GEOGEAPHIC DISTRIBUTION 



upon the particular grouping of plants and animals. 

 Maps are used to show zones of life. In like manner 

 the habitat of each species can be mapped out. (See 

 Figs. 170, ITT.) 



Modes of Distribution Little has been done in the 

 study of the geographic distribution of insects as a 

 class, because many of _ the groups are liable to be trans- 

 ported by accidental causes, so that there is a tendency 

 to consider their faunal areas quite unstable. Neverthe- 

 less, among insects there are many species whose hab- 

 itat is greatly localized, due in a measure to the 

 extreme narrowness of the life habits of the species ; 

 that is, they have become so adapted to a certain food 

 plant, certain climatic conditions, and to association 

 with certain other forms of animal life, that they can- 

 not exist away from these set conditions. In the study 

 of insect distribution, all these factors and conditions 

 are to be considered. 



The manner in which insects can be distributed in 

 a state of nature are: over landed areas, by flight or 

 travel, winds, transportation on birds, mammals or 

 other animals; across seas to new lands, by flight, (in- 

 sects have been met in flight three hundred miles from 

 mainland,) transportation in egg, larval or adult stages, 

 on driftwood. Commercial activities, instituted by 

 man, greatly facilitate the spread of insects. In the 

 first place, insects can be introduced intentionally, as 

 in the case of the gypsy moth. They can likewise be, 

 and more frequently are, accidentally transported. 

 The frequency of commercial exchanges between mari- 

 time powers, as well as the rapidity of the journey, 

 14 



