130 THE BIOLOGY OF THE SEASONS 



even divide into two parties, each of which draws up on a 

 half-circle, and both paddle to meet each other, just as if two 

 parties of men, dragging two long nets, should advance to 

 capture all fish taken between the nets when both parties 

 come to meet. As the night comes they fly to their resting- 

 places, always the same for each flock, and no one has 

 ever seen them fighting for the possession of either the bay or 

 the resting-place. In South America they gather in flocks of 

 from forty to fifty thousand individuals ; some enjoy sleep 

 while the others keep watch, and others again go fishing." 



Of shepherding, the only clear illustrations are to be found 

 among ants, some of which (e.g. Lasius niger and Lasius 

 bmnneus) keep aphides or green-flies, and others scale- 

 insects. This extraordinary habit was well known to 

 Linnaeus, who called the aphides the ants' cows (vacca 

 formicarum), and it has received considerable attention from 

 many observers. Perhaps it began in the simple fact that 

 the ants and the aphides frequented the same trees, dining, 

 as it were, at the same bountiful table. Then it was dis- 

 covered that the aphides would yield up some of their 

 " honey-dew " when licked or tickled, and the ants traded 

 on this. Gradually, perhaps, the ants began to take some 

 charge of their cattle, even building " aerial stables " for 

 them on the branches. The ants are accustomed to put 

 their pupae out to be sunned, and to carry them back again 

 when it rains ; perhaps this habit led on to what at first sight 

 is so startling, that the ants take aphides down into their 

 underground nests. " In Autumn the aphides lay eggs in 

 the cellars to which they have been brought by force or coax- 

 ing or otherwise, and these eggs the ants take care of, 

 putting them in safe cradles, and licking them as tenderly as 

 they do their own." It is probable that this very inter- 

 esting habit arose neither deliberately nor casually, but by 

 the gradual extension of habits previously established ; and 



