As the relations between ants and the various Homoptera have been regarded as mutualistic, it may be well to marshal the facts which seem to warrant this interpretation. The term "mutualism" as applied to these cases means, of course, that the aphids, coccids, and membracids are of service to the ants and in turn profit by the companionship of these more active and aggressive insects. Among the modifications in structure and behavior which may be regarded as indicating on the part of aphids unmistakable evidence of adaptation to living with ants, the following may be cited:
1. The aphids do not attempt to escape from the ants or to defend themselves with their siphons, but accept the presence of these attendants as a matter of course.
2. The aphids respond to the solicitations of the ants by extruding the droplets of honey-dew gradually and not by throwing them off to a distance with a sudden jerk, as they do in the absence of ants.
3. Many species of Aphididae that live habitually with ants have developed a perianal circlet of stiff hairs which support the drop of honey-dew till it can be imbibed by the ants. This circlet is lacking in aphids that are rarely or never visited by ants.
4. Certain observations go to show that aphids, when visited by ants, extract more of the plant juices than when unattended.
The adaptations on the part of the ants are, with a single doubtful exception, all modifications in behavior and not in structure.
1. Ants do not seize and kill aphids as they do when they encounter other sedentary defenseless insects.
2. The ants stroke the aphids in a particular manner in order to make them excrete the honey-dew, and know exactly where to expect the evacuated liquid.
3. The ants protect the aphids. Several observers have seen the ants driving away predatory insects.
4. Many aphidicolous ants, when disturbed, at once seize and carry their charges in their mandibles to a place of safety, showing very plainly their sense of ownership and interest in these helpless creatures.