Likeness Causeth Liking

Numerous myrmecologists have studied the reciprocal behavior of ants and plant-lice (aphids) which, in the course of evolution, seem to have become finely adapted to one another. Certainly, the behavior of both kinds of creatures is altered in each others’ presence. Nutritious food services have become readily available to ant species which extract honeydew from the excrement of the lice.

Aphids, by sinking their extended proboscis into the vascular tissue of the plants, gain direct access to their juices, and exude through the anus the surplus of the digested sap they have taken in. The liquid is rich in sugar (although poor in nitrogen). The worker ants crave to drink the unutilized, undigested honeydew. Their access to this nourishment is thus indirect: they “milk” the aphids; this means that the ants pat the abdomen of the aphids with their antennae tips, a tickling which causes the lice to exude a drop of syrup. Aphids thus managed by ants thrive. Little wonder that Linnaeus called the aphids ants’ cows.

Nobody quite understands why, instead of devouring the aphids, the foraging ants treat them with ritual courtesy insuring, in general, their survival. The way the ant massages the aphid resembles the way that it habitually begs for food from a nestmate. Intraspecific food-sharing behavior in Formica—how donor or acceptor initiates the sequence leading to regurgitation—has been analyzed in impressive detail, and it is now known that the pattern is a nearly universal one among Formicidae. What still remains uncertain is how the habit became extended into a setting involving two vastly different species.

In 1959, Werner Kloft made an astounding observation on the basis of which he then proposed a remarkable explanation. He noticed that the hind end of the aphid resembles the front end of the ant sufficiently to release the same begging behavior in a hungry ant as leads to soliciting behavior vis-á-vis a conspecific. As can clearly be seen in Figure 6—featuring the abdomen of an aphid on the left and the head of an ant to the right—the hind legs of the former represent the antennae of the latter, its siphuncles the mandibles, and its cauda, anus, surrounded by perianal hairs, the maxillo-labial tongue. Kloft demonstrated that while a round piece of wax is not tickled by ants, similar models with two bristle-like projections are. As for the aphids, the effect can also be produced by brushing their abdomens with an ordinary paint brush.

It is hard to escape the conclusion that this congenial relationship—which, by the way, has a marked benignant effect on aphids, greatly increasing both their intake of sap and their secretion of the honeydew—is the evolutionary aftermath of a global “misunderstanding.”

Thomas Sebeok, I Think I Am A Verb 143-144 (1987).

It seems to me that most creatures like to be rubbed.

Happy New Year all.