by Carl Strang
In a preliminary way I have begun to look at the question of how wing length affects song production in Roesel’s katydids. This seems pretty esoteric at first glance, but it has personal practical application, as I will explain. Roesel’s katydid is a European import which first appeared in the late 1940’s around Montreal, Quebec. They since have spread to the point that they can be found throughout northern Indiana and Illinois, and at least much of Michigan and Wisconsin. They come in a variety of wing lengths:
Short, as in this female in Kendall County, Illinois…
Intermediate, as in this male recently photographed at St. James Farm…
And long, as in this male at Blackwell Forest Preserve.
Researchers have noted that long-established populations in the European homeland are composed almost entirely of shorter winged, therefore flightless, individuals. The frequent observation of long-winged variations here has fostered the speculation that this has to do with dispersal as Roesel’s katydid expands its North American range. That may be true in the long run, but the edge of that range is far from the Kendall and DuPage County locations of the above individuals, and long-winged ones remain common.
Since I began my study of singing insects in 2006, I have been disappointed to find that each year my ability to hear the buzzing song of male Roesel’s diminishes. Furthermore, on hot days I can hear them fine in the cooler morning hours, but at some point I can no longer hear them. The pitch of their song rises with temperature, until it goes above my audible range. The SongFinder pitch lowering device proves they still are singing (incidentally, the buzz is steady when I hear it unaided, but through the SongFinder it has a superimposed vibrato which becomes more rapid with increasing temperature. This doesn’t happen with other singing insects).
Over the winter the question came to me whether the wing length of the males affects their song. Next year, retirement will allow time to pursue this in more detail, but for now I have an afternoon’s observations that seem to support a difference. I was following trails through north Blackwell’s meadows, occasionally listening with the SongFinder. The temperature was mid-70’s F. Many Roesel’s were singing, but I would not have known this without the device. Then I heard one unaided. I sought it out, and it was long winged. The same was true of two other individuals (the third is the one in the above photo).
Next year I want to make recordings of individuals with different wing lengths, and compare the parameters of their songs. Perhaps the analysis also will give some idea why the pitch-altered song has that vibrato. I want to think, too, about the possibility that there is an evolutionary dynamic going on here. If the song is different among males with different wing lengths, does that matter to females? If so, how? For instance, if long-winged females prefer long-winged males, and short-winged females prefer short-winged males, this could retain genes for long wings in the population for a reason other than dispersal advantage.