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Observations of the acoustic behavior of Hoplosphyrum boreale (Scudder): a common scaly cricket of Southern California.

Abstract.--We describe observations of the acoustic behavior of the scaly cricket Hoplosphyrum boreale (Scudder), a common scaly-cricket (Orthoptera: Gryllidae: Mogoplistinae) of southern California and the desert southwest. The long-range calling song used by males to attract females from a distance is compared to song produced in close-range courtship interactions between males and females. We also describe preliminary observations of mating behavior, and the song of a sympatric undescribed species.

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In comparison to field crickets (Orthoptera: Gryllidae: Gryllinae), ground crickets (Orthoptera: Gryllidae: Nemobiinae), and tree crickets (Orthoptera: Gryllidae: Oecanthinae), the acoustics and behavior of scaly crickets have received little attention. Scaly crickets are so named because small 'scales' cover nearly the entire body surface (Love & Walker 1979; Yang & Yen 2001). Although there are approximately 276 valid species currently recognized in 26 genera worldwide (Otte et al. 2002), other than taxonomic descriptions, there has been almost no ecological, evolutionary, or behavioral work done on this subfamily. Exceptions include Dambach and colleagues' behavioral and bioacoustic studies of an old world species Cycloptiloides canariensis (Dambach & Beck 1990; Dambach & Gras 1995) and Andrade & Mason's (2000) study of mating behavior in an Australian species, Ornebius aperta. The lack of attention paid this group of crickets probably reflects both their primarily pan-tropical and pan-subtropical diversification, as well as their occurrence in habitats that are "personally uncomfortable, physically impenetrable, or visually confusing to the collector" (Love & Walker 1979, p. 3). Fortunately, such is not the case in southern California, where Hoplosphyrum boreale (Scudder) is found in abundance in grassland and oak woodland habitats. In an effort to add to the knowledge of this group, we conducted field and laboratory observations of H. boreale. H. boreale is a small (ca. 1 cm length) brownish-gray colored cricket that inhabits the desert southwest (Fig. 1). Both sexes have a deep reddish-brown pronotum and head (Figs. 2A and B). Males have short wings covering nearly 1/3 of the abdomen. The wings are used to produce song to attract females for mating; females are wingless and do not produce song. The songs of other North American species of scaly crickets have been described in detail previously (Love & Walker 1979), however Love and Walker limited their detailed study to species found east of the 94th meridian. Thus the acoustic behavior of H. boreale and of a closely related, but as of yet undescribed species, has never before been published. The undescribed species also occurs within our study area. Here we confirm its existence, and include a description of its song, but do not formally describe and name the species as taxonomic work is forthcoming (R. E. Love, personal communication 2002).

[FIGURE 1-2 OMITTED]

Methods and Results

Males and females were observed and collected in the fall of 2001 and 2002 at the Las Virgenes View Park (North 34[degrees] 6', West 118[degrees] 43') in the Santa Monica Mountains National Recreation Area, Los Angeles County, California. Collections were made under permit SAMO-2001-SCI-0011 from the United States Department of the Interior, National Park Service to D. A. Gray. Collected individuals were brought to the laboratory at California State University, Northridge where further sound recording and behavioral observations were made. Crickets in the laboratory were held individually in 500 ml containers provided with food (Purina cat chow, Ralston Purina, St. Louis, MO) and water continuously available in cotton-plugged vials.

General Observations.

We have observed Hoplosphyrum boreale in grassland and mixed oak woodland areas. H. boreale produces a distinctive calling song consisting of a series of chirps normally composed of 2 to 3 pulses per chirp (Figs. 3 and 4). Cracks in the soil are used as refuges during the daytime hours, and to a lesser extent at night. In the evening and nighttime, males call from cracks in the soil as is common with field crickets in the genus Gryllus. However, males and females are both very mobile and are very frequently seen walking aboveground, presumably searching for food and/or mates. Peak abundance was in early to mid October. Few individuals were observed in September, but by October 2, 2001 density had increased and was informally estimated at 1 to 2 individuals per square meter in suitable habitat. This further increased to an estimated 2 to 3 per square meter by 19 October, 2001. Abundance had markedly decreased by November 17, 2001 and by January 11, 2002 only two males were heard calling. At peak abundance in early October it was common to find several males within 10 centimeters of each other. A similar pattern of peak abundance in mid-fall was observed in 2002. Although not conclusive, these observations suggest a single generation per year. If the H. boreale life cycle is generally similar to that of other crickets, this would suggest the likelihood of an egg diapause during the winter months (Masaki & Walker 1987; Alexander 1968; McIntyre 1978; Tanaka et al. 1999). Consistent with this idea, we have had success at rearing H. boreale in the laboratory by imposing a 4 [degrees]C diapause for 60 days (Rakshpal 1962); direct hatching of eggs at room temperatures has been unsuccessful (D. A. Gray & J. Saidy, unpublished observations).

