172 



PRACTICAL MICROSCOPY. 



delicate handling. It is not difficult to find and take away 

 the tracheal system (though the beginner will find this 

 easier to prepare from the silk-worm or caterpillar), and 

 the muscles which are used for raising and depressing the 

 wings during flight may be found without much trouble. 



FIG. 159. 



FIG. 160. 



The digestive apparatus varies somewhat in different 

 insects, and as an aid to the student the arrangement of 

 these organs is given as they appear in situ in the water- 

 beetle (Fig. 159) and the blow-fly (Fig. 160). 



The fly lives by suction, and therefore the food does not 

 require any unusual amount of mastication. It therefore 



