256 ZOOLOGY 



members of the other groups being relatively insignifi- 

 cant in numbers. Just as the insects seem with diffi- 

 culty to invade the waters, so the Crustacea appear to 

 find it hard to succeed on land. The isopod Crustacea, 

 called wood lice and pill bugs, are widespread, but not 

 very numerous in terrestrial species. Living in damp 

 spots and under stones, they manage to breathe air, and 

 in some cases have developed tracheal tubes correspond- 

 ing in function to those of the insects. Yet on the whole 

 they have no chance to compete with the insects, which 

 are so perfectly adapted for aerial conditions, 

 structure of It is characteristic of the Crustacea that they have 



insects and . r . , . ' 



crustaceans two pairs oi antennae, whereas the insects and myria- 

 pods have only one, and the arachnids are wholly with- 

 out these structures. Here, again, exceptions occur; 

 some Crustacea have only one pair of well-developed 

 antennae, and there are insects which have none. In- 

 sects never have more than six legs, except in some 

 larvae, such as the caterpillar. The other groups 

 usually have more than six, in many cases a much 

 larger number. Of all the groups, only the insects are 

 winged. Before reviewing the arthropods in detail, it 

 will be useful to give a summary of the principal groups. 



