niuil enough buoyancy is created to lift the spider and 

 tarry it away, sometimes distances of hundreds of 

 miles. Strong winds will often carry insects and even 

 hirds great distances away from their usual courses. 

 Crop pests may he blown North during the summer 

 and cause damage, but may never become permanently 

 established because they are killed by the northern 

 winter cold. Hurricanes are an important means of 

 colonizing islands far at sea with terrestrial species 

 (Elton 1925, Darlington 1938). There are authentic 

 records of rains of fishes and other aquatic species that 

 were sucked up and transported appreciable distances 

 by tornadoes (Gislen 1948). 



In studies done in England (Freeman 1946), it 

 was estimated that the number of insects drifting 

 through a rectangle 91 m (300 ft) high and 1610 m 

 (1 mi) long amounted to 12,500,000 per hour. The 

 number was highest during May, June, and Septem- 

 ber, at temperatures above 18°C. The aerial popula- 

 tion over the forests and swamps of Louisiana has 

 been measured (Click 1939) , by means of traps placed 

 on the wings of airplanes, and found to average the 

 following number of individuals per 1000 cubic meters 

 of air : 



Diptera were most numerous, followed during the 

 daytime by Coleoptera, Homoptera, Hymenoptera, 

 Araneida, Hemiptera, and others. Spiders and wing- 

 less insects were greatly reduced in numbers at night 

 because of the lack of vertical convection currents. 

 The density of insects in the air depends on the di- 

 urnal and seasonal activity rhythms of the animals to 

 give them exposure to air currents in their terrestrial 

 habitats. Those species tend to be most numerous 

 that have the greatest wing area per unit weight, 

 hence the greatest buoyancy. The insect population 

 in the upper air is not distinct from that flying in the 

 lower layers, but its density decreases with height in 

 a well-defined logarithmic manner up to and over 

 1500 m (5000 ft) (Johnson 1957, Taylor 1960). Im- 

 proved methods for obtaining aerial densities employ 

 a suction pump to strain known volumes of air per 

 unit of time (Johnson 1951). 



Other animals serve as vehicles for passive trans- 

 portation. Bits of vegetation, small animal life, and 

 the eggs of worms, entomostracans, rotifers, insects, 

 snails, and fish may cling to the feet of such water- 

 birds as ducks, rails, and herons and be carried many 



10 20 30 40 50 60 

 DISTANCE FROM CENTER OF SPLASH, cm 



FIG. 10-3 Decrease 

 5 cm from the cer 

 FaulweHer). 



mcentration of water 

 of splash (Wolfenbar 



opiets beyond 

 r 1946, after 



miles when the birds migrate. A species of Succinea 

 snail, native only to St. Croix and Puerto Rico, was 

 found alive in the feathers of a bobolink shot in Cuba. 

 Ferrissia snails have been found attached to the wing 

 covers of aquatic beetles and sphaeriid clams clamped 

 on their legs. These insects occasionally fly from one 

 body of water to another. 



Some dispersal movements are determined by the 

 manner in which the animals respond to environmen- 

 tal factors. Such, directed movements, or taxes (p. 12) 

 may induce the dispersal of animals upstream positive 

 to current rather than downstream. 



12 3 4 



DISTANCE OF RECOVERY, CORN ROW 



FIG. 10-4 Effect of increase in population at source (200, 500. 

 1000 eggs deposited) on the dispersal of European corn borer 

 larvae (Woifenbarger 1944, after Neiswander and Savage). 



Dispersal, migration, and ecesis 147 



