Tree Cricket Clarity

by Carl Strang

A year ago I was struggling with the identification of a group of four meadow-dwelling tree cricket species that reportedly live in northeast Illinois and northwest Indiana. They have been determined to be close relatives, and two of them had been established as sibling species by Thomas Walker of the University of Florida: the black-horned tree cricket and Forbes’s tree cricket are physically alike, and can be distinguished only by the pulse rate of their songs, which requires the analysis of sound recordings.

Forbes’s and black-horned tree crickets can be very black, as shown in this individual at Pinhook Bog in Indiana.

Forbes’s and black-horned tree crickets can be very black, as shown in this individual at Pinhook Bog in Indiana.

This is a paler representative of the species pair from the Great Marsh, also in the Indiana Dunes National Lakeshore.

This is a paler representative of the species pair from the Great Marsh, also in the Indiana Dunes National Lakeshore.

One member of this species group, the four-spotted tree cricket, is readily identified. It is pale, and has a distinctive pattern of spots on the basal segments of its antennae.

All four of the spots on the two lowest antenna segments of this four-spotted tree cricket in Fulton County, Indiana, are relatively small and well separated. The outer spot on the basal segment is nearly round.

All four of the spots on the two lowest antenna segments of this four-spotted tree cricket in Fulton County, Indiana, are relatively small and well separated. The outer spot on the basal segment is nearly round.

The final species is the prairie tree cricket. I thought I found this species last year at Mayslake Forest Preserve, again following the long-established focus on antenna spot patterns.

This individual was generally quite pale, and the antenna spots matched reference drawings for the prairie tree cricket.

This individual was generally quite pale, and the antenna spots matched reference drawings for the prairie tree cricket.

When I analyzed my sound recordings of the songs of prairie/black-horned/Forbes’s tree crickets from Mayslake and other DuPage County locations, however, they generally fell out as Forbes’s, though some recordings made at lower temperatures were somewhat ambiguous.

This confusion has been largely resolved, now, thanks to the recently completed Ph.D. thesis work of Laurel Symes. She traveled widely in her study of these species, collecting specimens and analyzing their songs and their genetic relationships. Though her focus was on female response to male songs, and the associated behavioral, ecological and evolutionary implications, the information she collected also is very helpful to my regional survey of singing insects.

Though she doesn’t say this herself in anything I have seen, it seems clear that we need to throw the time-honored focus on antenna spot patterns out the window for three of the four species, though it still holds for the four-spotted tree cricket. Laurel found a geographic separation between black-horned and Forbes’s tree crickets, with a zone of contact that may involve some hybridization. That zone is in Ohio, safely east of the Chicago region. Unless something new emerges in the future, I am following Nancy Collins in calling all our local ones Forbes’s tree crickets as a result of Laurel’s research.

As for the prairie tree cricket, it, too, seems not to be in the region, occurring well south and west of us. I will retain it on the hypothetical list, though, because Laurel’s results were less certain on this point.  In any case, as long as the temperature is warm (at least 68F), the remaining three species can be distinguished from sound recordings, so they need not be captured. All you need is the temperature, the graphs relating pulse rate to temperature, and a pulse rate count from the recording. At the standard temperature of 25C, four-spotted tree crickets have a pulse rate of 40 per second, prairie tree crickets 51 per second, and Forbes’s tree crickets 65 per second. Laurel found that selective pressures on male song and female choice keep these quite separate and narrowly defined, .

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Song Analysis

by Carl Strang

One of my goals this autumn was to begin sorting out the meadow dwelling tree crickets of the nigricornis species group at Mayslake Forest Preserve. In earlier posts I shared some photos that pointed to a mix of species, but now I have studied their songs, and the results prove to be complicated. One individual was straightforward. He was unique in having an abdomen that was black beneath.

I immobilized him briefly in the freezer for photos.

I immobilized him briefly in the freezer for photos.

