148 



ANALYSIS OF THE ENVIRONMENT 



which flight tends to lose its usefulness 

 without necessarily being harmful. Flight- 

 less ground beetles occur on mountains in 

 sheltered as well as in exposed habitats. On 

 islands, Sightlessness is correlated with the 

 presence of mountains and with coolness 

 rather than with exposure to winds. Every- 

 where Carabidae tend toward flightless 

 forms, not by selection against flight, but 

 mainly by the selection of the inherently 

 simpler, more viable, flightless beetles when 

 flight itself is not useful. In scattered habi- 

 tats, where the population density fluctuates 

 widely, flight has selection value, since it 

 allows the beetles to occupy a large num- 

 ber of the suitable niches and to keep them 

 occupied. In small areas, where populations 

 of these species are relatively dense, flight 

 is not essential, and selection hinges on 

 other factors, whether the animals live on 

 islands, or mountains, or in continental 

 areas. The whole situation as regards even 

 these ground beetles is too complex to be 

 compressed into a single paragraph; Dar- 

 lington's more complete statement should 

 be consulted. 



This information concerning flightlessness 

 among carabid beetles does not demon- 

 strate the absence of a simple negative se- 

 lection against winged forms of other types 

 of animals that have flightless representa- 

 tives in exposed, windy habitats. There are 

 many indications that such selection may 

 occur. The demonstration for the Carabidae 

 does show the necessity for a reexamination 

 of the evidence. 



Animal Distribution by Wind 



Ballooning spiders (p. 134) are but one 

 of a large number of organisms that are 

 regularly or sporadically carried aloft and 

 distributed by mild currents of air definitely 

 lacking storm force. Among other organ- 

 isms, pollens, plant spores, bacteria, sponge 

 gemmules, statoblasts of Bryozoa, encysted 

 rotifers, and various insects may be air- 

 borne for miles. Bacteria from sea water are 

 blown into the air surrounded by droplets 

 of water not much larger than the bacteria 

 themselves. A steady wind with a velocity 

 of but 10 miles an hour could carry such 

 a bacterium some 3000 miles before it could 

 fall from a height of only 100 feet. A slight 

 updraft would enable such a particle to 

 remain in the air almost indefinitely. Cur- 

 rents of air are also important as scent 



carriers, and odors are much more readily 

 detected down wind from their source. 

 These are matters of importance in preda- 

 tor-prey relations among mammals and in 

 the sex hfe of many animals, notably of 

 saturnid moths. 



In season there are milhons of insects, of- 

 ten including hairy larvae as well as adults, 

 above each square mile of suitable land sur- 

 face, and many are carried out to sea. They 

 are sometimes called aerial plankton, but 

 none pass complete hfe histories in the air 

 as do many plankton organisms in water. 

 One of the rich collecting grounds for in- 

 sects in the Chicago area is the drift hne 

 along Lake Michigan (see p. 534). Insects 

 flying at any given season are carried out 

 over the Lake; many fall into the water and 

 drift ashore, to be thrown up by the 

 waves in long and often dense rows near 

 the edge of the beach. 



Many air-borne organisms are killed by 

 desiccation, by sunlight, by ozone, and by 

 other adverse conditions. Despite such 

 hazards, aphids and syrphus flies have been 

 taken alive on Spitzbergen after an esti- 

 mated wind drift of some 800 miles (Elton, 

 1925). Diverse small animals are so car- 

 ried. In the air over Louisiana, spiders and 

 mites and representatives of eighteen orders 

 of insects were collected from aeroplanes 

 well above ground level. Diptera were most 

 abundant, with beetles next. Homoptera 

 and Hymenoptera were taken at 14,000 feet 

 and a spider was trapped a thousand feet 

 higher (GHck, 1939). These altitudes prob- 

 ably represent approximate rather than 

 extreme upper limits of the biosphere for 

 such forms under more usual air conditions. 



Even tiny snails may well be transported 

 by wind for considerable distances. If a 

 landfall of such snails becomes established, 

 it can undergo adaptive radiation and pro- 

 duce larger forms. Many groups of snails 

 have minute representatives. Such consider- 

 ations make one less certain that land con- 

 nections between regions now separated by 

 deep ocean water are necessarily required 

 in order to account for the known distribu- 

 tion of land snails and other small animals 

 (Gulick, 1932). As a final bit of evidence 

 of the potency of winds of more usual ve- 

 locity in animal distribution, attention may 

 be called to the greater number of aerial 

 waifs among American birds that make 

 landfalls on European shores, as compared 

 with the relatively few European birds that 



