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-w^ater forms 

 '^ ■ are almost sure to occur in any fresh- 



FiG. 4.— Crustacea : a, Cypris: b^ , _,. 



Cyclops: c^Daphnia. watcr aquarmm. in i^igure 4 are 



represented three of these, greatly enlarged. 



Among the Crustacea that live in damp places on land 

 the Sow-bugs, Omscidce (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 sow-bug. 

 of Crustacea occur. 



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

 Scorpions^ Harvestmen, Spiders^ Mites^ and others. 



The members of this class are air -breathing A rthropods, in 

 which the head and thorax are grown together, forming a 

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

 ings and which have no antertnce. 



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 



