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Factors influencing the pinworm community (Oxyurida : Nematoda) parasitic in the hindgut of the American… Noble, Stewart J. 1991

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FACTORS INFLUENCING THE PINWORM COMMUNITY (OXYURIDA NEMATODA) PARASITIC IN THE HTNDGUT OF THE AMERICAN COCKROACH PERIPLANETA AMERICANA by STEWART J. NOBLE A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February, 1991 Copyright by Stewart J. Noble 1991 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada DE-6 (2/88) ii ABSTRACT Large cockroaches, such as Periplaneta americana. typically harbour in their hindgut two or more species of parasitic pinworm (Nematoda: Oxyurida). Our laboratory colony was infected with three, possibly four species. The mechanism(s) permitting the sympatry of these potentially competing species were investigated by: i) repeatedly sampling over time hosts of various size to determine the structure, if any, in the pinworm guild and ii) infecting uninfected hosts with known doses of infective eggs and monitoring population changes via daily host dissections. Results indicate that chemically-mediated intraspecific interference competition maintains pinworm populations at densities well below the apparent carrying capacity of the majority of hosts. The concomitant reduction of interspecific pressures thus permits the co-habitation of multiple pinworm species in what is essentially a single niche. This intraspecific population limitation is likely a response to pressures produced by the large size of the parasite in relation the hindgut of early instar hosts. iii TABLE OF CONTENTS Page Abstract ii Table of Contents iii List of Tables iv List of Figures v Acknowledgements vi INTRODUCTION 1 MATERIALS and METHODS 3 Maintenance of Cockroaches 3 Longitudinal Distribution 3 Guild Structure-Mid Instar Hosts 4 Effect of Host Size 4 Effect of Host Moult 4 L. appendiculatum in Isolation 5 Experimental Infections 5 Statistical Analysis 6 Terminology 6 RESULTS 8 Longitudinal Distribution 8 Guild Over Time 8 Characterization of Infraguilds 13 Effect of Host Size 20 Species Combinations 33 Effect of Host Moult 36 Pinworm Fecundity 39 L. appendiculatum in Isolation 46 Experimental Infections 51 DISCUSSION 61 REFERENCES 82 iv LIST OF TABLES Table Page 1 Number of mid instar hosts infected with 0, 1, 2 or 3 pinworm species 15 2 Mean fecundity of each species 41 3 Effect of interspecific density on mean fecundity 45 4 Results of experimental infections with Thelastoma spp 58 V LIST OF FIGURES Figure Page 1 Longitudinal distribution of adult females from mid instar hosts 10 2 Prevalence and intensity over time of adult females from mid instar hosts 12 3 Species combinations of adult females from mid-instar hosts 17 4 Frequency distribution of adult females from mid-instar hosts 19 5 Prevalence of adult males from hosts of varying size 22 6 Frequency distribution of adult males from hosts of varying size 24 7 Intensity of adult males from hosts of varying size 27 8 Prevalence of adult females from hosts of varying size 29 9 Frequency distribution of adult females from hosts of varying size 31 10 Species combinations of adult females from hosts of varying size 35 11 Intensity of adult females from hosts of varying size 38 12 Effect of intraspecific density on mean fecundity 43 13 Prevalence of adult male L. appendiculatum from the isolate colony 48 14 Frequency distribution of adult female L appendiculatum from the isolate colony 50 15 Effect of L,. appendiculatum density on fecundity in the isolate colony 53 16 Results of experimental infections with L. appendiculatum 55 17 Results of experimental infections with Thelastoma spp. 60 18 Male and female Thelastoma spp. in situ 72 19 Changes in pinworm density with host size 74 i vi ACKNOWLEDGEMENTS I wish to thank my supervisor, Dr. Martin Adamson, whose aid, suggestions and generally angelic nitpicking made this a much better thesis. I would also like to thank Mr. Sam Gopaul for his meticulous care and maintenance of the cockroach colonies, Adrienne Buck and Fiona Hunt for their assistance with data collection and Robert C. Noble for spiritual guidance. 1 INTRODUCTION The american cockroach, Periplaneta americana. is found worldwide (Bell and Adiyodi 1981), and harbours a variety of parasites. At least 12 nematode species, representing 9 genera, are found worldwide in varying combinations within the hindgut of the american cockroach (Strand and Brooks 1977). Our colony contained E. americana infected with four species of "pinworm" (Nematoda, Oxyurida, Thelastomatidae) representing three genera: Thelastoma periplaneticola Leibersperger, 1960; T. bulhoesi (Magalhaes, 1900) Trayassos, 1929; Hammerschmidtiella diesingi (Hammerschmidt, 1838) Chitwood, 1932 and Leidynema appendiculatum (Leidy, 1850) Chitwood,1932. The occurrence of species with such apparently high degrees of niche overlap