His antennal spots clearly distinguished him as either a black-horned tree cricket or a Forbes’s tree cricket.

There was clear separation between the spots on the second antenna segment, and those spots were narrow.

There was clear separation between the spots on the second antenna segment, and those spots were narrow.

When I looked at my recording of his song in the Audacity sound analysis program (available as a free download), the pulse rate clearly was that of a black-horned tree cricket.

The program allows the pulse rate to be counted, in this instance the interval between the 6-second and 6.5-second point in the recording. The pulse rate of 36-37 per second at 65.5F is unambiguously that of a black-horned.

The program allows the pulse rate to be counted, in this instance the interval between the 6-second and 6.5-second point in the recording. The pulse rate of 36-37 per second at 65.5F is unambiguously that of a black-horned.

The other tree crickets were problematic. Here is the most ambiguous case. His antenna spots, along with his generally pale coloration, said he was a prairie tree cricket.

The spots on the second antenna segment were thick, and had a very narrow space between them.

The spots on the second antenna segment were thick, and had a very narrow space between them.

On the other hand, his song had a very rapid pulse rate.

A pulse rate of 48 at 68F is close to that of a Forbes’s tree cricket.

A pulse rate of 48 at 68F is close to that of a Forbes’s tree cricket.

All the other tree crickets I tested had songs most like Forbes’s, but only one had the antenna spots to match. The others either were closer to those of the prairie tree cricket, or were ambiguous. The literature I have seen cannot carry me any farther. My next step will be to look at nigricornis-group tree crickets at other sites in 2013, and hope for enlightenment.

Prairie Tree Cricket

by Carl Strang

Recently I shared the story of a tree cricket that I suspected may be a prairie tree cricket. Last week I caught a second one which confirmed the identification, at least as far as I can tell.

As before, I put him in the freezer for a few minutes to immobilize him for photos.

Like the first, he was pale, and his extremities were brown rather than the black of a black-horned or Forbes’s tree cricket.

Beneath, he was dusky on the underside of the abdomen, but not black.

The critical feature is the spotting on the first two antenna segments.

In this one there was none of the smudging that fuzzed one edge of one of the spots on the first cricket. The spots on the first segment were large and fused. The spots on the second segment also were thick.

I was concerned that the pixilation in the photos might be distorting the appearance of spacing between the spots on the second segment, so I took the added step of examining them through the magnifying glass. The spaces were indeed being exaggerated by the camera. This further supports the case for prairie tree cricket.

So now I have the impression that all four species in the nigricornis group of meadow-dwelling tree crickets are present (I have found the four-spotted tree cricket in a few locations), and all may be common, in northeast Illinois. The prairie tree cricket, once a western species, has spread east thanks to agricultural practice. After catching the subject of this post I have had a devil of a time trying to get more (they seem to be staying close to the ground for warmth), so I may not make more progress in quantifying the relative numbers of the species this year. Unfortunately the pulse rates of prairie and black-horned tree cricket songs are nearly identical across the range of temperatures in which they sing. Otherwise I could do the survey entirely with recordings. The four-spotted seems to prefer grasses, but song perches for the other three are goldenrods, asters and even woody plants, so there is little to distinguish them in habitat preference. As for monitoring practice, it looks like this will simply have to be a category “nigricornis group” rather than a count of the separate species.

A Cricket’s Adventure

by Carl Strang

At mid-day on Friday, a tree cricket sang from a Canada goldenrod plant near the eastern edge of Mayslake Forest Preserve.

The central character in today’s story.

At this point in the season, the long-trilling meadow-dwelling tree crickets are mainly members of the sibling species pair of black-horned tree cricket and Forbes’s tree cricket. The two species cannot be distinguished except through an analysis of their recorded songs. The critical feature of the song, the pulse rate, varies with temperature, and in the field the temperature can vary significantly with microsite at this time of year. So, I caught him and installed him in a cage. Back home, in the quiet of the evening, he began to sing and I got my recording.

