Many insects use plant-borne vibrations to attract a mate. Commonly a female responds to a signaling male not by approaching him (as would, for example, a female frog choosing a singing male) but with a signal of her own, usually different from and less elaborate than that of the male. As the two duet, the male searches for her, using her signals as a guide.
First, though, he must interest a female. Randy Hunt, of Indiana University Southeast, has studied the sexual signals of Graminella nigrifrons, a small, yellowish brown leafhopper that feeds on a variety of grasses in the eastern United States. The male of this species vibrates its abdomen to produce a complex song that, according to Hunt, begins with a piglike snort and progresses to repeated drumming with a Latin rhythm. Only females that happen to be on the same plant can pick up the signal, however, and if the male gets no reply, he quickly flies to another plant and tries again. This “call and fly” strategy is probably typical of many species.
Once a female answers a male’s calls, the challenge of finding her begins. The southern green stinkbug (so named because of the foul-smelling scent it releases when disturbed) appears to have an interesting technique. Andrej Cokl and colleagues at the National Institute of Biology in Slovenia placed male and female stinkbugs on ivy, a plant with many small stems. When a male stinkbug searching for a female comes to a branching point, he positions himself so that one set of legs touches one stem and another set touches the other. This position may enable him to detect differences between the two stems (in terms of the arrival time and amplitude of the female’s signals) and thus to determine which direction will take him to her.
Some insects, including many of the katydids and crickets that fill the summer night with their familiar songs, use both airborne sounds and plant-borne vibrations to communicate. When advertising for a mate, the male tropical cone-headed katydid (Copiphora rhinoceros), studied by Glenn Morris, of the University of Toronto, alternates between high-pitched airborne chirps and plant-shaking tremulations of its entire body. If a female walks onto his plant (in this species, the female moves toward the male), he switches from public chirps to the more private tremulations, thereby avoiding detection both by competing males on nearby plants, who might otherwise interfere with his courtship, and by bats that home in on acoustic signals.
The delicately beautiful green lacewing, common in meadows and gardens and along forest edges, signals by tremulating its abdomen, producing a low-frequency purr that travels through the plant stem. Some species of lacewing are unusual in that the male and female signals are virtually identical. The songs of all species, however, are quite distinctive—a characteristic that Charles Henry and Marta Wells, of the University of Connecticut, have used to identify a number of previously unrecognized lacewing species.
The diversity of plant-borne sexual signals is especially well illustrated in stoneflies. These insects spend their larval stages in water, most often in rocky streambeds, where their need for cool, clean, highly oxygenated water makes them good indicators of stream quality. After a male stonefly metamorphoses into adult form (which takes place out of the water), it begins signaling, usually by tapping or rubbing its abdomen on a stem or leaf. Studying these simple percussive mechanisms in different stonefly species, Kenneth Stewart and colleagues at the University of North Texas have discovered a tremendous variety of precise, intensely rhythmic signals.