[FIGURE 3-4 OMITTED]

Field Mating Behavior.

Males were observed to call from cracks in the soil, while walking over relatively bare surfaces, frequently from under dried leaves, and infrequently while perched on vegetation. Males thus appear to adopt an active search strategy of mate finding in addition to site-tenacious calling. On numerous occasions males were observed to encounter other males while walking on the surface (Fig. 5). Obvious overt male-male aggression typical of field crickets (Alexander 1961; Hack 1997; Tachon et al. 1999) was never observed in the field however. Males appeared tolerant of the presence of other males within very close proximity. On one occasion, 2 males were observed with 1 female under a single fallen leaf.

[FIGURE 5 OMITTED]

Only one mating interacton was observed in the field. A male and female were encountered positioned approximately 8 cm from each other. The male was facing the female and producing song. The song sounded similar in structure to the calling song males sing when alone, and so prompted further study described below. After approximately 20 seconds, the female approached the male, he turned to face the other direction, and the female mounted the male immediately. The female remained on top of the male for 1-2 seconds, and then dismounted and walked away. No nuptial feeding was observed. Unfortunately the observation conditions made it impossible to determine if a spermatophore had been transferred to the female.

In order to quantify song and determine if different songs are used for calling (long-range mate attraction) and courtship (close-range mating interactions), as is commonly the case in field crickets (Alexander 1961; Fitzpatrick & Gray 2001; Walker & Masaki 1989), we recorded a sample of males in the laboratory twice each: once when the male was alone and had been held individually isolated for a minimum period of one week, and once when paired with a female. Males and females were wild caught and thus had unknown mating histories. Males and females were held individually as described above, but for recording purposes the lids of the male containers were replaced with mesh screen lids.

Calling males were recorded using a digital tape recorder (TASCAM DA-P1, 20 Hz - 20 kHz [+ or -] 1 dB, all reported dB re: 20 [micro]Pa, TASCAM America, Montebello, CA) set at mono, 48kHz 16-bit sampling, fitted with a Sennheiser MKE 2-P-C microphone (20 Hz-20 kHz [+ or -] 3 dB, Sennheiser Electronic Corporation, Old Lyme, CT). The microphone was positioned 10 cm above the cricket to be recorded. We noted the temperature of each recording to the nearest 0.1 [degrees]C. Songs were transferred to a personal computer for analysis using CoolEdit 2000 (Syntrillium Software, Scottsdale, AZ). Laboratory recordings were made at temperatures ranging from 21.2 to 26.1 [degrees]C (Mean [+ or -] SD = 23.5 [+ or -] 1.3). We recorded a sample of 10 males two times each: once when the male was alone, and once after the introduction of a female; we additionally recorded calling song for another 5 male H. boreale and for 7 males of the currently undescribed species (Fig. 6). Song variables analyzed were the number of chirps per series (chirps/ series), the time from the end of one series to the start of the next (inter-series interval), the duration of a chirp (chirp duration), the time from the end of one chirp to the start of the next within a series (inter-chirp interval), the number of chirps per second (chirp rate, calculated as 1/the chirp period, which equals the chirp duration plus the inter-chirp interval), the number of pulses per chirp (pulses/ chirp), the duration of a single pulse (pulse duration), the number of pulses per second within a chirp (pulse rate, calculated as 1/the pulse period, where the pulse period equals the time from the start of one pulse within a chirp to the start of the next pulse within the same chirp), and the peak frequency of the chirp (frequency). Song data were adjusted to a common temperature of 20 [degrees]C by regression of song data on recording temperature, and then calculating the temperature adjusted data as [Song.sub.adj] = Song - (recording temp - 20)*slope of song on temperature for each song feature (Gray & Cade 1999, 2000; Martin et al. 2000). Several minutes of song were analyzed for each male (an average of 10 [+ or -] 7 chirp series for H. boreale).

