It is known that ants travel long distances from their nests, and yet return with unerring accuracy. It has been shown that they are able to do this through a marvellous sense of smell. The track left by the ant, as it leaves the nest, serves as a trail in returning to the starting-point. Moreover, it appears that the ant can pick out her own trail, even when it has been crossed by that of other ants. This means that she can distinguish the odor of her own trail from that of other members of the colony. The sense-organs by means of which the odor is detected lie in the antennæ. This fact accounts for certain actions of ants that have been described as showing that they have an affection for each other. Two ants, meeting, pat each other with their antennæ. In this way they are quickly able to distinguish members of their own nest from those of other nests. If they are of the same 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 member 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 probably 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 obviously of advantage to her and to the whole colony.