibly expelled from the intestine, serving to 

 propel the insect through the water if it so desires. The 

 metamorphosis is incomplete. 



5. Order Plecoptera. 



The Plecoptera, or Stone-flies (Fig. 236), are found in the 

 vicinity of water and have a somewhat elongated body, fre- 

 quently terminating in two long 

 cerci (Perla). The antennae are long 

 and filiform and the mouth-parts 

 adapted for biting, while the legs 

 are strong and used for walking. 

 Two pairs of wings occur almost 

 equal in size, but lacking the com- 

 plicated venation found in the Odo- 

 nata, and when at rest lie flat upon ^la. 286.-Stonb-j'lt. Perla. 

 the abdomen, completely concealing it. The larvse are 

 aquatic, and are usually to be found in considerable numbers 

 under stones in swiftly-running streams. They recall the 

 Thysanura in their appearance, and possess tracheal branchiae 

 on the under surface of the thorax, which in some forms are 

 retained in the adult. The metamorphosis is gradual or in- 

 complete according as these structures are or are not retained 

 in the imago. 



6. Order Corrodentia. 



The members of this group possess biting mouth-parts 

 and are sometimes destitute of wings. The Termites, or 

 White Ants, live in colonies and sliow a polymorphism. The 

 males and females, termed kings and queens (Fig. 237, A, B), 

 are at first provided with large wings resembling those of the 

 Plecoptera, but after the marriage flight settle to the ground 

 and become wingless. The workers select from the many pairs 



