Review Article

Journal of Orthoptera Research 2021, 30(2): 185-191

Substrate-borne vibrations used during acoustic communication and
the existence of courtship songs in some species of the genus Anaxipha
(Saussure) (Orthoptera: Trigonidiidae: Trigonidiinae)

WitBuR L. HERSHBERGER!

1 Hedgesville, West Virginia 25427, USA.

Corresponding author: Wilbur L. Hershberger (Wilhershberger@ mac.com)

Academic editor: Klaus-Gerhard Heller | Received 2 July 2021 | Accepted 17 September 2021 | Published 14 December 2021

http://zoobank.org/49303DCD-3FE3-42E2-A436-9403D3663640

Citation: Hershberger WL (2021) Substrate-borne vibrations used during acoustic communication and the existence of courtship songs in some species
of the genus Anaxipha (Saussure) (Orthoptera: Trigonidiidae: Trigonidiinae). Journal of Orthoptera Research 30(2): 185-191. https://doi.org/10.3897/

jor.30.70990

Abstract

Anaxipha (Saussure, 1874) are small, swordtail crickets found in much
of eastern North America. Many species within the genus Anaxipha were
only recently described and their calling songs characterized. However, lit-
tle is known about their courtship songs or use of substrate-borne com-
munication (drumming). This study is the first documentation of the
existence of courtship songs and substrate-borne vibrational communica-
tion in the genus. Courtship songs and substrate-borne vibrational com-
munication were first detected in the following species: Anaxipha exigua
(Say, 1825), A. tinnulacita Walker & Funk, 2014, A. tinnulenta Walker &
Funk, 2014, and A. thomasi Walker & Funk, 2014. When in the presence
of a conspecific female, males of all four species perform courtship songs
that are distinctly different in pattern of echeme delivery and syllable de-
tails compared to their respective calling songs. Additionally, males of all
four species exhibited drumming behavior during courtship singing and
variably during calling songs. Examination of video recordings of males
drumming during courtship singing showed that they are apparently us-
ing the sclerotized portion of their mandibles to impact the substrate on
which they are perched to create vibrations. Courtship song and drum-
ming bout characteristics were statistically different among the four spe-
cies studied here, although A. tinnulacita and A. tinnulenta were similar in
some measurements. Drumming during calling songs was common only
in A. tinnulacita, where drumming occurs predominately during the first
forty percent and last twenty percent of the long echemes of calling songs.
Additional study is needed to further explore the use of substrate-borne
vibrational communication in this genus.

Keywords

communication, courtship, courtship song, drumming, echeme, substrate-
borne vibrations, vibration

Introduction

Large numbers of species of small insects use substrate-borne
vibrations for near-field communication with conspecifics (Cald-
well 2014, Yack 2016). Many of these species inhabit herbaceous
and woody plants and use the plant’s stems and leaves to trans-
mit these vibrational signals (Michelsen et al. 1982, Eriksson et

al. 2011). Small, plant-inhabiting insects that produce acoustic
signals to attract mates most likely face challenges when trying to
locate a singing male once the female is within a meter or so of the
singer (Cocroft et al. 2000, Cokl and Virant-Doberlet 2003). With-
in the tangles of vegetation that these insects inhabit, the use of
substrate-borne vibrations, in addition to acoustic signals, would
be beneficial. This would allow for orientation toward and then
finding of potential mates once the searcher is on, or close to, the
stem or leaf from which the male is communicating (Hill 2001).
All the species of Anaxipha (Saussure, 1874) in this study are
tiny insects, less than 8 mm in length, and occur on a wide vari-
ety of plants and plant structures in North America (Walker and
Funk 2014). Males often sing from concealed perches, typically on
the underside of leaves or stems, and are difficult to locate visu-
ally. Males sing calling songs, both day and night, which are loud
and easily heard at distances of several meters (Walker and Funk
2014). Only at night can males and females be found foraging
on the top side of vegetation or wandering along stems. In this
study, the existence of courtship songs and drumming in the four
Anaxipha species found in West Virginia are documented.