leads to suppositions as to the mechanism(s) permitting the co-existence. Previous workers have attributed this sympatry to niche diversification within the pinworm species (Hominick and Davey 1972a, 1973). However, before evidence for the mechanisms pennitting stable co-existence can be proposed, repeated sampling of the pinworm fauna of P. americana over time and over a range of host sizes was necessary to determine the long term structure and stability of the pinworm guild. The colony of americana at U.B.C. has been maintained for twenty five years under uniform conditions of heat, light, relative humidity and food supply. This afforded an opportunity to repeatedly sample the cockroach population over time to determine the long term pattern of the pinworm distribution. Also, infective-stage pinworm eggs were collected and fed to uninfected roaches and the subsequent development of the pinworms studied via daily dissections of these experimentally infected hosts. This provided an interpretation of the long term pinworm distribution pattern in terms of the biological interactions which occur during development within the host. The co-occurrence of several pinworm species and therefore the possible effects of interspecies interactions on guild structure were also considered. To this end, a 2 colony of E. americana infected with one species of pinworm only (L. appendiculatum) was established, and the distribution of its pinworms compared to that found in the colony containing a mix of species. There are four morphological types of male pinworm present in our colony: £. periplaneticola. T\ bulhoesi. R diesingi and L appendiculatum. To date I have not been able to identify two morphological types of female Thelastoma spp. Data collected from the infected colony was combined with that from experimental infections to deterrnine whether this represents a male polymorphism or a cryptic female species, but at this point the answer remains unclear. It will be explained later that this makes little difference to the analysis. However it is awkward to repeatedly use "three or perhaps four" species when describing the pinworm guild. Thus the guild will usually be referred to as if it contained three species, but the presence of a fourth male type must be considered when discussing the male distribution. Pinworms have a direct life cycle (Dobrovolny and Ackert 1934). Gravid females in the hindgut lay eggs which are passed to the outside environment in the fecal pellets of the host. Within the egg, pinworms develop into first stage larvae which moult to produce ellipsoid second stage larvae. After ingestion by a suitable host, the second stage larva hatches in the midgut and immediately moults the third-stage which establishes in the hindgut. Here pinworms feed and develop, moulting twice to produce adults (fifth stage). Pinworms are haplodiploid (Adamson 1989, Van Luc and Spiridonov 1990, Zervos 1988b); males are haploid and develop from unfertilized eggs, while female are diploid and develop from fertilized eggs. v 3 METHODS and MATERIALS Maintenance of Cockroaches Three colonies of Periplaneta americana were maintained in standard plastic garbage cans, each covered by a clear plastic lid with mesh-covered holes for ventilation. Two colonies (#1 and #2) were infected with Thelastoma periplaneticola, !L bulhoesi. Hammerschmidtiella diesingi and Leidynema appendiculatum (Nematoda: Oxyurida: Thelastomatoidea). A third colony (#3) was infected with L appendiculatum only. Water and food were provided ad libitum. Food consisted of a ground mix of oat bran, brewers yeast, dry dog food and unsalted peanuts, with lettuce offered every two to four days. In this way, breeding colonies of 200 to 300 roaches were maintained over the study period. Colonies #1 and #2 have been maintained at U.B.C. for 25 years or more. Although they are housed in two containers, they can be considered a single colony for the purposes of this study. The population of roaches in a colony episodically declines to the point where it must be supplemented from the other, or a new colony must be established from the other. Thus roaches and their parasites from the two containers have been periodically mixed. Comparison of data from the two colonies revealed no significant differences in mean pinworm burdens or prevalences for any of the pinworm species, and therefore data were combined for this study. Longitudinal Distribution To investigate longitudinal distribution of the pinworms, 44 hosts (late instar and adult) were dissected and their hindguts quickly transferred to a liquid nitrogen bath for about 10 seconds. The frozen hindguts were then sectioned into 5 approximately equal parts and the adult female pinworm burden of each part was '!