12 THE STUDY OF INSECTS. 



except the Myriapoda in having many (more than four) 



pairs of legs. 



The illustrations named above are the more conspicuous 



members of the class ; but many 

 other smaller forms abound both in 

 the sea and in fresh water. Some of 

 the more minute fresh-water forms 

 are almost sure to occur in any fresh- 



FIG. 4. Crustacea : a. Cypris: t, 



Cyclops; c, Daphnia. water aquarium. In rigure 4 are 



represented three of these, greatly enlarged. 



Among the Crustacea that live in damp places on land 

 the Sow-bugs, Oniscidce (O-nis'ci-dae), are most often- 

 seen. These frequently occur about water-soaked 

 wood; and are often mistaken, by students begin- 

 ning the study of Entomology, for insects or Myria- 

 pods. Figure 5 represents a Sow-bug. 



On the sea-coast an immense number of forms 

 of Crustacea occur. 



Class ARACHNIDA (A-rach'ni-da). 

 Scorpions, Harvestmen, Spiders, Mites, and others. 



The members of this class are air-breathing Arthropods, in 

 which the head and thorax are grown together, forming a 

 cephalothorax, which have four pairs of legs fitted for ivalk- 

 ing, and which have no feeler-like antenna. 



The Arachnida abound wherever insects occur, and are 

 often mistaken for insects. But they can be easily distin- 

 guished by the characters given above, even in those cases 

 where an exception occurs to some one of them. The more 

 important of the exceptions are the following : In the Sol- 

 pugida the head is distinct from the thorax ; as a rule the 

 young of mites have only six legs, but a fourth pair are 

 added during growth ; and in the gall mites (Phytoptus) there 

 are only four legs. 



In the Arachnida we find only simple eyes. 



The cephalothorax (ceph-a-lo-tho'rax) bears six pairs of 



