CHAPTER XII 

 DISTRIBUTION 



I. GEOGRAPHICAL 



Importance of Dispersion. Dispersion enables species to miti- 

 gate the intense competition and the rigid selection that result from 

 crowded numbers; hence the tendency to disperse, being self -preserva- 

 tive, has become universal. Some species habitually emigrate in pro- 

 digious numbers: the African migratory locust, the Rocky Mountain 

 locust, and the milkweed butterfly, which annually leaves the Northern 

 states for the South in immense swarms, in autumn, and in the follow- 

 ing spring straggles back to the North. Vanessa cardui occasionally 

 migrates in immense numbers, as do also Pieris, some dragon flies and 

 some beetles, notably Coccinellidae. 



Wide Distribution of Insects. Insects have been found in almost 

 every latitude and altitude explored by man. Butterflies and mos- 

 quitoes 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 xanlhodice 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 ctf insects are world-wide in distribution, 

 the most richly distributed are Coleoptera, Thysanura and Collembola, 

 the last two feeding usually upon minute particles of organic matter in 

 the soil and being remarkably tolerant of extremes of temperature. 

 The four chief families .of butterflies occur the world over, as do several 

 families of beetles. Of species that are essentially cosmopolitan we may 

 mention the collembolan Folsomiafimetaria, and the butterflies Vanessa 

 cardui and Anosia plexippus, while among beetles no less than one hun- 



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