Materials and methods

Males and females of A. exigua (Say, 1825), A. tinnulacita Walk-
er & Funk, 2014, A. tinnulenta Walker & Funk, 2014, and A. thomasi
Walker & Funk, 2014 were collected in August and September dur-
ing the years of 2019 and 2020 from appropriate habitats (shrubs,
coarse weeds, and grasses at the edges or the understory of decidu-
ous woods) in Berkeley and Jefferson Counties in West Virginia,
USA. As in Hershberger (2021), all specimens were maintained
in plastic containers that were modified by removing most of the
plastic from the sides and lid. The open areas were then covered
with no-see-um netting held in place with high-temperature hot
glue. These singing cages are practically transparent to sound. The
crickets were fed iceberg lettuce, Fluker’s High-calcium Cricket
Diet (Port Allen, LA), and water ad libitum. Audio recordings of
calling songs were made with caged, individual males placed in
an anechoic room, with dim or no light. Temperatures were taken

JOURNAL OF ORTHOPTERA RESEARCH 2021, 30(2)

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W.L. HERSHBERGER

ee ee ee

Fig. 1. A typical drumming bout showing the low-frequency and brief nature of these sounds. The figure is the selection of a drumming
bout from a courtship song of A. thomasi showing the selection window spanning from the middle of the first tap to the middle of the

last tap. This tapping bout consists of 12 taps.

with either a Cooper Atkins DFP450W digital pocket thermometer
(Middlefield, CT, USA) or a Digi-Sense ZM-94460-78 receiver and
ZM-90205-10 transmitter (Vernon Hills, IL, USA). Recordings were
made with a Sound Devices 702, Sound Devices MixPre-6 (Reeds-
burg, WI, USA), or a Zoom F8n digital recorder (Hauppauge, NY,
USA) at a sampling rate of 96 kHz at 24-bit depth. The micro-
phone, a Sennheiser MKH 8020 RF condenser (Solred Stand,
Denmark), was placed within 8 cm of the singing males. Audio
recordings were processed using Adobe Audition CC 2020 (San
Jose, CA, USA) and examined using Raven Pro v 1.6.1 (Cornell
Lab of Ornithology). Data analysis was performed using Micro-
soft Excel (Redmond, WA, USA) and DataGraph software (Chapel
Hill, NC, USA).

Acoustic terminology follows Baker and Chesmore (2020): a
unit of sound produced by one closing stroke of the cricket teg-
mina is called a syllable; syllables are grouped into short echemes
that are organized into calling songs or courtship songs.

Courtship songs were recorded by placing a female into the
cage with the male, placing the cage in a dimly lit anechoic room,
and allowing the insects to interact. A minimum of three different
males were used for capturing courtship songs for each species.
Three to four females were used for different courtship sessions.
The recordings were made using the same equipment and tech-
niques described for recording calling songs.

Videos of courtship behavior were made by placing a male and
female of the same species into a clear, plastic container with a
removable lid. A hole was cut into the lid and covered with no-see-
um netting. Video was captured through the side of the clear en-
closure using a Canon 5DSR digital camera with a Canon 180 mm
macro lens with or without a 22 mm extension tube (Huntington,
NY, USA). Lighting was provided by a table lamp with a 40-watt-
equivalent LED bulb (to reduce heat). Audio was simultaneously
recorded using the Sound Devices 702 digital audio recorder and
a Sennheiser ME62 electret-condenser microphone (Solre@d Stand,
Denmark) placed within a few millimeters of the netting-covered
hole of the enclosure. Video was edited in Final Cut Pro X (Apple
Corp., Cupertino, CA, USA). All audio and video files are archived
at the Macaulay Library, Cornell Laboratory of Ornithology (audio
files are ML numbers 305982-306011. Video files are ML number
488780-488784).

Calling songs and courtship songs were processed in Adobe
Audition to amplify the frequency band from 0-2 kHz by 10-
30 dB to make it easier to detect the drumming bouts in Raven
Pro 1.6.1. Drumming bouts were selected by hand from the mid-
dle of the first tap’s signature through the middle of the last tap’s
signature in each tapping bout (Fig. 1). The tapping rate (taps/sec)

