248 



Beetles 



The characters made use of in separating species, genera, and even families 

 are so slight, obscure, and difficult to understand that the tables and keys 

 based on them chiefly result in wholly discouraging any beginner who 

 attempts to use them. And this is not so much the fault of the systematic 

 specialists as of the beetles themselves. When it is recalled that nearly 



brain 

 oesophagus 



Malpighian 

 intestine tubules 



oviduct' 

 accessory glands^-' rectum 



\receptaculum seminalis 

 FlG. 339. Dissection of female great water-scavenger beetle, Hydrophilus sp.; the 

 heart and air-tubes (tracheae) are cut away. (Three times natural size.) 



12,000 species of this order are known in North America north of Mexico; 

 that they represent nearly 2000 genera, grouped in 80 families; and that 

 much general similarity of structure as well as of habits prevails through- 

 out the order, it begins to be apparent why difficulties in classification are 

 inevitable. To find structural differences among these thousands of beetles, 



