Science: Selective Service Underground

How ant colonies draft their soldier armies and maintain balance in their incredible caste systems was cleared up last week by a Chicago zoologist. The secret: a “social hormone” exuded by some of the ants to inhibit the others.

Of all the social insects,* ants are the most highly developed. As many as eleven castes stratify some ant species: even the simplest colonies contain queens, drones, workers, soldiers. The drone caste contains all the colony’s few males, whose sole function is sexual. So is the queen’s. But the workers and soldiers are all sterile.

The origin of these non-sexual castes has long been debated by entomologists. Some claim they are born of different sorts of eggs. Others claim that all ants are equal when they hatch into the larval state and only during their further development acquire caste with its distinctive form, physiology and behavior. But, if so, how? Here, too, the scientists propose confusing answers:

>Through the quantity of food available. Ill-fed larvae become the small workers; well-fed larvae become soldiers; overfed larvae become queens.

> Through the quality of food. Larvae fed on special pollens become queens, etc.

> Through a secretion. Spread through the entire colony by the ants’ incessant grooming and antennae-waving, the exudate (“social hormone”) keeps the castes in balance as the body’s hormones keep its parts running smoothly together.

To confirm the last theory Zoologist Robert E. Gregg of the University of Chicago made 33 experiments with 9,537 ants of the species Pheidole morrisi. He arranged for ant larvae to be reared to maturity under the care of soldiers only and of workers only, and found: “Where the soldier caste is present in large numbers, there is a decrease in the number of soldiers that are produced. Conversely, in pure colonies of workers an excess of soldiers tends to appear over the number which normally develop in a control colony.”

To eliminate the food theories, Gregg fed all larvae the same kind of food, and all they could eat of it. The “social hormone” yet to be isolated is the only assumption, Gregg claims, that will explain his experiments. He believes that:

> The soldiers, acting as nurses to the larvae, suffuse them with their exudate, which prevents the development of more unnecessary soldiers.

>The workers give off no exudate and cannot inhibit the growth of soldiers, so too many appear.

>In a normal colony where the workers tend the larvae, the “social hormone” is passed from soldiers to workers to larvae. Presumably a similar queenly exudate inhibits all but a few of the soldiers in turn from developing into queens.

* Others: some species of bees, wasps, termites.