was determined by dividing the number of taps in the bout, minus
one, by the duration of the tapping bout in seconds. The syllable
rate for the calling and courtship songs was determined at several
haphazardly chosen times during the song. Syllables were selected
from the beginning of one syllable through to the beginning of the
eleventh syllable and dividing ten by the measured time (in sec-
onds). Several measurements were averaged, and standard devia-
tions were calculated for each recording. To show that taps within
an individual drumming bout increase in amplitude, I selected the
first two taps and the last two taps from each drumming bout in
several courtship songs for each species. This created an average
power (dBFS) for each selection. The means for the first two and
the last two taps were compared using a t-test of the means. To de-
termine the peak carrier frequency of the tapping bouts, I selected
all drumming bouts from several courtship songs for each species,
allowing Raven Pro v 1.6.1 to determine the peak frequency of
the sounds within the selections. These were averaged and stand-
ard deviations calculated in DataGraph. The data were not heavily
skewed, so means were compared using t-tests to calculate p-values
using SciStat.com comparison of means calculator (https://www.
scistat.com/statisticaltests/comparison_of_means.php).

Results

All four species produced calling songs consistent with the
descriptions in Walker and Funk (2014). Analysis of audio re-
cordings of calling songs revealed brief, low-frequency sounds,
from 2 kHz down, that occurred in groups of varying numbers
(detail of a drumming bout from A. thomasi, Fig. 1). Three of
the species in this study were found to perform these drumming
events during calling songs (A. tinnulacita, A. tinnulenta, and A.
thomasi) (Table 1). No drumming events were detected for A.
exigua in the nine major echemes of calling songs analyzed for
this species (these represent hours of recorded calling songs).

Table 1. Occurrence of drumming within calling songs of Anaxipha
species (average + SD).

Species Taps/bout Taps/s Syllables/s Temp.(°C) n

A. exigua ND ND 39.35 + 0.834 22.6 oh
A. tinnulacita 4.27+41.500 18.0041.525% 11.20 + 0.233 24.1 530
A. tinnulenta 2.83 40.578 17.36+2.829% 4.82 +0.045 22.7, 12
A. thomasi 7.28+0.813 17.39+0.504> 17.90 + 0.071 22.1 34
ND = not detected.

*p=0.161.

bp = 0.952.

* song segments analyzed, otherwise n = number of drumming bouts measured.

JOURNAL OF ORTHOPTERA RESEARCH 2021, 30(2)

W.L. HERSHBERGER

Drumming bouts within calling songs were rare except for A.
tinnulacita, which produced drumming bouts throughout long
periods of calling songs. The rate of tapping (taps/s) in these
calling song drumming bouts was somewhat similar across the
three species at approximately 18 taps/s (range 14.53-19.53
taps/s, n = 576), and the syllable rate of the calling songs varied
across these species in a typical, species-specific pattern (Walker
and Funk 2014) (Table 1). The range of the number of taps per
drumming bout during calling songs was different across the
three species, with A. tinnulacita giving 2-8 taps/bout, A. tin-
nulenta giving 2-4 taps/bout, and A. thomasi giving 6-9 taps/
bout (Table 2).

Table 2. Range of the number of taps per drumming bout by song
type. ND = not detected.

. Song Type
Species ; :
Calling Courtship
A. exigua ND 4-10 (33)
A. tinnulacita 2-8 (1502) 3-9 (21)
A. tinnulenta 2-4 (12) 4-9 (98)
A. thomasi 6-9 (31) 3-14 (92)

Number in parentheses = number of drumming bouts in the analysis.

Percent Usage

5 6
Number of Taps per Bout

Fig. 2. Percent usage of different length drumming bouts during
calling songs in A. tinnulacita. (n = 24 songs; 1,503 tapping bouts;
error bars are +SD of the means).

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A. tinnulacita produced numerous drumming bouts during
calling songs. Analysis of the different drumming bout types
(number of taps per bout) used during calling songs showed that
A. tinnulacita uses the three, four, and five taps per bout tap-type
most of the time (Fig. 2). From one major echeme of calling song
to the next, the use of drumming bouts was variable, but showed
a pattern of more drumming bouts occurring during the first 40
percent and the last 20 percent of these calling songs (Fig. 3).