[FIGURE 6 OMITTED]

The number of chirps per series appears to be the only song character notably different in calling and courtship song situations in H. boreale (Table 1). Song produced during courtship interactions has approximately one-half as many chirps per series as are produced when the male is calling alone. Statistical analysis confirms this impression (Z test of the difference between paired observations, N = 10 males, P < 0.005). Song produced in calling and courtship interactions did not differ in the inter-series interval, pulses per chirp, pulse duration, or chirp rate (all P > 0.05). Statistically significant differences were found in the inter-chirp interval, peak frequency, pulse rate, and the chirp duration (all P < 0.05), however given the magnitude of the differences which are small compared to the large difference in chirps per series, we question the biological significance of these differences.

The songs of the two species are likewise clearly distinct and non-overlapping (Table 1). The most distinctive features are the number of pulses/chirp ([F.sub.1,20] = 528.5, P < 0.0001), and therefore chirp duration ([F.sub.1,20] = 400.6, P < 0.0001), the inter-chirp interval ([F.sub.1,20] = 161.1, P < 0.0001), and the chirp rate ([F.sub.1,20] = 610.9, P < 0.0001), although other song features are also statistically significantly different (e.g., pulse duration, [F.sub.1,20] = 22.4, P < 0.0001 and peak frequency [F.sub.1.20] = 124.2, P < 0.0001). Pulse rate was the only song feature not to differ between species ([F.sub.1,20] = 0.0, P [much greater than] 0.05).

Discussion

Our observations quantify song properties used in communication, and suggest avenues for future field and laboratory work. From our observations of field behavior at high density, it appears likely that males adopt a mixed mate search strategy of actively calling for females, actively searching for females, and potentially acting as satellites of other calling males. The payoff for actively searching for mates in crickets increases in high-density situations as observed in other species of crickets (Cade 1979; Cade & Cade 1992; Hissmann 1990).

While our data add to the published knowledge of scaly cricket biology and acoustic communication, they highlight the need for future work with these and related crickets. Although the taxonomy of the group has recently been revised for eastern North America, additional distributional and taxonomic work on western species is likely (R. E. Love, personal communication 2002). Characterizing the interesting ecological, behavioral, and evolutionary questions could involve decades more work. Specifically it will be interesting to address intra-specific inter-sexual selection based on acoustic features, further characterize mating behavior, and examine behavioral and other forms of reproductive isolation between species where they are found in sympatry.
Table 1. Song data corrected to 20[degrees]C. Means [+ or -] SD's are
given for each song character for Hoplosphyrum boreale calling and
courtship songs, and for the calling song of an undescribed
Hoplosphyrum species.

 Inter-series
 Chirps/ interval
Species Song series (ms)

H. boreale Call 18 [+ or -] 15 939 [+ or -] 460
 (N = 15)
H. boreale Court 8 [+ or -] 4 828 [+ or -] 416
 (N = 10)
H. n.sp. Call * *
 (N = 7)

 Inter-chirp Pulse
 Pulses/ interval duration
Species chirp (ms) (ms)

H. boreale 2.7 [+ or -] 0.4 123 [+ or -] 17 12 [+ or -] 1
H. boreale 2.7 [+ or -] 0.5 148 [+ or -] 32 13 [+ or -] 1
H. n.sp. 7.4 [+ or -] 0.6 551 [+ or -] 132 15 [+ or -] 1

 Chirp
 Frequency Pulse duration
Species (kHz) rate (ms)

H. boreale 4.7 [+ or -] 0.2 49.6 [+ or -] 3.2 50 [+ or -] 7
H. boreale 4.2 [+ or -] 0.2 56.7 [+ or -] 3.2 44 [+ or -] 8
H. n.sp. 4.0 [+ or -] 0.1 49.6 [+ or -] 3.7 153 [+ or -] 18

 Chirp
Species rate

H. boreale 5.7 [+ or -] 0.4
H. boreale 5.4 [+ or -] 1.2
H. n.sp. 1.4 [+ or -] 0.3

* Note: chirps are not arranged in series in the undescribed species,
rather chirps follow a regular uninterrupted pattern.


Acknowledgements

Funding from the California State University, Northridge was used to defray research expenses.

References

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Jonelle Saidy and David A. Gray * Department of Biology, California State University, Northridge, California, USA

* Correspondence: Email: dave.gray@csun.edu.

Accepted for publication 22 July 2003.
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Author:Saidy, Jonelle; Gray, David A.
Publication:Bulletin (Southern California Academy of Sciences)
Date:Apr 1, 2004
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