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 an 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 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 Ecua- 

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

 ft., Pieris xanthodice ranges above 15,000 ft., and dragon flies, Hymen- 

 optera 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 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 fam- 

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

 mention the collembolan Isotoma fimetaria, and the butterflies Vanessa 

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

 dred species are cosmopolitan or subcosmopolitan, including Tenebrio 



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