I Differences in age structure among field cricket populations (Orthoptera; Gryllidae): I possible influence of a sex-biased parasitoid

Anne-Marie Murray and William H. Cade

Abstract: This study examined age structure in adult populations of three species of field cricket, Gryllus veletis, G. pennsylvanicus, and G. integer. Adults were aged by counting growth layers in cross sections of tibiae. The study species differ in several life-history traits including the likelihood of parasitism by Ormia ochracea, a tachinid that orients to calling males. Gryllus integer is parasitized whereas G. veletis and G. pennsylvanicus are not. Such differences between the species should result in different age patterns. Data from field collections demonstrated that adult G. veletis and G. pennsylvanicus had similar maximum life-spans of about 4 weeks, and males were similar in age or slightly older than females. The maximum age for female G. integer was also about 4 weeks, but few males >20 days old were encountered. Moreover, male G. integer were significantly younger than females in five out of six samples. This pattern in G. integer, evident in 2 successive years, could be consistent with sex-biased mortality by Ormia ochracea. The results are discussed in relation to differential longevities and the intensity of sexual selection on male mating behaviour.

Resume : Nous avons ktudik la structure selon l'ige de populations adultes de trois espkces de grillons, Gryllus veletis, G. pennsylvanicus et G. integer. L'ige a kt6 dktermink par dknombrement des couches de croissance dans des coupes transversales de tibias. Les espkces ktudikes different par plusieurs aspects de leur biologie, notamment par leur susceptibilitk au parasitisme d'Ormia ochracea, un tachinide qui recherche les miles chanteurs. Gryllus integer sert d'h8te au tachinide, alors que les deux autres espkces ne sont pas parasitkes. I1 est lkgitime de s'attendre alors ce que ces espkces diffkrent aussi par la structure selon 1'8ge de leurs populations. Les rksultats des rkcoltes sur le terrain ont For personal use only. dkmontrk que les adultes de G. veletis et de G. pennsylvanicus ont des longkvitks maximales semblables d'environ 4 semaines et que les miles sont d'iges kquivalents et lkgkrement plus igks que les femelles. L'ige maximal des femelles de G. integer a kgalement kt6 kvaluk a 4 semaines, mais peu de miles de plus de 20 jours ont kt6 rencontrks. De plus les males de G. integer se sont avkrks significativement plus jeunes que les femelles dans cinq des six kchantillons. Cette tendance, observke chez G. integer au cours de 2 annkes conskcutives, est compatible avec la mortalitk causke par 0. ochracea qui affecte un sexe plus que l'autre. Ces rksultats sont examinks a la lumikre des effets de la longkvitk diffkrentielle et de l'intensitk de la sklection sexuelle sur le comportement reproducteur des miles. [Traduit par la Rkdaction]

Introduction Considering its significance to a wide range of theoretical and applied biology, relatively little is known about age Knowledge of age structure is important in many ecological structure in insect populations. This is due in large part to and behavioural studies. Differential mortality rates within a lack of quantitative methodology. Qualitative methods Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 and between populations can select for different life history abound (reviewed in Southwood 1978), but allow only broad patterns (Stearns 1976, 1992). Comparisons of age distribu- age categories to be defined. Neville (1963, 1965) was the tions between males and females can detect sexual bimatur- first to use a simple but accurate way to determine adult ism, an important component of sexual selection (Thornhill insect age. He described growth layers in locust cuticle that and Alcock 1983) and age has also been implicated as an were deposited daily in response to circadian rhythm and important factor in female choice (Halliday 1978, 1983). photoperiod. Similar growth layers have since been documented in representatives of &any orders (reviewed by Received November 11, 1994. Accepted March 3 1, 1995. Neville 1983). This work was undertaken to examine age structure within A.-M. Murray1 and W.H. Cade. Department of Biological three species of field crickets (Orthoptera, Gryllidae). We Sciences, Brock University, St. Catharines, ON L2S 3A1, used the method originally described by Neville and first Canada. applied to field crickets by Zuk (1987a, 1987b, 1988). I Author to whom all correspondence should be addressed. d~llusveletis and G. pennsylvanicus are common in the

Can. J. Zool. 73: 1207 - 1213 (1995). Printed in Canada / ImprimC au Canada Can. J. Zool. Vol. 73, 1995

