Wild Side: Long-winged katydids

Why did the katydid grow such long wings?

The straight-lanced meadow katydid, Conocephalus strictus, is a very common insect on Martha’s Vineyard, inhabiting dry habitats with a mix of grasses, other herbaceous plants, and, often, low shrubs. In favorable habitats, I sometimes find this species at densities of several adults per square meter.

Females sport a long spike, called an ovipositor, that extends from the tip of the abdomen. Significantly longer than the body of the insect, this ovipositor serves its owner as a device for positioning eggs in suitable spots. It serves human observers by making females of this species very easy to identify!

The ovipositor is the “straight lance” that gave C. strictus its common name. Other katydids all have ovipositors of some description, but in none other of our species does this organ approach such a glorious length. Males, alas, are harder to identify, though they, too, have distinctive (if much smaller) appendages on their abdomen, and, with practice, have a fairly recognizable overall “look.”

If the straight-lanced meadow katydid is overendowed with ovipositor, it’s generally deficient in the wing department. On typical adults of either sex, the wings are reduced to little nubs. On males, the winglets, when rubbed together, make a faint, whirring sound that is the mating call of this insect. But as you’d expect, these wings are utterly useless for flying (though like all katydids, this species can hop prodigiously).

But like a few other Orthoptera species, Conocephalus strictus occasionally appears in a long-winged form (“macropterous,” as opposed to the normal “brachypterous” or short-winged form). Long-winged individuals are generally very rare, constituting far less than 1 percent of the population on the Vineyard. Until recently, I had seen exactly one in 10 years of watching this species, during which time I surely saw many hundreds, or even thousands, of individuals.

Recently, however, while bugging at one of my favorite sites in Edgartown, I came across a population of C. strictus in which about half the individuals were long-winged. The wings, when they’re present, turn out to be fully functional: These things flew competently when disturbed, often traveling 30 or 40 feet before dropping back into the grass. Over the course of an hour or so of poking around, I probably saw a total of 50 individuals, adding up to a large number of a form I had supposed was always very rare.

A localized bloom of macropterous katydids, it turns out, is a well-documented phenomenon in some species. In Roesel’s bush cricket, for example, a distant relative of C. strictus that does not occur on the Vineyard, high incidence of long-winged individuals is associated with either very crowded conditions or with hot, dry conditions that limit the amount of food available. Long wings may also be associated with newly established populations, in which the insects presumably face uncertain conditions. But in any case, long wings appear to be aimed at helping some of the population disperse to seek better foraging, more temperate conditions, or less crowding.

In other words, many, most, or maybe even all individuals in these species have the genetic potential to produce long wings. Mostly, they don’t. But conditions that stress the species presumably trigger some kind of hormonal change that turns on the switch for growing long wings.

Mobility comes at a cost, however. Those glorious long wings represent a substantial investment in protein and other nutrients, which the insect could apply to other needs if it didn’t grow long wings. In particular, long-winged females appear to sacrifice egg production; protein that could go toward reproduction goes instead toward helping the female search for more congenial conditions.

This has certainly been a warm, dry summer, the kind of conditions that one might suppose would stress a small katydid, and perhaps stimulate wing growth. But if that were the reason for the dozens of long-winged katydids I found, I’d expect that other populations around the Island would show the same response, since the entire Vineyard has been subjected to the same summer weather. I haven’t observed that, so weather can’t be the only explanation.

The property with the long-winged katydids on it, though, was also treated with a prescribed burn early in the year. And that’s true of few if any of the other sites where I see Conocephalus strictus. Perhaps some residual chemical effect of the burn influenced wing development? Or perhaps the transient heat of the burn, which occurred before this year’s generation of C. strictus had hatched, affected eggs, and somehow turned on the mechanism for growing wings?

The “fire effect” is just a hypothesis at this point, one that I’ll try to investigate on burn sites in future seasons. But I can’t discern any other stimulus that might have stressed these insects just at this particular location. So this is one more of my ever-growing list of questions to keep in mind.