CLASSES OF ANIMALS 



451 



3. Class Arachnida. Most of the true spiders are useful be- 

 cause of their constant warfare upon insects. The ticks are 

 altogether parasitic ; the mites are responsible for the itch, for 

 sheep scab, for the chicken-mite disease, and for damage to 

 other domestic animals, as well as to greenhouse and orchard or 

 field plants. In most cases the best thing for protecting plants is 

 powdered sulfur. The jigger, or harvest mite, usually lives upon 

 plants, but sometimes makes itself at home on human beings. 



4. Class Insecta. The insects constitute the most numerous 

 class of animals. The many species show^ adaptations to a 



Fig. 185. How the lobster breathes 



The featherlike gills of these crustaceans are protected by an extension of shell which 



incloses them almost completely. By the action of appendages connected with the 



mouth organs a constant current of water is made to pass over the gills through the 



space under the shield, moving from the back edge forward 



remarkably wide range of conditions. From the human point 

 of view they furnish many enemies, being in a sense our keen 

 competitors for the possession of the earth (see Chapter XLIX) ; 

 but we have among them also a few good friends. 



In certain parts of Africa and Asia, as well as in South 

 America, Mexico, and Central America, the natives are said to 

 use various species of locust and caterpillar as food. Ants and 

 termites, cicadas, the grubs of beetles, and the eggs of water 

 beetles are also consumed. The Chinese sometimes eat the pupa 

 of the silk moth after the silk has been removed from the 

 cocoon. In so-called civilized countries the only insect that 

 supplies food to man is the honeybee. 



