60 ANIMAL INTELLIGENCE. 



a number of naturally hostile species all living together 

 after the manner of the ' happy families ' of the showmen. 

 Habit of keeping Aphides. It is well known that 

 various species of ants keep aphides, as men keep milch 

 cows, to supply a nutritious secretion. Huber first ob- 

 served this fact, and noticed that the ants collected the 

 eggs of the aphides and treated them exactly as they 

 treated their own, guarding and tending them with the 

 utmost care. When these eggs hatch out the aphides are 

 usually kept and fed by the ants, to whom they yield a 

 sweet honey-like fluid, which they eject from the abdomen 

 upon being stroked on this region by the antennae of the 

 ants. Mr. Darwin, who has watched the latter process, 

 observes with regard to it, 



I removed all the ants from a group of about a dozen aphides 

 on a dock plant, and prevented their attendance daring several 

 hours. After this interval, I felt sure that the aphides would 

 want to excrete. I watched them for some time through a 

 lens, but not one excreted ; I then tickled them with a hair in 

 the same manner, as well as I could, as the ants do with their 

 antennae ; but not one excreted. Afterwards 1 allowed an ant 

 to visit them, and it immediately seemed, by its eager way of 

 running about, to be well aware what a rich flock it had dis- 

 covered ; it then began to play with its antennae on the abdo- 

 men, first of one aphis and then of another ; and each, as soon 

 as it felt the antennae, immediately lifted up its abdomen and 

 excreted a limpid drop of sweet juice, which was eagerly de- 

 voured by the ant. Even quite young aphides behaved in this 

 manner, showing that the action was instinctive, and not the 

 result of experience. 



The facts also show that the yielding of the secretion 

 to the ants is, as it were, a voluntary act on the part of the 

 aphides, or, perhaps more correctly, that the instinct to 

 yield it has been developed in such a relation to the re- 

 quirements of the ants, that the peculiar stimulation sup- 

 plied by the antennae of the latter is necessary to start the 

 act of secretion ; for in the absence of this particular stimu- 

 lation the aphides will never excrete until compelled to do 

 so by the superabundance of the accumulating secretion. 

 The question, therefore, directly arises how, on evolutionary 