Niagara region. Both are univoltine with an obligative dia- and around the campus of Brock University, St. Catharines, pause. Gryllus veletis overwinters as a nymph and the pri- and in various locations in the Niagara region of southern marily micropterous adults are found from late May to late Ontario. Gryllus veletis adults were collected between 2 June July. Gryllus pennsylvanicus overwinters as an egg and and 8 July 1992. Most males collected were calling just prior adults are evident from late July to October. Macropterous to capture. Samples of G. pennsylvanicus were captured individuals occur infrequently (Alexander and Bigelow between 25 July and 17 September 1991. Males were 1960; Alexander and Meral 1967; Alexander 1968). Gryllus labelled callers if they were calling just prior to capture and integer occurs in central Texas, has a high frequency of noncallers if they were silent. Data from calling males and macropterous individuals, requires no obligative diapause, noncalling males were analysed separately. and seems to have two population peaks a year (Alexander Gryllus integer adults were collected around the Bracken- 1968; Cade 1979a). ridge biology field station in Austin, Texas, over the periods The three species also differ with respect to selection by 25 June - 22 September 1991 and 20 May - 8 October a sex-specific parasitoid. Males of all three species call and 1992. Most individuals were captured by attracting flying attract receptive females for mating. Calling carries a heavy males and females to broadcasts of tape-recorded conspecific cost in the Texas species, G. integer, as calling males also male song (for a description of song broadcasting see Cade attract an acoustically orienting dipteran parasitoid, Ormia 1989). Phonotactic individuals landed near the loudspeaker ochracea. Infested males die within 7 - 10 days and infesta- and were then collected. Calling males in the vicinity were tion rates as high as 80% have been recorded (Cade 1975, also captured. Samples collected using the different methods 1979b, 1984). Female G. integer are very rarely parasitized were analysed separately. (Cade 1979b; A. -M. Murray, unpublished data). Gryllus Three to five samples were taken across the season for veletis and G. pennsylvanicus are not affected by such a sex- each species so that seasonal trends in age structure could be specific parasitoid, and this might influence male-female determined. Each sample consisted of 8 - 100 individuals age patterns among the species. It is predicted that males of captured over a short time interval (1 -6 days). All speci- G. integer will have significantly shorter life-spans than con- mens were immediately labelled and preserved in 70% specific females and that such differential longevities may ethanol until they were sectioned. not be apparent between the sexes in either G. veletis or Ages in days are given as means f 1 SD. Data proved nor- G. pennsylvanicus. mal and homoscedastic for variance, so subsequent data analy- ses were carried out using standard parametric methods. Methods Two-way ANOVAs were used to determine the effects of date and sex on age variation. Post hoc Tukey's tests were Age was estimated by counting chitin layers in cross sections used for multiple comparisons. of tibiae. Individuals of known age from laboratory cultures were first examined to establish the accuracy of the method for the study species. Samples of crickets were then gathered Results

For personal use only. across the 1991 and 1992 field seasons and compared. Accuracy of the ageing method Figure 1 illustrates the significant linear, relationships Ageing method between number of growth layers in cross sections of tibiae The hind leg, from the distal end of the femur down, was and actual age in days for laboratory-reared adult G. veletis, excised and held between the fingers, and thin sections of G. pennsylvanicus, and G. integer. Curvilinear relationships tibia were cut using a razor blade under a binocular micro- were also examined but did not improve the relationships. scope. Six to 10 sections were taken per adult. Tibia1 sections Results for males and females were pooled, as there were no were arranged in a drop of water on a slide, allowed to dry, significant differences between the sexes in either slope and then mounted in Canada Balsam. The growth layers visi- (G. veletis, t = 0.24, p > 0.05; G. pennsylvanicus, t = ble in 4-6 of these sections were later counted using a Leitz 0.25,~> 0.05; G. integer, t = 0.24,~> 0.05) or elevation light microscope ( x 400). The maximum number of layers in (G. veletis, t = 0.3 1, p > 0.05; G. pennsylvanicus, t = any one section was used in subsequent analysis. 1.77, p > 0.05; G. integer, t = 0.13, p > 0.05) of lines. The slopes and elevations of the three regression lines do not Accuracy of the ageing method differ among ,the three species (F[2,2361= 0.31, p > 0.05;

Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 To test the accuracy of the method, large groups of nymphs F[2,236,= 1.87, p > 0.05). were reared to adulthood in the laboratory under a tempera- ture and photoperiod regime similar to ambient conditions. Age distributions in field populations Individuals were then killed at a known adult age (1 -34 Figure 2 illustrates frequency distributions of ages of males days) and preserved in 70% ethanol until they were sec- and females for all G. veletis captured over the study period. tioned. Slides were assigned a random number by a third Both sexes ranged in age from 2 to 28 days. Figure 3 illus- party so that subsequent estimations of age by the authors trates similar data for G. pennsylvanicus categorized into were carried out without prior knowledge of the actual age three groups, calling males, noncalling males, and females. of the insect. Estimated age was later regressed on actual age All three categories had similar maximum ages of about 4 to examine the relationship. weeks. Newly moulted adults (1 -3 days) were found in the field until mid-August. All calling males were estimated as Age patterns in field populations 9 days old or more. Frequency distributions for G. integer Samples of G. veletis and G. pennsylvanicus were taken in captured in 1991 and 1992 are presented in Figs. 4 and 5, Murray and Cade

Fig. 1. Relationship between number of growth layers in Fig. 2. Frequency distributions of estimated age for all adult cross sections of tibiae and actual age in days of laboratory- male and female G. veletis collected over the 1992 study reared crickets. (A) G. veletis, r = 0.60, p = 0.0001; n = period. 60. (B) G. pennsylvanicus, r = 0.89, p = 0.0001; n = 110. (C) G. integer, r = 0.89, p = 0.0001; n = 72.

u 6 m 1 FEMALES I

ADLILT AGE (DAYS)

Fig. 3. Frequency distributions of estimated age for all adult G. pennsylvanicus (calling males, noncalling males, and females) collected over the 1991 study period.

6 1 MALES For personal use only.

ADULT AGE (DAYS)

respectively. All individuals in both years were at least 5 days old when captured. Males had a maximum age of 20 (1991)or 23 (1992)days and females 27 days. Table 1 illustrates variations in mean age for male and female G. veletis in three separate periods across the field season. Age varied with date (F[2,1091= 59.85, p = 0.0001).Adults captured in early June were younger than those captured in late June (q = 15.4,p < 0.001)and early

Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 July (q = 1 1.12,p < 0.001),with no difference in mean age L between the last two dates (q = 1.37,p > 0.50).Age did 3 1 FEMALES I not vary between the sexes (F,l,lo9,= 2.85, p = 0.09), 10 n = 120 neither was there an interaction between date and sex (F[2,1091= 0.56, p = 0.57). Data on within-season trends in mean age for the three categories of adult G. pennsylvanicus are given in Table 2. Age varied with date (F[2,2291= 109.6, p = 0.0001).Adults showed successive increases in mean age from first to last samples (July vs. August, q = 8.2,p < 0.001 ; August vs. 2 6 10 14 18 22 26 30 September, q = 13.2, p < 0.001).Mean age also varied with category (F[2,2291= 10.55, p = 0.001),females being ADLILT AGE (DAYS) younger than both calling (q = 5.2, p < 0.001)and noncall- Can. J. Zool. Vol. 73, 1995

Table 1. Ages of male and female field-captured Gryllus veletis. 2 June 25 June 8 July

n Mean k SD Range n Mean k SD Range n Mean f SD Range Males 29 8.8k3.8 2-16 19 16.2f4.4 9-27 5 17.8f 3.7 14-23 Females 22 6.8k3.4 2-14 32 14.7k4.7 8-28 9 15.3k3.0 11-19

Table 2. Ages of male and female field-captured Gryllus pennsylvanicus. 25 July 13 August 12 September

n Mean + SD Range n Mean f SD Range n Mean +_ SD Range Calling males 15 12.8f3.3 9-19 22 14.4k3.5 9-25 8 19.4k4.9 13-27 Noncalling males 22 10.3+_4.1 3-19 31 14.2k5.8 1-25 17 20.4f 3.4 14-25 Females 47 7.7f3.2 2-18 44 12.3k4.2 2-20 29 20.2f4.0 12-32

Table 3. Ages of male and female field-captured Gryllus integer from 1991. 25 June 3 August 22 September

n Mean + SD Range n Mean f SD Range n Mean + SD Range

Calling males - - - 16 11.4k3.2 7-18 - - - Flying males 16 11.3k2.4 8-15 28 13.2k2.6 7-17 50 11.1k2.6 7-18 Flying females 22 13.5k3.6 7-21 32 18.1f 4.5 7-27 50 13.5k2.7 7-20

Fig. 4. Frequency distributions of estimated age for all adult Fig. 5. Frequency distributions of estimated age for all adult G. integer (calling males, flying males, and flying females) G. integer (calling males, flying males, and flying females) collected over the 1991 study period. collected over the 1992 study period. 12- FLYING FLYING MALES n = 94 10- MALES For personal use only. n = 62 8 -

v, 6 - 1 4 3 4- -n 2 - I I CALLING >- h I MALES n = 16 Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13

