go ANIMAL ECOLOGY 



V 9. Most animals have more or less definite migratory 



movements during the twenty-four hours of day and night, 

 and in some cases these are regular and rhythmical, but not 

 necessarily correlated exactly with light and darkness. The 

 result of these movements is to alter the composition of animal 

 communities in any one place. Sanders and Shelford ^^ found 

 that among the animals of a pine wood in North America there 

 was a certain amount of diurnal migration up and down 

 in a vertical direction. For instance, one species of spider 

 (Tetragnatha lahoriosa) was to be found among low herbs at 

 4.30 a.m. and among shrubs at 8.30 a.m., while another species 

 (Theridium spirale) occurred in trees at 4.30 p.m. and in herbs 

 at 8.30 p.m. Many insects (especially flies) occurred at 

 different heights in the vegetation, depending upon the time 

 of day. There exist similar vertical migrations among plankton 

 animals in fresh-water lakes and in the sea. A number of 

 species of, e.g., Crustacea come nearer to the surface during the 

 night. 



10. We may repeat here that the distinction between day 

 and night communities is not necessarily a very sharp one, 

 and that there are a number of animals which come out both 

 by day and by night {e.g. the common black bear of North 

 America ^8), and others whose time-limits are determined by 

 other factors {e.g. the slugs mentioned later on). The length 

 of dusk varies throughout the year ; in England it is longest 

 at midsummer and midwinter, and shortest in spring and 

 autumn. Again, the amount of light at night is tremendously 

 influenced by the state of the moon and the occurrence of 

 cloudy weather. In fact, the distinction between day and night 

 communities may turn out to be less marked than we might at 

 first sight suppose ; but enough has been said to show that the 

 alternation of day and night communities is a very important 

 phenomenon and that it affects animal society profoundly in 

 nearly all parts of the world. 



1 1 . In the polar regions there is no such alternation of day 

 and night except during the spring and autumn ; and, since 

 at these times the temperature is too low or the ground too 

 snowy to support much animal life, the species living there are 



