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 
muscles 
oesophagus 
Anglia 
f'Mlimentary 
canal 
ventral nerve 
\ chain 
m — wing 
‘vagina 
oviduct' 4 * 1 ' egg-tubes 
accessory gland£' Yectum 
jL^ Malpighian 
intestine tubules 
Fig. 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, 
