150 Trans. Acad, Sct. of St. Louis 
fiy at night, while loaloa is transmitted by Tabanid flies which 
bite by day. 
Most animals have more or less definite migratory movements 
during the twenty-four hours of the night and day, and in some 
eases these are regularly rhythmical but not necessarily cor- 
related 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 
animals of a pine woods there was a certain amount of diurnal 
migration in a vertical direction. For instance, one species of 
spider was to be found among low herbs at 4:30 a. m. and 
among shrubs at 8:30 a. m., while another species occurred in 
trees at 4:30 p. m. and in herbs at 8:30 p. m. 
The distinction between day and night communities is not 
necessarily a sharp one. The length of dusk varies throughout 
the year; in England it is longest in midsummer and mid- 
winter, and shortest in spring and autumn. Again the amount 
of light at night varies regularly with the moon and intermit- 
tently with cloudy weather. In fact, the distinction between day 
and night communities may turn out to be less marked than we 
might at first suppose. 
In polar regions there is no such alternation of day and night 
except during spring and autumn; and since during these times 
the temperature is too low or the ground too snowy to support 
animal life, the species living there are nearly all typical day- 
light ones. Conversely, below a certain depth in the sea, or in 
large lakes and in subterranean waters, and inside the bodies of 
animals, there is continual darkness, so the animals living there 
also form homogeneous and permanent communities. Sometimes, 
however, the bodies of animals reflect the rhythm of their 
outer environment, and cause corresponding differences in thelr 
parasitic fauna. Probably the most conservative, smooth work- 
ing and perfectly adjusted communities are those living at @ 
depth of several miles in the sea, for there can be no rhythms 
in the environment such as there are on land.’ 
As we pass from poles to equator, the night fauna begins to 
appear and becomes gradually more elaborate and important, 
until in such surroundings as are found in a tropical forest it 
*Ecology 3:306. 
