186 INTRODUCTION TO ZOOLOGY. 



to render a proper decision where an alternative is 

 offered. There are two reasons for this : First, the 

 student may not know the meaning of the terms 

 employed, or have a satisfactory image of the things 

 they represent. A study of the glossaries and the 

 structure of tlie insect will help to overcome this diffi- 

 culty. Second, the distinguishing mark is often one 

 of comparison, expressed by longer, shorter, thicker, 

 slender, somewhat, etc. This can be aided only by 

 comparison of many insects, and will be overcome by 

 experience. 



Every teacher and every student ought to know 

 how to use a set of analytical tables. That in itself 

 is a valuable acquisition. Let us study how to use 

 these. We should always make a careful examination 

 of the specimen in hand before turning to the tables. 

 Suppose each student has a beetle in hand. The 

 first division of the Coleoptera is marked — 



A— Head not distinctly prolonged into a beak. 



It is or it is not prolonged. If it is prolonged, we 

 omit all divisions under A — and go to AA — ; but if it 

 is not prolonged, we read the next division, — 



B— Hind tarsi ■vritli the same'number ot joints as the others. 



The hind tarsus either has or has not the same num- 

 ber as the others, so we say "Yes" or "No," accord- 

 ing as it agrees or disagrees with the characterization 

 in B — . If we decide " No," Ave turn at once to — 



BB— Front and middle tarsi 5-jolnted; hind tarsi 4-Jointed. 



If our previous decision was right, it must fit here. 

 Then we read — 



0— Anterior ooxal eavitles closed behind. 



They are either closed or open. If open, we omit 

 all intervening divisions and go at once to — 



CC— Anterior ooxal cavities open behind. 