The next day I wanted good photos for documentary purposes. I put the cricket in a jar, and the jar in the freezer for 10 minutes. This immobilized the little guy long enough for me to get photos.

The dorsal view shows that this individual is very pale. The antennae and tarsi are dark, but not the black typical of the two species. There is no stripe on the pronotum.

There is some dusky pigmentation on the underside of the abdomen, but this too can be much darker in black-horned/Forbes’s tree crickets. He looks dead, but he fully revived within 15 minutes or so.

The spots on the first two antennal segments are very helpful in tree cricket characterization. Here the spots are large, dark and close together.

The cricket’s adventure was nearly complete. I had a public night hike Saturday night, and I took him in a jar, releasing him with an explanation of his story and the reason I put him through it.

It was only later, when I studied the photos closely, that I began to have some doubts. The heavy, close-together markings correspond more closely to those of a third species, the prairie tree cricket, which I had been seeking but not found. The literature and on-line information seem to suggest that, nevertheless, this could be a black-horned/Forbes’s. The critical question seems to be, how wide is the space between the two bars on the second antennal segment? If it is less than a third the width of the inner bar, then this is a prairie tree cricket. If more, it’s a black-horned/Forbes’s. As best I can tell, it is very close to this critical width. I will continue to pursue the question for this individual, but all of these species show some variability, and my best response is to find more crickets in Mayslake’s meadows, and see what they tell me.

The Meaning of “Range”

by Carl Strang

This is the third installment of a weekly series on singing insect species that supposedly occur in northeast Illinois or northwest Indiana but which I have yet to find after several years’ field work. Today I will consider two crickets representing different groups. They have in common a certain peculiarity in their range maps. We’ll start with the melodious ground cricket.

As you can see, the shaded area on this map (from the Singing Insects of North America website, or SINA) places nearly all of Illinois within the range of this species. However, that shaded area is computer generated from the only concrete records, which are represented by the black dots. Note that there are no dots anywhere in Illinois. As far as the SINA database goes, the species never has been found in Illinois, and there are only two records from northern Indiana. Illinois is included in the range thanks to a single record from northern Missouri. The species first was described from Ohio in 1957 by Edward S. Thomas and Richard Alexander. It is very similar to the Carolina ground cricket physically, but its song is described as a more melodious trill (lacking the Carolina’s discordant overlay of tones), and its habitat is narrower, limited to bogs and marshes. Even in Ohio there are few locations. The map shows melodius all over Florida, where Thomas Walker (who runs the excellent SINA site) is located.

The next example is the prairie tree cricket.

Northeast Illinois and northwest Indiana are included in the range for this species thanks to two records for Cook County, Illinois, from 1934 and 1935. On the other hand, two records from Iowa probably would have led the computer to shade our region even without those old Cook County records. I mentioned in an earlier post this year that I am looking for prairie tree crickets in sweep samples from meadow and prairie areas, but so far have found only four-spotted, and black-horned/Forbes’s tree crickets in that sampling.

These examples underline the need to be careful in thinking about the geographic range of species. Some singing insects are relatively general in their habitat choice, or tolerant of human alterations in the landscape, or simply have been fortunate to have passed through the sieve of history unscathed. They are the common ones. Shaded areas in range maps like SINA’s represent them best. Other species are much pickier, or their lower densities have made them subject to more frequent local extinctions over time. They are best represented by dots. The best example I have encountered here is the sphagnum ground cricket, which appears indeed to be limited to the narrow confines of sphagnum moss areas. These were more ubiquitous in the broad zone which trailed the last continental glacier north, but then in southern parts became isolated in little bits here and there.

I am not removing melodious ground crickets or prairie tree crickets from my hypothetical list for our region, but until I find them I will not list them as definitely occurring here today.

Meadow Tree Crickets

by Carl Strang

Some tree crickets live in trees, others live among the trees in the forest understory, but there are a few species that inhabit meadow and prairie areas. Last week I looked at some of these when I did some sweep sampling in two locations. The first was Springbrook Prairie Forest Preserve.

