THE CLASSIFICATION OF INSECTS 71 



over the earth depend upon its habits and food supply. Some 

 species are exceedingly limited in their distribution, - - as, for in- 

 stance, the little butterfly Oencis scmidca, which inhabits only the 

 highest peaks of the White Mountains, - - while others are quite 

 cosmopolitan, living in many distant parts of the world and with 

 quite different food habits, an example being the bollworm (Heli- 

 otliis obsoleta), which is found on every continent. Some species 

 are so rare that but one or two specimens have ever been taken, 

 while others occur in such countless myriads as to become the 

 worst pests of crops. 



Genus. As a species is composed of individuals of similar struc- 

 ture, so a genus is formed of a number of species having some 

 common characteristics which make them more nearly related to 

 each other than to any other species. In the same way genera are 

 grouped together into families, which have some common charac- 

 teristics distinguishing them from other families of genera, and 

 families are likewise grouped into orders. Frequently, various other 

 subdivisions are made for the purpose of bringing out certain re- 

 lationships, which are evident but which do not seem to warrant 

 definite rank. It should be observed that no standard exists as to 

 what structural characters are sufficient for establishing a species, 

 genus, or family, and that structures which will separate species in 

 one order are of sufficient importance to separate families in another 

 order, this all depending upon the constancy and relative importance 

 of the character. Thus, orders are commonly divided into sub- 

 orders, families into subfamilies, and genera into subgenera, while 

 we recognize varieties and races of individuals within a species, as, 

 for instance, the varieties and races of garden plants and domestic 

 animals. In each case the subgroup is composed of a portion of 

 the larger group, which has some common characters distinguish- 

 ing it from the other subgroups of the same rank. Such terms as 

 sections, divisions, tribes, and scries are also used in the same sense. 



Inasmuch as some three hundred thousand species of insects have 

 been described, it would evidently be impossible for any one person, 

 or any one library, to have all the descriptions which are scattered 

 throughout the scientific books and journals of all countries and 

 languages. Hence most entomologists acquire a general knowledge 

 of the larger groups and then make a special study of some one 