FLYING 6 1 FEMALES I n = 43

2 6 10 14 18 22 26 30 ADLILT AGE (DAYS) 2 6 10 14 18 22 26 30 ADULT AGE (DAYS) Murray and Cade

ing males (q = 4.5, p < 0.005). No difference in age was detected between the two categories of males (q = 0.81, p > 0.50). A significant interaction was found between date and category (F[2,2291= 3.4, p = 0.03). Further analysis revealed that in the first sample, both calling (q = 6.9, p < 0.001) and noncalling (q = 4.1, p < 0.001) male groups were significantly older than females but this difference was not seen in later samples. Over 42% of females in the first sample were estimated to be less than 7 days old compared with 13.5% of males. Data for G. integer from the 1991 season are reported in Table 3. Flying individuals varied in age across the season (F[2,1921= 20.71, p = 0.0001). Individuals captured in August were significantly older than those caught in either June (q = 5.1, p < 0.001) or September (q = 6.1, p < 0.001). Age also varied with sex (F11,1921= 35.71, p = 0.0001), males being younger than females, and there was a significant interaction between date and sex (F[2,1921 = 5.37, p = 0.005). Males were significantly younger than females in August (q = 8.6, p < 0.001) and September (q = 4.77, p < 0.01) but not in June (q = 3.6, p < 0.05). Both calling and flying males were captured in August, but there was no difference in mean age between these categories of males (t = 1.7, p > 0.05). Data on G. integer captured in 1992 are given in Table 4. Flying adults of both sexes were captured on three dates, in June, August, and October, but no variation in age with date was evident (F[2,861= 1.68, p = 0.20). However, differ- ences did exist between the sexes (F[1,861= 19.65, p = 0.0001), males being significantly younger than females. There was no interaction between sex and date (F[2,861= 0.02, p = 0.98). Samples of calling males were collected in May, July, and October. Mean age varied across dates for callers (F[2,401= 4.3, p = 0.02), but post hoc comparisons For personal use only. failed to detect discrete differences between pairs of means. Two samples in 1992 contained both flying and calling males. No significant difference in age was apparent between males exhibiting either behaviour prior to capture in either July (t = 0.88, p > 0.20) or October (t = 1.5, p = 0.15).

Discussion All three species exhibited significant positive relationships between growth layers in the tibiae and actual age in days. Similar regressions suggest no differences among the species with respect to the rate of deposition of such layers, though considerable scatter was evident for G. veletis. Increased variance was evident when ageing older specimens of all

Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 three species, a trend also noted by Zuk (1987~)and by workers on other insects (Ellison and Hampton 1982). This increase in variance may be due to the difficulty of discern- ing individual layers in thicker, older sections. It is also likely that growth layer deposition ceases after a certain time. Neville (1963) found that in locusts, cuticular maturation, and thus growth layer deposition, was completed after about 3 weeks. A maximum of 39 layers were counted in this study, correlating with a 35-day-old G. pennsylvanicus male. If we had examined crickets older than 35 days, a levelling off might have been seen, corresponding to the maximum number of layers laid down in the cuticle. This limit to the deposition of layers does not alter the results presented here, Can. J. Zool. Vol. 73, 1995

