Economic Importance of Animals 543 



PHYLUM 1 1— ARTHROPOD A (CRAYFISH, LOBSTER, 

 CENTIPEDE, MILLIPEDE, INSECTS, TICKS, MITES, 

 SPIDERS, ETC.) 



The arthropods are so numerous and of so many varieties that a short 

 discussion of their economic importance is quite difficult. The more 

 representative examples of each of the classes of arthropods will be 

 discussed. 



Class Crustacea 



Crayfishes (Figs. 128, 129, 130, and 307) and lobsters are used as 

 food by man and other animals. There are two distinct genera of cray- 

 fishes in the United States: the genus Cambarus east of the Rocky 

 Mountains and Astacus west of the Rockies. 



The materials which crayfishes use as food vary greatly. Probably 

 the materials most abundant and convenient are most frequently used 

 by them for foods. The following have been used as sources of food 

 by different species of crayfishes at various times: dead fish, clams, 

 adult and larval insects, frogs, eggs of salamanders, toads, and frogs, 

 eggs and adults of other crayfishes, dead leaves, such vegetable matter 

 as young bean plants, young corn, potatoes, onions, buckwheat, and 

 many other young plants. 



The materials which lobsters use as food might be listed as follows: 

 long-neck clams, hard-shell clams, conchs, dead and living fishes, eel- 

 grass, etc. 



The enemies of the crayfish include man, certain fishes (especially the 

 black bass), many birds (such as the eagle and kingfisher), certain water 

 snakes, common box turtle, and the larger salamanders. 



The crayfish acts as a scavenger, thus cleaning many pools and 

 streams which otherwise might retain their contained materials. They 

 also injure dikes, dams, reservoirs, and levees by burrowing in them. 

 Rather discouraging and unsuccessful methods for their extermination 

 include drainage of the infested areas, scattering of unslaked lime over 

 the infested area, pouring carbon bisulfide into their burrows. 



Crayfishes have been used extensively for laboratory studies in neu- 

 rology, homology (Fig. 307), reactions and behavior, and habits and 

 activities. They may eventually take the place of our diminishing sup- 

 ply of lobsters as a source of food. 



Certain Crustacea, such as Daphnia (Fig. 134), Copcpods, and many 

 other similar types, are a great source of foods for fishes and other 



