CURRENTS OF AIR AND OF WATER 



147 



American midwest and midsouth. Torna- 

 does may uproot stands of large forest trees 

 with catastrophic destruction of the whole 

 biotic community. A tornado in Western 

 Iowa during July, 1940, killed an estimated 

 1000 birds in a tract of about 100 acres 

 (McClure, 1945; see p. 339). 



More Routine Results of Wind Action 



In addition to the large-scale climatic 

 eflFects produced by winds, such as have al- 

 ready been suggested, air currents exert 

 important controls on the micro-climates of 

 habitat niches. For example, winds modify 

 temperature both directly by transport of 

 air of changed temperature and by influenc- 

 ing the evaporating power of the air. The 

 strength of the wind in the habitat niches 

 in which most small animals carry on the 

 major part of their life activities is greatly 

 reduced as compared with that in the open 

 air a few inches, feet, or yards above. The 

 wind intensity to which most insects are ex- 

 posed is on the order of 10 per cent or less 

 of the air movement measured by the 

 meteorologist. Air drift near the floor of a 

 Panama rain forest is known to average only 

 1 mile a day at a time when the winter 

 trades speed over the forest roof at a rate 

 575 times greater. Geiger (1927) gives a 

 brief generalized discussion of the eflFect of 

 forests on air movement. 



The wind creates major as well as minor 

 habitats. It is responsible for the formation 

 of the sand dunes of the world, whether of 

 the desert or long shore lines. The capture 

 of dunes by vegetation and animals is one 

 of the well-worked chapters in successional 

 ecology (p. 566). The formation by wind 

 action of the great, fertile loess beds of 

 China and of central North America is even 

 more important. The sheet erosion by winds 

 in deserts, in semiarid and drought-stricken 

 dust bowls of the world, fills, in part, the 

 other side of the picture. Wind dissection 

 of regions characterized by stronger relief, 

 as in the arid southwestern United States, 

 yields picturesque landscapes, often of great 

 beauty and of marked poverty of animal 

 life. 



Animal Habits Affected by Wind 



The strong winds in exposed habitats 

 are a special handicap, particularly for ani- 

 mals with weak powers of flight. Habits of 

 life are frequently changed from those 

 shown by related forms in less windy re- 



gions. Birds hide behind wind-breaking 

 ledges of rock or more casual stones. In the 

 windy desert in high Tibet, as in other 

 deserts, some birds build a rampart of peb- 

 bles on the side of the nest that is usually 

 to windward; various larks in Algeria, 

 Palestine and Iraq, and Tibet show this be- 

 havior pattern. 



Insects meet such conditions in various 

 ways. Some are confined to sheltered niches 

 and show a relatively increased tendency to 

 burrow in the ground. Certain butterflies 

 and moths flatten themselves on the ground 

 and attempt flight only in relatively calm 

 air; others make rapid darts from shelter to 

 shelter, while some insects— Psetidabris bee- 

 tles, for example— show a death-feigning 

 reaction as the wind shakes their food 

 plants. They fall to the ground, only to "re- 

 vive" when the wind slackens, and the bee- 

 tles then nm over the ground to a nearby 

 food plant (Meinertzhagen, 1927; Kings- 

 ton, 1925). 



Animal Structure in Relation to Wind 



A much-discussed relation between body 

 form and wind action is found in the obser- 

 vation that, for many insects, regions of 

 strong winds and of circumscribed habitats, 

 such as are found in islands and mountains, 

 have an unduly large proportion of winsjless 

 forms. Wollaston (1854) reported that a 

 third of the native species of beetles of 

 Madeira were flightless, and Hingston 

 (1925) emphasized a similar condition 

 among the grasshoppers of the high, windy 

 plains of Tibet, although winged forms oc- 

 cur at lower levels. Darwin, in the Oiigin 

 of Species, advanced an explanation for 

 such relationships. He thought that reduced 

 wings and the tendency toward being 

 flightless in exposed habitats results from 

 natural selection. The winged forms were 

 supposed to be blown away and perish. This 

 explanation has only recently been seriously 

 questioned, and then, directly, only for re- 

 stricted groups. 



It now appears that in the family of 

 ground beetles (Carabidae), the relations 

 between reduced wings, or winglessness, 

 and a given habitat are much more com- 

 plex than was suggested by Darwin's 

 theory. The evidence and argument are 

 given by Darlington (1943). In summary: 

 carabid beetles with reduced wings not 

 suitable for flight (hereafter called flight- 

 less) occur on continents in habitats in 



