408 Evolution and Adaptation 
nest, they separate quietly ; if of other nests, they may fight. 
If an ant from one nest is put into another nest, it is instantly 
attacked and killed —an act that appears to be injurious 
rather than useful, for the ant might become a valuable mem- 
ber of the new colony. If, however, an ant is first immersed 
in the blood of a member of the community into which she 
is to be introduced, she will not be attacked, and may soon 
become a part of the new community. By her baptism of 
blood she has no doubt acquired temporarily the odor of the 
new nest, and by the time that this has worn off she will 
have acquired this odor by association, and become thereby a 
member of another colony. 
Numerous stories have been related of cases in which an 
ant, having found food, returns to the nest with as much of 
it as she can carry, and when she comes out again brings with 
her a number of other ants. This has been interpreted to 
mean that in some mysterious way the ant communicates her 
discovery to her fellow-ants. A simpler explanation is proba- 
bly more correct. The odor of the food, or of the trail, 
serves as a stimulus to other ants, that follow to the place 
where the first ant goes for a new supply of the food. The 
fact that the first individual returns to the supply of food 
seems to indicate that the ant has memory, and this is obvi- 
ously of advantage to her and to the whole colony. 
The peculiar habits of some of the solitary wasps, of sting- 
ing the caterpillar or other insect which they store up as 
food for their young, is often quoted as a wonderful case 
of adaptive instinct. The poison that is injected into the 
wound paralyzes the caterpillar, but as a rule does not kill it, 
so that it remains motionless, but in a fresh state to serve 
as food for the young that hatch from the egg of the wasp. 
A careful study of this instinct by Mr. and Mrs. Peckham 
has shown convincingly that the act is not carried out with the 
precision formerly supposed. It had been claimed that the 
sting is thrust into the caterpillar on the lower side, a ventral 
