Even after a quarter-century in California, I still miss lightning bugs—especially in late summer. By some quirk of biogeography, they never made it west of the Rockies. We have a few species of glowworms, with luminescent wingless females and larvae, but no fireflies as such. And I also miss the nocturnal chorus of the katydids: what Sue Hubbell called “the audible essence of a summer night.”
Not that we don’t have katydids; I found two a couple of weeks ago, lurking in a pelargonium. But they don’t have the classic call. (The eastern true katydid has borne that name since colonial times; the first documented reference is by the botanist John Bartram in 1751. There are various stories about who Katy was and what she might have done, most involving a romantic triangle that ended badly). Although I’m not sure about the fine points of identification, the katydids in the garden, big green insects with long gawky hind legs and rhomboid wings, were probably either greater angle-wings (Microcentrum rhombifolium) or California angle-wings (M. californicum). The brand new Field Guide to Grasshoppers, Katydids, and Crickets of the United States describes the song of rhombifolium as “a loud lisp repeated every 2-4 seconds and a series of ticks that sounds much like someone slowly running a thumbnail along the teeth of a pocket comb.” Californicum’s song is a two-part lisp at 2.5-second intervals, without the ticks.
You can hear both songs, and many others, at the Singing Insects of North America web site (http://buzz.ifas.ufl.edu). Katydid songs tend to be high-pitched, with a frequency of 8 to 20 kiloHertz. Given the normal loss of high-frequency hearing, folks my age are often deaf to most of them. I was relieved that I could still register the performances of the common virtuoso katydid (Amblycorypha longinicta), said to be the most complex of any North American singing insect’s, and its relative the Cajun virtuoso (A. cajuni). Some entomologists use ultrasonic detectors to translate these high-frequency songs into audible range.
It’s not a vocal performance, of course. Katydids (and crickets) make music by rubbing their forewings together—a process called stridulation. Grasshoppers also stridulate, using their legs and hindwings. Near the base of a katydid’s left forewing is a specialized vein with a series of downward-projecting ridges, the stridulatory file. The right forewing has a sharp, upward-projecting structure called the scraper. Opening and closing the wings brings the scraper and the file into contact and produces the calling song.
Among true katydids, only the males sing. But Microcentrum is a member of the false-katydid subfamily in which males call and females respond with brief ticks, produced by a less complex apparatus. The song of each species is a unique combination of frequencies and rhythms. Entomologists have found that apparently identical populations with variant songs are in fact genetically distinct; the songs are part of a shared recognition mating system, ensuring that the callers find appropriate partners. In some, males move in the direction of a female’s tick calls; in others, the two move toward each other. They listen with the knees of their forelegs; how’s that for Intelligent Design? It’s a risky process; predators and parasitic flies may be eavesdropping.
In addition to the calling song, some katydids, according to the field guide, make a distinctive noise when threatened or harassed. This is called a “protest song,” but is otherwise not described. I doubt that it sounds like “We Shall Not Be Moved,” though.
Human musicians have picked up on the idea of stridulation, of course. The frottoir or rubboard of Louisiana Creole music is a pretty close analogy to the katydid’s file. It used to be an actual washboard hung around the player’s neck until 1946, when Cleveland Chenier asked his welder friend Willie Landry to make him a special model with built-in shoulder straps. Chenier, brother of accordionist Clifton, played with a handful of bottle openers and other picks and could get amazing echo effects on the rubboard. Puerto Rican jibaro musicians use a gourd rasp called the guiro; and in Kenya, the ridged surfaces of Fanta soda bottles are or were used to similar effect. But the most remarkable parallel to katydid music, and one that was only recently discovered, is a product of evolutionary convergence: a stridulating bird.
In the forests of Ecuador there’s a small red-headed bird called the club-winged manakin, a member of a family noted for elaborate male song-and-dance displays. The club-wing waves its wings over its back, producing a loud, clear, violin-like tone. Cornell ornithologist Kimberly Bostwick noticed that one feather on each of the bird’s wings had a series of seven ridges on its central vane and another had a stiff, curved tip. Equipped with a camera that could record a thousand frames a second, Bostwick filmed the manakin in action. She found that the bird shakes its wings a hundred times a second. With each shake, the scraper feathers hit the feathers with the ridged vane, just like the action of katydid wings or a rubboard player. That produces 14 sounds per shake, with a frequency of 1,400 cycles per second.
Bostwick calls the modified wings of the male manakin a tribute to the power of sexual selection—the same process that drove the evolution of the songs of the katydid: “Darwin would have loved it if he had known.” In fact, Darwin did write about the specialized music-making wing feathers of a species of manakin in The Descent of Man, among a long catalog of secondary sexual characteristics. He just didn’t have the technology to go beyond description to functional analysis. Darwin with a high-speed camera—there’s a thought. A scanning electron microscope would have been nice, too.