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Greece, they are also ferocious in combat. In some species the raiders are armed with powerful sickle-shaped mandibles capable of piercing the bodies of their opponents. During my research on ant slavery I found one species that uses a radically different method. The raiders carry a hugely enlarged gland reservoir in their abdomen (the rear segment of the three-part body) filled with an alarm substance. Upon breaching the victim’s nest, they spray large quantities of the pheromone through the chamber and galleries. The effect on the defenders of the allomone (or, more precisely, pseudo-pheromone) is confusion, panic, and retreat. They suffer the equivalent of our hearing a thunderously loud, persistent alarm coming from all directions. The invaders do not respond the same way. Instead, they are attracted to the pheromone, and as a result they are able easily to seize and carry away the young (in the pupal stage) of the defenders. When the captives emerge from the pupae as adults, they become imprinted, act as sisters of their captors, and serve them willingly as slaves for the rest of their lives.
    Ants are possibly the most advanced pheromonal creatures on Earth. They have more olfactory and other sensory receptors on their antennae than any other known kind of insect. They are also walking batteries of exocrine glands, each of which is specialized to producedifferent kinds of pheromones. In regulating their social lives they employ, according to species, from ten to twenty kinds of pheromones. Each one conveys a different meaning. And that is just the beginning of the information system. Pheromones can be discharged together to create more complex signals. When released at different times or in different places, their meaning changes yet another way. Still more information can be transmitted by varying the concentration of the molecules. In at least one American species of harvester ant I have studied, for example, a barely detectable level of pheromone evokes attention and movement by workers toward the source. A somewhat higher concentration causes the ants to search excitedly back and forth. The highest concentration of the substance, that occurring close to the signaling worker, causes a frenzied attack on any foreign organic object in the vicinity.
    Plants of some species communicate by pheromones. At least they are able to read the distress of neighboring plants by responding with actions of their own. A plant attacked by a serious enemy—bacteria, fungus, or insect—releases chemicals that suppress the invader. Some of the substances are volatile. They are “smelled” by their neighbors, who make the same defensive response even though they themselves are not yet under attack. Some species are assaulted bysap-sucking aphids, insects especially abundant in the north temperate zone and capable of wreaking heavy damage. The plant-generated airborne vapor not only stirs neighboring plants to secrete defensive chemicals, but also reaches small wasps that parasitize aphids, drawing them to the vicinity. A few species use yet another defensive line. The signals are transmitted from plant to plant along the strands of symbiotic fungi that entwine the roots and connect one plant to another.
    Even bacteria order their lives with pheromone-like communication. Individual cells come together, at which time they trade DNA of special value to one or the other. As their populations increase in density, some species also engage in “quorum sensing.” The response is triggered by chemicals released in the liquid around the cells. Quorum sensing results in cooperative behavior and formation of colonies. The best studied of the latter process is the construction of biofilms: free-swimming cells gather, settle on a surface, and secrete a substance that surrounds and protects the entire group. These organized micro-societies are all around and inside us. Among the most familiar are the scum on unwashed bathroom surfaces and the