CHAPTER XII 



DISTRIBUTION 



i. GEOGRAPHICAL 



Importance of Dispersion. Dispersion enables species to 

 mitigate the intense competition and the rigid selection that 

 result from crowded numbers ; hence the tendency to disperse, 

 being self-preservative, has become universal. Some species 

 habitually emigrate in prodigious numbers : the African migra- 

 tory locust, the Rocky Mountain locust, and the milkweed but- 

 terfly, which annually leaves the Northern states for the South 

 in immense swarms, in autumn, and in the following spring 

 straggles back to the North. Vanessa cardui occasionally mi- 

 grates in immense numbers, as do also Pieris, some dragon 

 flies and some beetles, notably Coccinellidse. 



Wide Distribution of Insects. Insects have been found in 

 almost every latitude and altitude explored by man. Butter- 

 flies and mosquitoes occur beyond the polar circle, the former 

 in Lat. 83 N., the latter in Lat. 72 N., and a species of 

 Emesa closely allied to our common E. longipes is recorded by 

 Whymper from an altitude of 16,500 ft. in Ecuador, where, 

 according to the same traveler, Orthoptera occur at 16,000 

 ft., Pieris xani ho dice ranges above 15,000 ft., and dragon flies, 

 Hymenoptera and scorpions reach a height of 12,000 ft., while 

 twenty-nine species of Lepidoptera range upward of 7,300 

 ft. A very few species of insects inhabit salt water, Halobates 

 being found far at sea; some kinds live in arid regions and a 

 few even in hot springs, while caves furnish many peculiar 

 species. In short, insects are the most widely distributed of 

 all animals, excepting Protozoa and possibly Mollusca. 



While all the large orders of insects are world-wide in dis- 

 tribution, the most richly distributed are Coleoptera, Thys- 



366 