however, as very few individuals over 25 days were encoun- females were collected, male G. integer were significantly tered. Counting growth layers in adult tibiae is a useful younger than females. Thus, males of this species exhibit method for ageing field crickets. However, the method reduced longevity compared with conspecific females. should be calibrated each time a different species is exam- Gryllus integer is known to be a host of the sex-specific ined, as predictability may vary among species. parasitoid Orrnia ochracea. Gravid female flies are attracted New teneral adults of both northern species were encoun- to calling males and deposit larvae on or near the individual tered in the field. All G. integer captured were 5 days or (Cade 1975, 1979b). Female G. integer are very rarely older, as most were trapped phonotactically . Laboratory parasitized (Cade 1979b; A.M. Murray, unpublished data) trials suggest that crickets do not begin to exhibit phonotaxis and thus one might expect this differential parasitism to be to male song until they are >4 days past eclosion (Sakaluk reflected in differential longevities between the sexes, as was 1982; personal observation). The maximum age recorded observed in this study. from field adults was about 4 weeks. Few data on age structure are available from comparable Distinct differences in age patterns were evident among gryllid species. Simmons and Zuk (1992) aged adults of the the three species under study, both within and across sea- European field cricket G. bimuculatus in Spain. In southern sons. The mean age of both males and females increased Europe G. bimuculatus has several characteristics in com- across the season for G. veletis and G. pennsylvanicus, mon with the G. integer population studied here. It too has reflecting the fact that both species are univoltine. The mean an almost continuous breeding season, is characterized by age of male and female G. integer showed no consistent trend predominantly macropterous individuals, and engages in within the seasons. Gryllus integer is a more or less continu- mass dispersal flights, but it is not affected by a sex-specific ous breeder and seasonal fluctuations in age reflect this. parasitoid. Male G. bimuculatus were found to be signifi- No difference in age was discernible between the sexes in cantly older than females. More recently, populations of G. veletis, but male G. pennsylvanicus tended to be older Teleogryllus oceanicus were examined (Simmons and Zuk than females. This difference was especially pronounced 1994). In two populations, one on mainland Australia and early in the season. In fact, by a week or two into the field one on the island of Moorea, males were significantly older season, only female nymphs were observed, most males than conspecific females. The exception to the pattern having already reached adulthood (personal observation). No occurred on Hawaii, where males and females were similar such disparity between the sexes was reported by Zuk in age. The population on Hawaii is subject to predation by (1987~)for her population of G. pennsylvanicus in Michi- 0. ochracea, and Simmons and Zuk (1994) suggest that the gan. However, Zuk combined data across the entire season difference in age patterns among populations is consistent when comparing the sexes, and within-season patterns might with that expected as a result of sex-biased mortality caused have been masked. by the dipteran parasitoid. Earlier male eclosion, or protandry, has been described in Differences in age structure were revealed both among many insects and may benefit males in some groups through and within species in this study. Male G. integer consistently the increased probability of mating with virgins (Wiklund exhibited reduced life-spans compared with conspecific For personal use only. and Fagerstrom 1977) or through first male sperm prece- females and such a pattern was not evident in either G. veletis dence (Wedell 1992). Females of most gryllids mate multi- or G. pennsylvanicus. Differential male longevities should ply, however, and sperm mixing has been reported (Backus affect the force of selection acting on male mating behaviour, and Cade 1986). Protandry may be important in competition but this has never been considered. Previous field and model- for territories prior to mating. Early-eclosing males of the ling studies on G. veletis, G. pennsylvanicus, and G. integer desert grasshopper, Liguorotettix coquilletti , occupy higher have revealed similar trends in the intensity of sexual selec- quality territories than later eclosing males, and also exhibit tion on males. Selection was often weak or absent and longer life-spans and higher female-encounter rates (Wang affected by fluctuations in population density and sex ratio et al. 1990). Male field crickets are territorial and male spac- (French and Cade 1987; Cade and Cade 1992; Rowel1 and ing patterns are achieved through acoustical and physical Cade 1993; Souroukis and Cade 1994). This research, interactions (Alexander 1961; Campbell and Shipp 1979; however, involved cross-sectional studies in which differ- Cade 198la; Campbell 1990). Early eclosion and acquisition ences in longevity between individuals or between species of high-quality calling sites may be important to male were not considered. Moreover, research has also demon- G. pennsylvanicus, especially if such sites are limited. strated the significance of the age of males in choice by

Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 Observations while collecting indicated that specific sites females (Zuk 1987b, 1988). Future work should involve lon- were rapidly colonized by a new male after the capture of the gitudinal studies of closely related species that differ in age original resident (personal observation). It has also been sug- structure to evaluate how differences in longevity can affect gested that protandry may arise because of selection on the force of selection acting on male reproductive behaviour. females to delay reaching adulthood (Thornhill and Alcock 1983). Females would thus gain larger adult size, with cor- Acknowledgements related increases in fecundity. In G. integer, a difference in age between the sexes was This research was supported by an operating grant (No. also noted. But in contrast to G. pennsylvanicus, male A6 174) to W.H.C. from the Natural Sciences and Engineer- G. integer were younger than conspecific females. In fact, ing Research Council of Canada and by a postdoctoral males over 20 days old were encountered in only one of eight fellowship to A.-M.M from Brock University. We thank samples collected over the 2-year period. A closer analysis J. Crutchfield and L. Gilbert for access to Brackenridge revealed that in five of six samples where both males and Field Laboratory, University of Texas at Austin. We thank Murray and Cade