When males were placed in the same container with a conspe-
cific female, all four species performed courtship songs. The males
for the species in this study held their wings up, nearly perpen-
dicular to their bodies, during calling and courtship singing. The
courtship songs were different from calling songs in their over-
all amplitude and the amplitude envelope over time. Courtships
songs were also broken into smaller echemes, particularly after the
first several seconds of song during any particular courtship song
bout (Fig. 4). Courtship songs also differed from calling songs in
the shape and duration of the individual syllables that make up
the echemes, with an increase in lower amplitude sounds between
the louder syllables. These weaker sounds must be associated with

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Fig. 3. Mean number of drumming bouts performed during differ-
ent portions of calling songs in A. tinnulacita showing standard error
of the means. The occurrence of drumming was variable from one
calling song to the next. Consistently, more drumming bouts were
given during the first 40 percent and the last 20 percent of the major
echemes of calling song (n = 14 songs, 1218 drumming bouts total).

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Fig. 4. Oscillograms of the calling and courtship songs of the four species of Anaxipha. A1. A. exigua calling song; A2. A. exigua courtship
song; B1. A. tinnulacita calling song; B2. A. tinnulacita courtship song; C1. A. tinnulenta calling song; C2. A. tinnulenta courtship song;
D1. A. thomasi calling song; D2. A. thomasi courtship song. All audio files were normalized to -3 dBFS for comparison. Y-axis represents
amplitude and is analogous to dBFS (full scale).

JOURNAL OF ORTHOPTERA RESEARCH 2021, 30(2)

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Fig. 5. Audio spectrograms of three seconds of calling and courtship songs of the four species of Anaxipha. A1. A. exigua calling song;

A2. A. exigua courtship song; B1. A. tinnulacita calling song; B2. A. ti

nnulacita courtship song; C1. A. tinnulenta calling song; C2. A. tin-

nulenta courtship song; D1. A. thomasi calling song; D2. A. thomasi courtship song. Compared to calling song, the individual syllables
of courtship songs are shorter, patterned differently, with more sounds associated with wing-opening movements (wing-dragging).

wing-opening movements (wing-dragging) (Fig. 5). The syllable
duration within courtship songs was always significantly shorter
when compared to the syllable duration of the calling songs for
each species (Table 3). Due to the high skewness of the untrans-
formed data, p-values were calculated from t-tests of Log10 trans-
formed data comparing the sample means.

Table 3. Syllable duration comparison between song types (aver-
age + SD). The syllable duration is statistically significantly shorter
in courtship songs across the four species in this study. Numbers
in parentheses are the number of syllables analyzed from each
song type. Due to the significant skewness of the raw data, p-values
were calculated from t-tests of Log10 transformed data comparing
the sample means.

Syllable Duration (ms)

Species ; ; p-value
Calling Courtship
A. exigua 17.61 + 2.662 (961) 2.83 + 0.468 (102) < 0.00001
A. tinnulacita 28.06 + 0.875 (480) 12.63 + 4.764 (480) < 0.00001
A. tinnulenta 36.11 + 1.967 (583) 5.34 + 1.805 (103) < 0.00001
A. thomasi 16.85 + 1.625 (540) 13.61 + 5.655 (362) < 0.00001

All four species in this study also produced numerous drum-
ming bouts during courtship songs. The range of the number of
taps per drumming bout was higher in courtship songs than in
calling songs, with A. exigua giving 4-10 taps/bout, A. tinnulaci-
ta giving 3-9 taps/bout, A. tinnulenta giving 4-9 taps/bout, and
A. thomasi giving 3-14 taps/bout (Table 2). The average number
of taps/second in courtship song drumming bouts showed that
the rates were statistically different between all species (p-value <
0.0022); the syllable rate of the courtship songs varied across spe-
cies, similar to their individual calling song rates (Table 4). While
drumming bouts were given throughout courtship singing for A.
exigua and A. thomasi, it was found that drumming bouts were

JOURNAL OF ORTHOPTERA

Table 4. Average taps/bout and taps/s within courtship songs of
the four Anaxipha species compared to the average syllable rate of
their respective courtship song during these tapping bouts (+ SD).

