APPLIED ENTOMOLOGY 



forming an outside shell or external skeleton (exo-skeleton), there being 

 practical^ no internal supporting structures except as ingrowths from 

 the outside. In the possession of this external skeleton these animals 



have a seeming resemblance to the 

 shells (Mollusca), but the materials 

 of which it is composed are quite 

 different, being largely calcium car- 

 bonate in the Mollusca, and chitin 

 which somewhat resembles horn in 

 its nature, sometimes with calcareous 

 salts deposited in it, in the Arthro- 

 poda. In its simplest members the 

 Arthropod body is also practically 

 bilaterally symmetrical, though this 

 condition is concealed somewhat by 

 secondary changes in many of the 

 group. The possession of a bilaterally 

 symmetrical body consisting of a 

 series of segments; an exoskeleton of 

 chitin, and the presence of jointed 

 legs, are then, distinctive features of 

 the Arthropods. 



To separate the various groups of Arthropods, other characters must 

 be used. Aside from several small sections not often seen, there are five 

 large and important divisions which call for recognition. These are the 

 Crustacea, including the lobster, crab, beach flea, sow bug and many 



Fig. 1. — Crayfish (Crustacea); about 

 one-half natural size. (Original.) 



Fig. 2. — "Sow-bug; a crustacean 

 living on land; about natural size. 

 (Original.) 



Fig. 3 



-Millipede (Diplopodn) 

 (From Folsom.) 



latural size. 



others; the Diplopoda or Millipedes; the Chilopoda or Centipedes; the 

 Hexapoda or Insects; and the Arachnida, including the scorpions, pseudo- 

 scorpions, spiders, mites, ticks, etc. 



