One of the most endearing characteristics of grasshoppers is their ability to produce sound. Some of the most wonderful memories of my childhood include sitting in a meadow bursting with sounds of insects and watching grasshoppers use their hind legs to produce soft, rhythmical songs, and not realizing that a seed that would eventually blossom into a full-blown career in entomology is sprouting in my brain.
Despite the strangely persistent misconception, the sound of grasshoppers is not produced by rubbing their legs together (in fact, no insect makes sound in this way), but rather by dragging the inner side of the hind femur against a thick vein on the front wing (depending on the group, either the femur or the vein is armed with a row of stridulatory pegs). But this ability to produce loud songs is far less common among grasshoppers than it may appear to somebody who grew up in Europe, one of the few places in the world where members of the vociferous subfamily Gomphocerinae dominate the grasshopper fauna. I was surprised how few grasshoppers sing in North American or Australian meadows, and tropical grasshoppers of South America and Africa are almost all silent.
I was therefore quite startled when I caught yesterday in Gorongosa a beautiful grasshopper Cataloipus cognatus, and the insect responded to this violation of its freedom by producing loud and persistent squeaks. It took me a while to discover how the sound was produced. At first I thought that it was using its hind legs to make the sound, but this lineage of grasshoppers (Eyprepocnemidinae) lacks stridulatory pegs on their legs, and besides, I was holding it by the legs and thus it couldn’t use them even if it wanted to. Looking closely I realized that the grasshopper’s sound was coming from its mouth. I knew of a katydid species that was capable of stridulating with its mandibles, but had no idea that some grasshoppers could also do it.
Just to be sure I caught a few more individuals, and some made the sound while others didn’t. Then I noticed that the silent ones were all females, while all males were producing the sound. Since I don’t have a microscope here in the Chitengo camp (yet, one is coming soon, fingers crossed), I could not look at the structure of the sound-producing apparatus. But I recorded the sound and looked at its oscillogram, which revealed a clean, evenly spaced pattern of pulses, which is indicative of the presence of a distinct stridulatory file. This, combined with the fact that only males produce sound, seems to suggest that the sound might be used not only as a defensive signal, but rather that it may play a role in courtship. If this true, and I will try to confirm it by watching the courtship behavior of this species, it would make a very interesting case of independent evolution of courtship stridulation in eyprepocnemidine grasshoppers.