TIME AND ANIMAL COMMUNITIES 85 



the extraordinary effect upon insects when a passing cloud 

 covers the sun. The drop in temperature slows down their 

 movements or actually stops them altogether. The return of 

 the sun starts them all off at high speed once more. It is 

 worth while to watch a big ant-hill under such conditions. In 

 the sun the whole place swarms with hurrying ants, carrying 

 sticks, caterpillars, or each other, with restless energy. When 

 it gets suddenly cooler they all stop working fast and do every- 

 thing vdth painful slowness. The larvse of a species of locust 

 which periodically undertakes great migrations in the Northern 

 Caucasus has similar reactions, which have been worked out 

 rather carefully by Uvarov.^s On the first stages of the 

 journey the larvae march along on the ground in great droves 

 (they do not grow their wings until a later stage in their travels) ; 

 but they never march at night, and if the temperature falls 

 below about 13° to 15° C. their movements cease and they 

 have to stop wherever they happen to be. In the same way 

 they will stop for only a tiny passing cloud. It is interesting 

 to note that there is also a higher limit of temperature above 

 which they will not continue to march, so that they halt some- 

 times in the middle of the day. We see, then, that this locust 

 is able to remain active only under certain optimum conditions 

 of temperature, and many other examples of the same kind of 

 thing could be given. 



3. With many animals the coming of nightfall has precisely 

 the same effect as a cloud over the sun, but the stoppage of 

 work is longer and may require the taking of more elaborate 

 precautions. And in addition we find that another set of 

 animals adapted to a different set of conditions comes out and 

 takes the place of the others. If the changes in conditions are 

 greater, or last longer, many animals migrate away altogether to 

 a more suitable locality or else tide over the bad period in some 

 special way ; for instance, by renouncing all outside feeding 

 and living upon their own fat like a hibernating marmot. 



Animal communities are therefore organised into a series V 

 of separate smaller communities, each of which is in action at 

 a different time. There are " day and night shifts," wet and 

 dry weather sets of animals, communities of winter and 