• jl 4 recorded. The location of the pinworm head was used to assign it to a particular fifth of the hindgut. « Guild structure in mid-instar hosts i To investigate the infraguild structure of a "typical" host, samples of five mid-instar roaches (4th-6th instar) were collected approximately weekly from the colonies infected with all three pinworm species, dissected, and the numbers of adult female pinworms of each species recorded. Host instar was approximated from rear femur length (mean femur length=5.1mm for mid instars). Samples were collected from September 1987 to February 1990 and data were later combined into semi-monthly samples to study changes in prevalence and intensity over time. A total of 328 mid-instar roaches were examined. Effect of host size on guild structure Repeated samples of four roaches each were collected to investigate the effect of host size and age on adult male and adult female pinworm burden. Each sample consisted of an adult female (mean femur length=10.0 mm), an adult male (mean femur length=9.9 mm), a late instar (8th-9th instar, mean femur length=8.2 mm) and an early instar (2nd-3rd instar, mean femur length=2.1 mm) cockroach. In all, 192 hosts (48 of each class) were examined and the structure of their guild compared. The egg burden was measured in pinworms from 90 hosts examined consecutively to avoid bias as to the selection of hosts for pinworm fecundity measurement. Mean fecundity for each species was calculated for each host. Effect of Host Moult To investigate the effect of the host moult on adult female pinworm burden, 38 pairs of roaches, each pair consisting of a newly moulted roach and a fully tanned ! 5 control roach, were removed from the colony and dissected. The burden of adult female pinworms of each species was recorded. Cockroaches which have recently moulted are recognizable by their white colour. After approximately two days the cuticle tans and the roaches regain their typical brown colour. The cast hindgut lining of recently moulted roaches, although separated from the new underlying hindgut lining, is not passed out of the host until approximately two days after the molt. For this reason, recently molted hosts and their controls were held for three days before being dissected. Leidynema appendiculatum in isolation Sets of four roaches each, consisting of an adult female, an adult male, a late instar and an early instar, were collected from the colony infected with U appendiculatum only and their burden of adult male and adult female pinworms recorded. In all, 76 (19 of each class) hosts were collected from this colony. Fecundity of the adult female pinworms present was measured for 48 roaches examined consecutively and the mean fecundity calculated for each. Experimental Infections Uninfected roaches were obtained by placing oothecae, which had been washed in water and rinsed briefly in 70% ethanol, in 1 gallon plastic containers covered with mesh lids. After hatching, the roaches were maintained in the same containers and fed as above. Nematode eggs were obtained by dissecting roaches from the infected colonies and removing gravid female worms. Worms were put in distilled water and placed in an incubator at 29 C for three to four days, after which eggs containing infective third-stage juveniles were recovered. These were placed on a small piece of apple and 6 offered to roaches from which food had been withheld for three to six days. After the apple had been consumed, roaches were placed in an incubator at 29 C. In the first set of infections, twenty cockroaches were given thirty eggs each. Roaches were then dissected two per day on days 1, 2, 3, 4, 5, 6, 9, 10, 11 and 12. In the second set, sixteen roaches were each given twenty eggs. Roaches were dissected two per day on days 2, 6, 9, 13, 14, 16, 18, and 21. In the third set, twelve roaches each received twenty eggs. Dissections were performed, two per day, eight hours after infection and on days 3, 6, 9, 12, and 19. In the fourth set, fifteen roaches each received thirty eggs. Dissections were performed, three per day, on days 12, 15, 17, 19 and 20. Statistical analysis Data were analysed on an IBM Olivetti computer using the "Pipestat" data manipulation program (version 5.3, Gary Perlman, Wang Institute of graduate studies, Tyngsboro, MA.). Unless otherwise stated, all tests were evaluated at the 0.05 level of significance. Terminology Difficulties in terminology often arise when parasite distributions are described in the same terms as free living populations. Further confusion arises when the entire complement of pinworms in a host (the guild) is differentiated from the members of a specific species of pinworm present (the population). To avoid such confusion, the terminology of Margolis et. al. (1982) has been used. Thus "infrapopulauon" refers to the pinworms of a particular species within an individual host, and "suprapopulation" to the summation of all such infrapopulations in the host population under study. "Infraguild" refers to pinworms from all species present in an individual host, and "supraguild" to the summation of all infraguilds. i 