210 INTELLIGENCE IN INSECTS 
spreads like a conflagration till it has seized on most or all 
the members of a community.” It is in their attacks upon 
enemies that imitation in ants is especially marked. An 
attack by an ant is the signal for others to join in the fray. 
Sometimes a strange ant may be tolerated in a nest until 
it happens to arouse the animosity of one of the members, 
when various others fall to and help to dispatch the intruder. 
Wasmann states that several beetles of the species Dinarda 
dentata were received as guests in a nest of Formica sanguinea 
and had lived there for some time and propagated. He 
then placed in the nest a specimen of an allied species of 
Dinarda which was attacked and killed. This aroused the 
killing propensities of the other ants which fell upon the 
Dinarda dentatas, and the guests which had been kept for 
so long met their fate. That ants in their treatment of 
aphids are influenced by imitation is indicated by the fact, 
signalized by Forel and Adlerz, that Formica sanguinea, 
which very rarely make use of honey dew as food, readily 
adopts the custom of its slaves upon perceiving them solicit 
the aphids for their sweet exudation. Slave making ants 
readily adopt guests which are received in a friendly manner 
by their slaves, although they would otherwise be apt to 
attack them, and slaves in turn are disposed to be friendly 
to the guests which they perceive to be tolerated by their 
masters. 
In treating of insect intelligence we shall find it instructive 
to consider its failures as well as its exceptional manifesta- 
tions. As Forel has remarked insects are exceedingly stupid 
' r as regards everything not closely related to their instinctive 
interests. But even when the latter are involved, they 
usually fail to make the simplest and most obvious inferences. 
A striking case is furnished by the Amazon slave-making 
ant, Polyergus rufescens, which on account of the remarkable 
