If katydids represent the insect world in disco contests, stoneflies would play the drums in insect rock bands. Members of the grasshopper and beetle orders drum, and ants and other insects communicate by vibrating the substrate, but no arthropod comes close to the rhythmic virtuosity of stoneflies. Unlike crickets and katydids, male stoneflies actively search out their mates. A male of one species makes the first move, a brief crescendo of dull taps: “ba da da Da DA DA Dum.” The female responds with a comparatively quiet “ba da da da da dum.” Another species marches to a different drumbeat: “ta TA TA TA TA,” and the female responds with a slightly spaced-out “ta-ta-ta-ta-ta.” Some species repeat only this two-part pattern. Others use a three-part pattern: male calls, female responds, male confirms, and some use even more complex patterns of call and response. Stonefly specialists have studied about 150 species, each with its own pattern. Lacking drumsticks, stoneflies use their abdomens, tapping or rubbing the ground, or merely shaking their bodies to vibrate the substrate. Some species sport special abdominal appendages for drumming.
The number of beats, the shape of the beat—that is, how sharp the sound is—the interval between drumbeats, and the evenness of the rhythm all contribute to the specificity of the song. Some species favor a two-beat-per-second rhythm, others up to 20 beats per second. And the rhythm appears to arise, as in crickets, from genetically determined nervous system wiring.
Stonefly
© 2008 Stephen Hart
A computer-generated “drum solo” gets the attention of a female, but only if it comes close to matching the species-specific rhythm. Vary the program too much and the female ignores it. Furthermore, stoneflies of a single species living in Alaska and Colorado beat out and recognize a slightly different rhythm. The existence of dialects in these two populations suggests to scientists that they may be in the process of evolving into separate species.
Males and females can converse over almost a whole nine yards of wooden rod in laboratory tests, and through different twigs of the same limb. When researchers give the two stoneflies separate drumheads—for example, a paper cage in the laboratory, a dry leaf or dry bark in the wild—males and females could only communicate over about two yards. On a solid rock surface, with no drumhead, they couldn’t communicate at all.
Only virgin females respond to the male’s drumming, and once the conversation begins, the couple keep at it until the male finds his bride. The more vigorous their duet, the faster the male finds the female and they can mate. A typical male search in a horizontal laboratory arena might go like this: The male wanders randomly, calling occasionally until he hears a “come hither” response and begins the duet. He walks a short distance to his left and calls again. He notes the direction of the response and turns sharply right, calling again after aiming in the direction he thinks he should travel. Within a few turns, he finally finds the female. No one has studied male stonefly searching in the wild, where a typical “arena” would consist of a the many twigs of a highly branched tree. Scientists think the male may make some sort of innate triangulation to locate the female, and that she could judge his fitness as a mate by how long it takes him to find her. To facilitate the search, she stands her ground while communicating, but may move after a while to avoid slow males—and spiders eavesdropping on her conversation. If the male’s search proves successful, the pair mates immediately. Better communicators presumably stand a better chance of mating and laying eggs before a predator finds them. And passing genes on to the next generation is, after all, the name of the game, no matter what kind of communication the animals use.