Species Taps/bout Taps/s Pulses/s Temp. (°C) n
A. exigua 6.61+1.340° 19.914+0.941 39.15+0.952 22.6 33
A. tinnulacita 6.19+41.5407 20.67+40.667 13.21 40.067 23 21
A. tinnulenta 6.93 +0.865* 17.58+41.130 4.63 +0.053 22.1 98
A. thomasi 8.41 + 3.210 23.15+0.715 20.04 + 0.221 23.5 92:

n = the number of drumming bouts analyzed.
‘ p-values from t-test comparisons of means shows these means to be statistically
similar. All other comparisons were statistically different at the 95% level.

typically given later in courtship songs for A. tinnulacita and A.
tinnulenta (Fig. 6). The peak audio frequency of the taps of drum-
ming bouts during calling and courtship songs were similar within
each species and were no doubt related to the substrate that the
males were tapping on (plastic containers). However, the taps cre-
ated by A. exigua were consistently of a significantly lower peak
carrier frequency than those produced by the other species in this
study (p < 0.0001), with the carrier frequencies of A. tinnulacita
and A. tinnulenta (p = 0.21) and A. tinnulacita and A. thomasi (p =
0.367) being statistically similar (Table 5).

During courtship songs, tapping within any particular drum-
ming bout increases in peak amplitude in all four species, with
the ending of a drumming bout being statistically louder than the
beginning, with a p-value of < 0.0001 for each comparison (Fig. 7).

A. thomasi was the only species in this study that performed
jumping motions during the silent periods between echemes
within courtship songs. The male would repeatedly jump back-
wards slightly, with irregular, rapid motions creating sounds when
coming to rest on the substrate. These motions created low-fre-
quency sounds similar in carrier frequency to those in typical tap-
ping bouts, but the pattern of these sounds was very different from
the tapping bouts given during courtship or calling song echemes

RESEARCH 2021, 30(2)

W.L. HERSHBERGER

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Fig. 6. Audio spectrograms of courtship songs of four species of Anaxipha showing drumming bouts (below 2 kHz) and tegminal
sounds (above 4 kHz). Low frequencies (below 2 kHz) were enhanced to better show the low-frequency drumming bouts. A. Anaxipha
exigua, dark areas below 1 kHz are drumming bouts; B. Anaxipha tinnulacita, fine, dark lines after 1:05 (min:sec) are the drumming
bouts; C. Anaxipha tinnulenta, the dark areas after 5 sec and below 2 kHz are the drumming bouts; D. Anaxipha thomasi, the compact
dark areas below 2 kHz are the drumming bouts. The clustered, individual taps between drumming bouts are sounds made by the
insect jumping repeatedly and rapidly between echemes during the courtship display. Note that the time scales for each courtship song

are different, as each species courtship song length is different and variable.

(Fig. 6D at 9, 11.5, 15, 19.5, 20, 22.5, and 25 sec) (a video file of
this behavior is at ML488781). These movements and sounds were
not studied further here.

Also noted during close examination of these species’ court-
ship interactions were palp-drumming sessions performed by
the males typically during non-singing portions of the courtship
interactions when the male and female were in close proximity
(video files of this behavior are at ML488780, ML488782, and
ML488784). These palp-drumming sounds were of extremely low

amplitude and required significant amplification of the audio to
hear them; they were not studied further here.

Discussion

This is the first report showing that members of the genus
Anaxipha use substrate-borne vibrations as a possible method
of communication and the first to demonstrate that these trigs
produce courtship songs that are distinctly different from their

JOURNAL OF ORTHOPTERA RESEARCH 2021, 30(2)

190

-65

-60 -

| >

_55

Average Power (dBFS)

-50

A. exigua ‘A. tinnulacita A. tinnulenta A. thomasi

Fig. 7. Examination of the means of the average power of the com-
bined first two and the combined last two taps of drumming bouts
in courtship songs +SD. In nearly all instances, tapping becomes
louder during an individual drumming bout within courtship
songs across all four species. Numbers closer to the abscissas are
louder. a = average of the first two taps, b = average of the last
two taps in bouts of four taps or more. * = t-tests comparing the
means of first two taps to the last two taps, within each species,
showed the p-values were all < 0.0001, showing that the taps are
significantly louder at the end of drumming bouts (A. exigua n = 5
songs, 45 drumming bouts; A. tinnulacita n = 4, 29; A. tinnulenta n
= 4, 98; A. thomasi n = 5, 75).

Table 5. Average of the peak frequencies in Hz of highest ampli-
tude frequency of taps of drumming bouts for the specific con-
tainer type in which these insects were kept.

Species Peak Frequency + SD n
A. exigua 89.8 + 35.59 5,48
A. tinnulacita 277.0 + 58.12 533
A. tinnulenta 266.5 + 27.84? 5,73
A. thomasi 287.8 + 55.96? 5) 22.

