INSECTS AND THEIR NEAR RELATIVES 



these four classes are based largely upon the manner in which 

 the different segments are grouped together to form compact and 

 distinct parts of the body, and by the number and position of the 

 appendages. 



The Crustacea include the lobsters, crabs, crayfish, shrimps, bar- 

 nacles, sow-bugs, etc., and are primarily distinguished from all other 

 arthropods by the fact that they breathe by means of gills and live 

 either in the water or in damp places. The body is divided into 

 two main regions, the 

 anterior segments be- 

 ing usually covered by 

 a single large shell 

 forming the head-tho- 

 rax, or cephalothorax, 

 while the remaining 

 segments form the ab- 

 domen. Each segment 

 usually bears a pair of 

 appendages. On the 

 head are found two 

 pairs of antennae, and 

 on the thorax and ab- 

 domen are numerous 

 appendages fitted for 

 walking or swimming. 



The only crustacean commonly mistaken for an insect is the little 

 sow-bug, or pill-bug, found in greenhouses, under boards, or in 

 damp places (Fig. 4). These rarely do any damage and may be 

 readily distinguished from insects by the two pairs of antennae 

 and the numerous appendages. The gills are to be found under 

 plates on the lower side of the abdomen. 



The Arachnida include the spiders, scorpions, ticks, and mites, 

 and are almost entirely terrestrial. The body is divided into the 

 cephalothorax and abdomen, as in the Crustacea, but there are no 

 antennae and but four pairs of legs. Although ticks and mites are 

 not insects, yet they are so nearly related, and their injuries to plants 

 and animals are so similar to insect depredations, that they are com- 

 monly included in economic entomology. Spiders are, if anything, 



FlG. 4. Sow-bug, or pill-bug (Porcellio laevis) 

 Enlarged. (After Jordan and Heath) 



