Thursday, January 12, 2017

Identify Yourself!




The break between fall and spring semesters was my long-awaited opportunity to begin editing the backlog of insect song sound files I recorded last fall. It’s far more than analytical work. When I’m listening, I’m inevitably immersed in what I heard, saw, and felt as I made those recordings. I emerge feeling as though I’ve actually been back in the meadows, and it takes a few minutes to readjust to the snow and ice outside my windows. Only four months until the first Spring Field Crickets begin to sing in NE Ohio…

In going through those sound files, I’ve focused on sorting out the Black-horned Tree Crickets (Oecanthus nigricornis) from the Forbes’s Tree Crickets (Oecanthus forbesi).  They look identical and live in the same kinds of habitats, so visual cues aren’t helpful, and their exact ranges aren’t known. Doesn’t that sound like the kind of challenge I’d find engaging?

Here are two Black-horned Tree Crickets from Novak Sanctuary State Nature Preserve in Portage County, which is southeast of Cleveland. The male is singing, and the female is ovipositing. You can see the black legs and antennae, and these crickets’ heads are typically partially or entirely black.




Now here are two Forbes’s Tree Crickets. As I noted above, they have black legs and antennae, and their heads are typically partially or entirely black.





The only way to tell them apart is to measure the number of sound pulses per second in their songs. A pulse of sound is generated when a singing cricket closes his wings, engaging the file on one with the scraper on the other. (“Closing” doesn’t mean “lowering – the wings are still raised.) The number of pulses is measured against the temperature at which he is singing in order to get the “pulses per second.” 


You may recall from other posts that crickets and katydids sing higher and faster when it’s warm and slower and lower in cooler temperatures. Therefore, pulses per second will be affected by temperature: there will be more pulses per second at 78 degrees than at 68. 

Got it? I promise it won’t be as tedious to read this post as it is for me to count all those pulses per second on my sonograms. 


Now for the song comparison.

The first recording is a Black-horned Tree Cricket on 9-22-16 at Novak Sanctuary State Nature Preserve in Portage County, which is southeast of Cleveland. He’s singing at 47 pulses per second at 72.5 F. I’ve highlighted a one-second area of the sonogram.



The next recording is a Forbes’s Tree Cricket from Bath Nature Preserve in Summit County, which is south – but not southeast – of Cleveland. He was an old guy who was getting a little creaky when I recorded him in his goldenrod on 11-1-16. He’s singing at almost exactly the same temperature – 73 degrees F – but he sings at 59 pulses per second. That means there are more wing strokes per second in his song, and to my ears, it sounds a little denser or more concentrated. (You can also see in the sonogram that an occasional wing stroke isn’t as strong as the others. Remember, tree crickets in early November are elderly!)




So who lives where, and what’s the actual overlap zone? Is the Forbes’s moving east, or have they been here all along? Do the two species coexist, or does one replace the other? And if it’s changing over time, what’s the baseline as of now?

Forbes’s Tree Crickets had been documented as far east as the Sandusky Bay area on Lake Erie’s central Ohio shore and also in the Columbus area. Now I’m documenting them here in NE Ohio. I go through the pulse-counting process for all the Black-horned/Forbes’s I record at a specific location, then enter the data in a graph for that particular place. 


Dr. Thomas Walker, entomology professor emeritus at the University of Florida, created a graph on which a Forbes’s or Black-horned’s song can be placed by pulses per second at a given temperature. The upper line would be the Forbes’s, as they sing at more pulses per second at a given temperature. The Black-horned Tree Crickets would fall along the bottom line, as their songs have fewer pulses per second at the same temperature. In my graphs, the upper line is orange and the lower line is blue.

So far, I’m finding Forbes’s Tree Crickets east along the Lake Erie shoreline counties and a little inland all the way to the eastern edge of Lake County and into Geauga County. Here are three examples:

Lake Erie Bluffs, below, is near the Lake/Ashtabula County line. I have only recorded Forbes’s Tree Crickets there, and this chart shows five of them.


I recorded only Black-horned Tree Crickets at Novak Sanctuary/State Nature Preserve in Portage County near the Summit and Geauga County borders.


At Bath Nature Preserve, which is the location of the University of Akron Field Station, I found both Forbes’s and Black-horned Tree Crickets. Some crickets’ songs fell in between the two. I had no idea when I recorded them that both species were present!



(These two crickets are from the Garden Bowl wetland at Bath Nature Preserve. He is a Forbes's and she could be as well.)






There isn’t too much time to gather data, as these crickets mature in early August and their lives end when freezing temperatures arrive. I can record at night while nights are warm, but these crickets sing only in the afternoon when days get shorter. As adequate warmth becomes increasingly unreliable, I bring some home to warm terrariums and record them at a constant temperature right here.

Take this old guy, for example. He was already a senior when I brought him home, but he was still singing quite a bit. You may be able to hear - and perhaps even see in the sonogram - that his song wasn't as rich and full anymore and he sounds rather crackly and creaky. Apparently, this did not diminish his attractiveness in the terrarium.






I decided to bring a female home for him, and did they ever hit it off! They were together all the time, often sharing the same leaf. It almost looked like an autumn romance. 


But they were old, as I said. After she died, I brought home another female. Within 15 minutes, their courtship was underway. 

 


I didn’t think he still had it in him. He died a few days later, though she lived for another few weeks.

I had two other Forbes’s males as well. The one who lived in a kitchen terrarium near the refrigerator instead of in the dining room had a rich, strong song that sounded like he could have been younger, though I have no idea if that could possibly be true. (Some human singers “keep their voices” later in life. Could something comparable be true for crickets?) 

      
     (Yes, that's him. I brought him home from Lake County.)





I think what I’ll try to do next year is have a Forbes’s, a Black-horned, and a Four-spotted in separate terrariums here so that I can really working on refining my listening skills! In the meantime, here is my work-in-progress Google map of Forbes's and Black-horned Tree Crickets in northern Ohio. Purple is Forbes's, orange is both, and blue is Black-horned.



I just wish I didn’t have to wait until next August to record more of them!


Many thanks to Neha Reddy, who made it possible for me to enter all these cricket songs using Dr. Walker’s regression lines.

6 comments:

  1. Thank you, Lisa. The time you spend on understanding these two species is admirable.

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    1. And thank you for your inspiration and guidance in separating these two!

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  2. Love your photo of the mating pair, with the female on top, licking the gland on the male's back. I have great video of that behavior for a pair of Broad-wingeds.

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    1. Thanks, Lang! Your video must be quite splendid.

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  3. Lisa, great that you are documenting the range and sounds of these tree crickets, great photos and recordings! we would love to archive some of your recordings if you are interested.

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  4. Beautiful photos and fascinating post! I love nature and the sounds are soothing and therapeutic.

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