4 p-values show no statistically significant difference between these two species (p =
0.21).

> p-values show no statistically significant difference between these two species (p =
0.367).

p-values for comparisons of A. exigua to the other three species were all p < 0.0001.

n = number of songs, total number of drumming bouts.

typical calling songs. The use of courtship songs in other Grylli-
dae is well known (Alexander and Otte 1967), but little is known
about the acoustic courtship behavior of the Trigonidiinae, with
mating behavior reported in only a few species (Spooner 1972,
Ingrisch 1977, Mendelson and Shaw 2002, Fergus et al. 2011,
Shaw and Khine 2004, Funk 2016, Centeno and Zefa 2019).
Males of all four species in this study produced songs that were
statistically different from their calling songs when in the pres-
ence of conspecific females, and all four species exhibited drum-
ming during courtship singing. While calling songs consisted of
long, rarely broken echemes that crescendo quickly to full vol-
ume, courtship song echemes crescendo more slowly and are
broken into more discrete intervals, typically in the later portion
of the courtship song.

The high amplitude of calling songs would seem to make it
harder for females to locate a displaying male in the tangle of
vegetation typical of the habitat types used by these insects. The

W.L. HERSHBERGER

use of substrate-borne vibrations (drumming) certainly would
make it easier for searching females to find courting males (Hill
2001, 2009, Eriksson et al. 2011, Caldwell 2014, Yack 2016). Fur-
ther investigations into this mode of communication within the
Anaxipha will be required to determine the range and efficacy of
these vibrations to facilitate the orientation of searching females.
The use of laser-vibrometers or sensitive transducers may be ideal
for this work.

Three of the species studied here used drumming during call-
ing songs. While these substrate-borne vibrations would certainly
aid searching females looking for a perspective mate, it is unclear
why A. exigua did not appear to perform drumming during call-
ing song bouts, while drumming bouts were readily detected in
the other three species studied here. Also, it is interesting that
drumming bouts were easily detected in the courtship songs of
A. exigua. Further study of A. exigua’s calling songs in a different
container type would be illuminating to ensure that some aspect
of the structure of the singing cages used here was not preventing
the insect from producing these particular sounds.

It appears that the combination of calling song syllable rate,
courtship song syllable rate, and the tapping rate in drumming
bouts would clearly define each of these four species as distinct
and separate to listening females. The change in the syllable length
and syllable loudness in courtship songs along with the enhanced
use of wing-closing sounds, the change in the pattern and ampli-
tude envelope, and the increase in production of longer drum-
ming bouts (more taps/bout) would clearly signal to the female
the presence of a receptive male of the correct species.

Additional maneuvers and vibrations were only observed in A.
thomasi, with males jumping (irregular, rapid, backward motions)
during the silent portions of courtship encounters. It appeared that
the sounds were created by his feet hitting the substrate as he lands
from the jumping motions. These jumping sessions were only per-
formed when he was not creating sounds with his tegmina. Also,
males did not appear to drum during these jumping displays. The sig-
nificance of these additional displays requires further investigation.

In all the species in this study that were videoed during court-
ship displays, males were observed to perform palp-drumming
during quiet portions of the courtship display. These sounds were
extremely faint. Future study of palp-drumming will require very
sensitive equipment, such as laser-vibrometry, to fully detect what
the males and the females are communicating to one another dur-
ing these easily overlooked courtship interactions.

Acknowledgements

I am grateful to the Potomac Valley Audubon Society for
granting me permission to study these species at their preserve
and for the permits to collect specimens. I am grateful to Dr.
Thomas J. Walker, Professor Emeritus, University of Florida, for
his generous sharing of his knowledge and data. I would like to
thank Dr. David Funk for sharing information and comments
about these species and their songs. I am thankful to the Or-
thoperists’ Society for providing funding to publish this paper.
Thanks also to the anonymous referees. I am also grateful for
the assistance of Dr. Tracy Leskey and the JOR editor for invalu-
able improvements to this manuscript. All audio recordings as-
sociated with this project are archived at the Macaulay Library,
Cornell Laboratory of Ornithology, ML numbers 305982-306011
and 488780-488784.

JOURNAL OF ORTHOPTERA RESEARCH 2021, 30(2)

W.L. HERSHBERGER

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