78 INTRODUCTION TO ZOOLOGY. 



Logical Definition. 



An insect is an arthropod tuhich is composed of sev- 

 enteen body segments grouped into three body divisions; 

 has a chitinous exosheleton, one pair of antennse, and two 

 sessile compound eyes; breathes by spiracles u'Jiich open 

 into trachese ; has three pairs of legs and tivo pairs of 

 wings ; can move by flying ; and lives in the air. 



From your table of differences, make a logical 

 definition of an arachnid, a myriapod, and a crusta- 

 cean. 



