ants and constitute the whole,
or, at any rate, an important part of the food of certain species. In
return the Homoptera and caterpillars receive certain services from the
ants, so that the relations thus established between these widely
different insects may be regarded as a kind of symbiosis. These
relations are most apparent in the case of the aphids, and these insects
have been more often and more closely studied in Europe and America.

The consociation of the ants with the aphids is greatly facilitated by
the gregarious and rather sedentary habits of the latter, especially in
their younger, wingless stages, for the ants are thus enabled to obtain
a large amount of food without losing time and energy in ranging far
afield from their nests. Then, too, the ants may establish their nests
in the immediate vicinity of the aphid droves or actually keep them in
their nests or in "sheds" carefully constructed for the purpose.

Some ants obtain the honey-dew merely by licking the surface of the
leaves and stems on which it has fallen, but many species have learned
to stroke the aphids and induce them to void the liquid gradually so
that it can be imbibed directly. A drove of plant lice, especially when
it is stationed on young and succulent leaves or twigs, may produce
enough honey-dew to feed a whole colony of ants for a considerable
period.

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-de