60 
ANIMAL INTELLIGENCE. 
a number of naturally hostile species all living together 
after the manner of the 4 happy families 5 of the showmen. 
Habit of keeping Aphides . — It is well known that 
various species of ants keep aphides, as men keep milch 
cow^s, 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 during 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 
