For many kinds of animals, hearing may be more important than vision, especially for governing social interactions. Birds, for example, have excellent eyesight, generally much better than ours, but the social world of many birds is more a world of sound than of vision. Their call notes maintain contact with associates, convey alarm, or express levels of arousal; their songs attract mates and mark the limits of territory.
Birders long ago realized that knowing the sounds made by different species was a powerful identification method, and accomplished birders can reliably identify hundreds of species by their songs and call. After all, it’s how the birds sort themselves out!
In the insect world, likewise, vision may be crucial for avoiding danger, locating food, or dodging obstacles while in flight. But some groups of insects have evolved complex anatomical structures aimed at producing sound for social purposes. The cricket calling in your yard is a male, advertising his presence to rivals and potential mates alike. If the weather is warm enough, the cricket may call nearly continuously for days on end, investing a high percentage of his energy into making noise.
The structures involved in insect sound production vary from group to group, but for crickets and katydids, the mechanism is a simple one. These insects rub rough areas at the bases of their wings together. The resulting scratches resonate across the leathery forewings of the insect, gaining volume, and some species also position themselves on leaves (sometimes dry, dead ones) so as to broadcast still more loudly.
But unlike the sound-producing mechanisms of a songbird or human, the so-called stridulatory apparatus of a cricket allows for very little variation. These insects have about as much flexibility in their sound-making as you do when you run your fingers along the teeth of comb. You can move your finger faster or slower, but you have almost no control over the pitch or tone of the teeth as you pluck them. The noise an insect produces, in other words, is largely determined by the configuration of its stridulatory structure. And since those structures vary from species to species, the sound you hear is in a sense a representation of certain details of the insect’s anatomy.
As a birder who has branched out into insect observation, I inevitably began trying to learn the songs of bugs. Some calls I’ve learned the hard way, tracing them to their source and then visually identifying the singer. But it’s also possible to learn songs from recordings, which are available on websites such as Singing Insects of North America (entnemdept.ufl.edu/walker/buzz) and Cornell University’s Macaulay Library (macaulaylibrary.org/), or on CDs that accompany some insect field guides.
Simply listening to recordings of known identity is one good way to learn insect calls; the human ear, as any musician will tell you, is a sensitive and discriminating instrument. But I’m also interested in using recording insect songs to produce physical documentation of the presence of different species. Moreover, because the production of sound is so central to the biology of crickets and katydids, I believe that analysis of their calls can help us understand the relationships among insect populations. Differences in the anatomy of insects will translate to differences in the sounds they produce, and careful enough examination of enough songs may lead to discoveries that other methods of observation have overlooked.
I’m currently experimenting with a field recording rig consisting of a basic “shotgun” microphone (designed to focus on sound coming from a very small area), a small digital audio recorder, and a set of cheap headphones to monitor the recording process. Good results are not easy to achieve: even with the shotgun mic, you need quiet, windless conditions and a close-range “listen” to get good recordings. But with care and patience, you can generally capture a few seconds of clean sound.
I use a free program called Audacity, a basic sound file editor, to snip out the best five or six seconds of a recording. Then I process the resulting file in another program, Raven Lite, to produce a graphical representation, called a sonogram, of the sound. (Raven Lite is available for free from the above-mentioned Macaulay Library website.) Sonograms have been used for years to visually portray bird songs, and more recently to illustrate the calls of bats. But the advent of digital sound formats, approachable processing software, and inexpensive recording equipment has made sonogram analysis available to amateur observers like me.
With the graphical representation of a song in front of you, you can see the timing of individual pulses of sound, examine their shape, determine what the primary frequency of the song is, and discern whether there are overtones or undertones. By comparing an unknown song to sonograms made from identified songs, you can find a good match and put a name to your anonymous singer. In effect, this technology allows you to eavesdrop on insects, capturing their conversations in a form that makes sense to a human mind.