E.S. Cade, M. Ciceran, K. Souroukis, D. Belme, S. Adamo, Behavioural ecology. An evolutionary approach. Edited by and T. Pisaric for help in collecting crickets and two review- J.R. Krebs and N .B. Davies. Blackwell Scientific Publications, ers for their helpful comments. Oxford. pp. 180-213. Halliday, T.R. 1983. The study of mate choice. In Mate choice. Edited by P. Bateson. cambridge University Press, Cambridge. References pp. 2-32. ~eville,A.C. 1963. Daily growth layers for determining the age of Alexander, R.D. 1961. Aggressiveness, territoriality, and sexual grasshopper populations. Oikos, 14: 1-8. behavior in field crickets (Orthoptera: Gryllidae). Behaviour, Neville, A.C. 1965. Circadian organization of chitin in some insect 17: 131-223. skeletons. Q. J. Microsc. Sci. 106: 315 -325. Alexander, R.D. 1968. Life cycle origins, speciation, and related Neville, A.C. 1983. Daily cuticular growth layers and the teneral phenomena in crickets. Q. Rev. Biol. 43: 1-41. stage in adult insects: a review. J. Insect Physiol. 29: 2 11 - 2 19. Alexander, R.D., and Bigelow, R.S. 1960. Allochronic speciation Rowell, G.A., and Cade, W.H. 1993. Simulation of alternative in field crickets, and a new species Acheta veletis. Evolution, male reproductive behavior in field crickets. Ecol. Modell. 65: 16: 443-466. 265 -280. Alexander, R.D., and Meral G.H. 1967. Seasonal and daily chirp- Sakaluk, S. 1982. Onset of phonotaxis and age at first mating in ing cycles in the northern spring and fall field crickets, Gryllus female house crickets, Acheta domesticus (Orthoptera; Grylli- veletis and G. pennsylvanicus. Ohio J. Sci. 67: 200 -208. dae). J. N.Y. Entomol. Soc. 90: 136-141. Backus, V.L., and Cade, W.H. 1986. Sperm competition in the Simmons, L.W., and Zuk, M. 1992. Variability in call structure field cricket Gryllus integer (Orthoptera: Gryllidae). Fla. and pairing success of male field crickets, Gryllus bimaculatus: Entomol. 69: 722 -728. the effects of age, size and parasite load. Anim. Behav. 44: Cade, W.H. 1975. Acoustically orienting parasitoids: fly 1145-1 152. phonotaxis to cricket song. Science (Waashington, D.C.), 190: Simmons, L.W., and Zuk, M. 1994. Age structure of parasitized 1312. and unparasitized populations of the field cricket Teleogryllus Cade, W.H. 1979a. Field cricket dispersal flights measured by oceanicus. Ethology. 98: 333 -338. crickets landing at lights. Texas J. Sci. 31: 125 - 130. Souroukis, K., and Cade, W.H. 1994. Reproductive competition Cade, W.H. 1979b. The evolution of alternative male reproductive and selection on male traits at varying sex ratios in the field strategies in field crickets. In Sexual selection and reproductive cricket, Gryllus pennsylvanicus. Behaviour, 126: 45 -62. competition in insects. Edited by M.S. Blum and N.A. Blum. Southwood, T.R.E. 1978. Ecological methods. 2nd ed. Halstead Academic Press, New York. pp. 343 -379. Press, New York. Cade, W.H. 1981a. Field cricket spacing and the phonotaxis of Stearns, S.C. 1976. Life history tactics: a review of the ideas. crickets and parasitoid flies to clumped and isolated cricket Q. Rev. Biol. 51: 3-47. songs. Z. Tierpsychol. 55: 365 - 375. Stearns, S.C. 1992. The evolution of life histories. Oxford Univer- Cade, W.H. 1984. Effects of fly parasitoids on nightly calling dura- sity Press, New York. tion in field crickets. Can. J. Zool. 62: 226-228. Thornhill, R., and Alcock, J. 1983. The evolution of insect mating Cade, W.H. 1989. Nightly and hourly rates of attraction of flying systems. Haward University Press, Cambridge, Mass. field crickets, Gryllus integer, to conspecific song. Can. J. Zool. Wang, G-Y., Greenfield, M.D., and Shelly, T.E. 1990. Intermale

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Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 vanicus and G. integer. Behav. Ecol. Sociobiol. 21: 157 - 162. tion. Evolution, 42: 969 -976. Halliday, T.R. 1978. Sexual selection and mate choice. In