Tuesday, June 26, 2012

The high mathematics of grasshopper identification

We are in serious drought now, last summer repeating itself. One hundred degrees was forecast for the day. It took me a long time to learn, but I have, finally, learned that on this kind of day there is no point driving a long distance to one of my favorite spots to look for insects, because they won't be there, and I can knock myself out dragging through the heat, and I won't find a thing. Butterflies won't be flying, because in the heat the flowers shut down and produce no nectar. Robber flies won't come out, because there will be nothing out for them to catch. Tiger beetles will stay buried under the sand, dragonflies will roost in the shadows. Grasshoppers, good old grasshoppers, will be out, but they tend to stay in the open barren places of straight overhead sun where you can't stand to be yourself.

I decide to go to some place nearby, and just go for the morning when the heat is still in the bearable 80's. I go to the Jonesboro Nature Center, and walk out on the prairie. It's alive with grasshoppers, and there are three species I find myself concentrating my attention on. Oddly, these all require geometry to identify.

Let me show you a picture of one of the three species, and then I'll explain how it works.

This is the rather inappropriately named Northern Green-striped Grasshopper. Females are green and no doubt have a green-stripe somewhere; males like this one are brown and are green-stripeless. You identify this species by a line through the eye that gives it a sort of sad, droopy-eyed expression, by the narrow ridge on the pronotum (the pronotum, on grasshoppers, is the sort of saddle-like cover that goes over the thorax), and by a sort of sharp narrow ridge on the upper side of the big hind femur (the upper long joint of the leg). As a final confirmation, this species has brown hind tibiae. The tibia is the skinny second long joint of the leg. On grasshoppers, the hind tibia is hidden under the femur. It is often a bright color, and is revealed during sexual displays. Dull brown on this species, and, as I say, very difficult to spot.

Now, the Northern Green-striped Grasshopper is one of the most common grasshoppers in the United States, found everywhere except in the West, and in Florida.  It has, however, a lookalike twin, the Southern Green-striped Grasshopper. According to the published range maps, that one doesn't occur in Arkansas, it occurs only in the Gulf states and in Florida. Well, I carefully examined many pictures I had taken of the Northern last year, and realized that one that I took a picture of was the Southern Green-striped, completely out of its known range. Here's how I could tell, and here's where the geometry comes in. If you look at the above picture again and look at the back edge of the pronotum, you'll see it makes a sharp-pointed angle facing backwards. The angle is acute, which means it is narrower and more pointed than a right angle would be. On the Southern Green-striped the shape is a right angle, and if you think you see that right angle, you can get confirmation by getting a peek at the hind tibia, which, in the Southern, is bright blue. Here's the picture.

What I want to find out this year is just how common this Southern version is. I know it is much scarcer than the Northern. On this day I was trying to check all Northerns that I saw, and I didn't find any Southerns. Maybe they don't come out until later in the year.

Luckily it was a Monday, and the Nature Center is closed on Monday, so there weren't any people around to wonder what I was doing out in the middle of the prairie crawling around on my hands and knees.

On this day, there were two more species I was interested in, and by odd coincidence, each of them had a lookalike twin, and in each case the twin species could be identified by the same geometry.

The first of them was the Orange-winged Grasshopper. I think I have mentioned this one on previous blogs. The Orange-winged has a twin, the Coral-winged, which, according to the books, occurs at the same times and in the same places as the Orange-winged, but I had never knowingly seen it, so it was one of my major targets for this summer. When I had gotten a little way into the prairie I chased up a large grasshopper with bright orange wings. It didn't fly far, and landed out of sight down in a thicket of sticks and dried grass stalks. I searched carefully and finally spotted it. It was a large female, and they would rather hide than fly, so this one stayed still while I lifted the sticks, and bent the grass stalks, out of the way so I could get a shot at it with my camera.

The Orange-winged, a very common species, comes in a green phase, and in a gray-brown phase. This one was a reddish brown such as I had never seen, and it occurred to me that maybe the Coral-winged came in such a color. So very carefully, trying not to startle it, I positioned myself for a directly overhead shot.

The hind femur on the right side is slightly out from the body so I could catch a glimpse of the blue at the base of the leg on the inside. This is diagnostic for Orange-winged, as the Coral-winged does not have this feature. But the blue is not always revealed, and often when you try to get in this position, the grasshopper takes off before you can photograph it. But I didn't really need to see the blue, because in this case it is the Coral-winged that has the angle at the back of the pronotum acute. The angle on the Orange-winged, as you can plainly see here, is a right angle.

So, I must go on looking for the Coral-winged.

The third species I was concerned with, and this one was the most fun, was the Ridgeback Sand Grasshopper. Like the other two it is in the group known as the Band-winged Grasshoppers. These are typical grasshoppers of open places, long-winged strong leapers and flyers, often with bright colored wings, and a tendency to go clickety-click as they fly. Except for the wings, mostly they are in the colors of the sand and rocks and dry grass they live in. The Ridgeback Sand is a typical member, looking like he is made out of the sand he is sitting on. He is noteworthy, however, for the extremely elevated crest on his pronotum.

If you look at that very high ridge on the pronotum you will notice, about a third of the way back from the head, a deep furrow, or sulcus. Well, this furrow is very deep, in fact it cuts half way through the ridge. So this was my fun: I spent all morning chasing them around trying to get at exactly the correct angle so that I could take a picture right through the the cut. Here's what I finally managed.

There were a lot of these present on the prairie, and courtship was going on. A number of males would approach a female all doing their wild courtship dance which mainly consisted of flashing their hind tibiae and waggling them about. Like many grasshoppers that blend invisibly into their drab colorless backgrounds, their usually concealed (repressed?) hind tibiae were their one chance to dazzle the females with brightness. Here's a shot of one I got that was almost going too fast to keep in the view-finder.

This species, in keeping with my theme, also has a lookalike twin. The new wrinkle is, neither one of them should be found here. The Ridgeback Sand Grasshopper, according to the range maps, is found in the southwest corner of Arkansas, and here it is in the northeast in Jonesboro. Its nearly identical twin, the Mottled Sand Grasshopper, is only found to the north of us, in fact, north even of Missouri. Since neither of them belonged here, I had to make sure which one I was seeing. And the trick once more is to look for the angle on the back of the pronotum. In this case, it's the Mottled that has the right angle, and the Ridgeback that has the acute. Here it is: I'll let you decide for yourself.

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