This low area dominated by big bluestem grass was part of the habitat I surveyed.

I went there in hope of finding more of the cicadas, tentatively identified as swamp cicadas, that I heard there last year. At the time the song seemed definitive, but I since have learned that Linne’s cicadas sometimes have songs that are similar, and so I was hoping to find one to photograph. All was quiet, however. That’s not necessarily a bad sign. If this is indeed a new population (DuPage is north of the published range for swamp cicadas), they might not be emerging every year. In any case, while I was waiting for singing cicadas I did some sweep sampling, and turned up several tree crickets.

Here is one of the individuals I caught in a goldenrod-dominated area. It has a dark stripe down the top of the head and pronotum, as well as dark antennae. Despite the otherwise pale color, these features point to the black-horned/Forbes’s pair of tree cricket sibling species.

Proper identification of these tree crickets requires an examination of spotting on the basal two antenna segments.

Here the spots on the first, basal segment are very large and smudged. Those on the second segment likewise, to the point where the entire segment looks black at first glance. Also note the dark area on the underside of this tree cricket’s abdomen. All these features point to black-horned/Forbes’s.

The next cricket, from the big bluestem area, is much paler, and shows a different antennal spot pattern.

Here the spots all are smaller, and the outer ones on the basal segments are round and have smeared edges. This is a four-spotted tree cricket.

Another Springbrook tree cricket was more ambiguous.

This individual happily nibbling my finger shows spotting that falls within the range of overlap for four-spotted and black-horned/Forbes’s tree crickets. The most definitive spot is the outer one on the basal segment. It appears just large enough to rule out four-spotted.

The next day I did some sweep sampling at Mayslake Forest Preserve.

The sample included several pale tree crickets like this.

The cricket in that last photo had the following spot pattern.

This is a clear indicator of four-spotted tree cricket, the outer spots on both segments small, round, and faded looking.

A final example showed even fainter spots.

Again I identified this one as a four-spotted tree cricket.

There is a fourth meadow species that I have been watching for but so far have not found. The prairie tree cricket is generally pale, like the four-spotted. Its antenna spots are heavy and close together, but without the blurred smudging of the black-horned/Forbes’s species pair.

Tree Cricket Ambiguity

by Carl Strang

During a recent Take Your Kids To Work Day program at Forest Preserve District headquarters in Danada Forest Preserve, I was one of the teachers in an entomology unit. An adult tree cricket turned up in a sweep net sample, and I decided to try for photos.

As seems common with tree crickets, this one was interested in palpating my finger (often they’ll nibble as well). The shot I was seeking was a photo of the insect with its antennae held back, preferably taken from a quartering angle. I did get a usable one.

Those dark spots on the first two antenna segments often are distinctive enough for species identification. The most recent authoritative drawings and descriptions I have seen were contributed by a leading researcher on tree crickets, Tom Walker, in the Field Guide to Grasshoppers, Katydids and Crickets of the United States (2004, Cornell University Press), which he co-authored with John L. Capinera and Ralph D. Scott. The Danada cricket falls into a middle ground in the drawings, conceivably fitting four-spotted, prairie, or black-horned/Forbes’s.

Until I saw the photo I didn’t expect this. The cricket overall was very pale, and I had ruled out the black-horned/Forbes’s sibling species pair because they typically have dark areas on the head, pronotum (top of thorax), and underside of the abdomen. If I had to choose, though, based on antennal spotting alone, I would say this individual was a black-horned or Forbes’s tree cricket, albeit at one extreme end of their range of variation (for more on that species pair, see my post of last autumn).

In previous years I have examined a number of tree crickets from the area where this one was caught, and all have been in the black-horned/Forbes’s darker color pattern. The only clear result is that I need more experience with these four prairie-to-shrubby-meadow tree cricket species.

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