7 Prevalence defines the proportion of hosts that are infected with the specified parasite, and abundance the number of parasites in each host. Intensity is the number of parasites in each infected host. 1 8 RESULTS Longitudinal Distribution in the Hindgut Most adult female pinworms were found in the anterior portion of the host hindgut (Fig.l). Regardless of the number or combination of species present, the ileum always contained pinworms in infected hosts. Although proportionately more Thelastoma spp. females were found more posteriorly in the hindgut as compared to H. diesingi or L. appendiculatum (Fig.l), Thelastoma spp. females found in the more posterior region were always accompanied by pinworms located in the ileum and there was a significant correlation between the number of Thelastoma spp. females present and their tendency to be located in a more posterior position in the gut (rM).26, P<0.001). There was no evidence that any of the species preferred a more posterior position whether in mixed or single-species infections. The Guild Over Time-Mid Instar Hosts Samples of five mid-instar roaches, collected approximately weekly, were combined into semi-monthly samples representing two months each. Changes in prevalence and intensity are shown in Figure 2. Thelastoma spp. was the most prevalent adult female pinworm in 9 of 14 of these semi-monthly samples (Fig.2). HL diesingi was most prevalent in the remaining 5 samples. Four of the five samples in which R diesingi was most prevalent occurred consecutively from March 1988 to Nov. 1988 (Fig.2). L. appendiculatum was the least prevalent pinworm in 10 of the 14 samples. Thelastoma spp. had the highest mean intensity in 13 of 14 semi-monthly samples, the only exception being Nov. 1988 (Fig.2). In 10 of the 14 samples, the relative order of mean intensity was Thelastoma spp. > EL diesingi > L appendiculatum (Fig.2). 9 Figure 1. Longitudinal disnibution of adult female pinworms in the hindgut of 44 hosts dissected to investigate longitudinal distribition. Location refers to the percentage of the host hindgut which was anterior to the head of the pinworm A-Anterior hindgut (Ileum) P-Posterior hindgut (Colon/Rectal constriction) Thelastoma spp. H.diesingi Lappendiculatum 61-80 81-100 11 Figure 2. Changes in prevalence (top) and intensity (bottom) over time of adult female pinworms in mid instar hosts. Time is represented on the X-axis by the first letter of the first month in each sample, thus S=September/October, N=November/December, J=January/February etc. Prevalence refers to the percentage of hosts infected. Intensity refers to the mean number of adult female pinworms per infected host. Dark circles-Thelastoma spp. Open circles-H,. diesingi Open diamonds-L. appendiculatum Zl 13 Characterization of Infraguilds-Mid Instar Hosts 96.1% of 328 hosts contained at least one gravid female pinworm (Table 1). 75% (247) of hosts contained Thelastoma spp., 62% (205) contained H. diesingi and 40% (120) contained L,. appendiculatum. Hosts were classified according to number of pinworm species present. Observed numbers of hosts harbouring 0, 1, 2 or 3 species did not differ significantly from expected values. Expected values were calculated by multiplying the proportion of hosts harbouring the stated combination of species, based on their independent distributions, by the total number of hosts (Table 1, chi2=5.95, P>0.05, df=3). However, the 3 species were not distributed independently of one another. FL diesingi occurred more often than expected in combination with Thelastoma spp. or L. appendiculatum (Fig.3, chr*=7.3, P<0.01, df=3) and there were fewer hosts than expected containing the combination of Thelastoma spp. with L. appendiculatum (Fig.3, chi2=9.8, P<0.005, df=3). Abundance ranged from 0 to 29 gravid female pinworms per host, averaging 7.2 (SE 1.8). Overall mean intensity was 6:5 (SE 0.34), 2.9 (SE 0.17) and 1.9 (SE 0.20) gravid females per host for Thelastoma spp., H. diesingi and L. appendiculatum respectively. Relative order of intensity did not vary whether hosts were infected with one, two or three species of pinworm. Thelastoma spp. had the highest mean intensity in all three categories of infected host (Fig.3). In hosts containing two species (Table 1) the overall mean intensity was 5.6 (SE 0.27), 2.6 (SE 0.28) and 2.2 (SE 0.32) for Thelastoma spp., H. diesingi and L. appendiculatum respectively. This order of relative intensity (Thelastoma spp. > HL diesingi > L.appendiculatum) was mirrored in the mean intensity of hosts containing three species (Fig.3). Frequency distributions based on abundance were positively skewed for all species (Fig.4). Thelastoma spp. were found over a greater abundance range than the 14 15 Table 1. Number of mid-instar hosts (n=328) infected with 0, 1, 2, and 3 species of pinworm. Expected values are based on the independent distributions of the pinworm species. Host Class Number of Hosts (number of pinworm species) " Observed Expected chi2 0 11 (3.4%) 19 3.54 1 110(33.5%) 102 0.63 2 159 (48.5%) 150 0.50 3 48 (14.6%) 56 1.27 Total chi^ 5.95, P>0.05 16 Figure 3. Relative proportion of each combination of adult female pinworm recovered from mid instar hosts. Each section of the pie represents the percentage of mid instar hosts which harboured the specified combination of pinworms. The bar diagrams show the mean number of adult female pinworms within each category of host. T-Thelastoma spp, H-H diesingi L-L appendiculatum H.diesingi Thelastoma spp. T L 18 Figure 4. Frequency distribution of adult female pinworms from mid instar hosts. Abundance refers to the number of adult female pinworms in the host. Frequency is the number of hosts harbouring the specified number of pinworms. AVThelastoma SPJL B) -£L diesingi C) -L. appendiculatum Frequency 20 other species. 102 hosts contained more than 5 gravid female Thelastoma spp. each, whereas 20 hosts contained more than 5 JL diesingi and only 8 contained more than 5 L. appendiculatum (Fig.4). Effect of Host Size Four categories of host were sampled and compared: adult female, adult male, late instar and early instar. Numbers of adult male and adult female pinworms were recorded. Thelastoma spp.: Males Overall prevalence (i.e. prevalence produced by treating the four host groups as a single group) was 41% for male T bulhoesi and 32% for male T. periplaneticola. Prevalence of male T. bulhoesi did not differ significantly amoung the four host groups (Fig.5, chi2 =2.35, P>0.5, df=3). Adult female and adult male hosts did not differ significantly with respect to prevalence of male £. periplaneticola. although adult females were more commonly infected (Fig.5, chi2 = 2.3, P>0.1, df=3). Prevalence was significantly greater in adult female hosts as compared to late or early instars (Fig.5, chi2 = 5.28, P<0.025, df=l for adult females versus late instars). Prevalence of T. periplaneticola was significantly lower in early instar hosts than in all other host classes (Fig.5, chi2 = 5.63, P<0.025, df=l for early instars versus late instars). Adult male and late instar hosts did not differ significantly with respect to prevalence of infection with male T. periplaneticola. although adult males were more commonly infected (Fig.5, chi2 = 0.54, P>0.25, df=l). T. periplaneticola males were found over a much greater range of abundance than were the other species (Fig.6). 29 hosts contained 4 or more male T periplaneticola and as many as 86 T periplaneticola males were found in a single host. T. periplaneticola was the only male pinworm found 21 Figure 5. Prevalence of infection with adult male pinworms, showing the percentage of each host class infected with the specified species of male pinworm example approximately 70% of adult female hosts contained adult male H periplaneticola. AF-Adult female hosts AM-Adult male hosts LI-Late instar hosts El-Early instar hosts Percent of Hosts Infected ro cn co o o o o o zz 23 Figure 6. Frequency distribution of adult male pinworms from early instar, late instar, adult male and adult female hosts combined. Abundance refers to the number of adult male pinworms in the host. Frequency is the number of hosts haroouring the specified number of pinworms. A) - ! periplaneticola B) -£L diesingi C) -L. appendiculatum DVT, bulhoesi Frequency 25 in numbers greater than 10 per host (Fig.6). Mean intensity was 24.4 (SE 4.1) male 31 periplaneticola per infected host, and the variance to mean ratio of 48.4 indicated overdispersion. T. periplaneticola males were most numerous in adult female and late instar hosts (Fig.7). Approximately 80% of the 31 periplaneticola males recovered were from adult female hosts. 17 of the 22 hosts containing more than 6 male T. periplaneticola were adult female roaches, 4 were late instar and one was an adult male. Male 31 periplaneticola mean intensity was significantly higher in adult female hosts than in any of the other three host classes (Fig.7, Q statistic (proposed by Dunn 1964, used in non-parametric multiple comparisons where there are not equal numbers of data in each group being tested)=3.69, P<0.002 for adult female versus late instar hosts). Mean intensity was not significantly different in adult male and late instar hosts (Fig.7, Q=0.47, P>0.5) despite the high intensity in late instars. 31 bulhoesi was modally distributed at 1 male per infected host (SE 0.04) (Fig.6). Mean intensity was not significantly different in any of the host classes for male 31 bulhoesi (Fig.7, He (Krukal-WaUis nonparametric test statistic corrected for ties)=2.42, P>0.25). I Females Overall prevalence of female Thelastoma spp. was 68%. Prevalence was significantly lower in early instar hosts than in other host classes (Fig.8, chi2=9.98, P<0.005, df=l for adult male versus early instar hosts). Prevalence was not significantly different amoung adult female, adult male or late instar hosts (chi2=0.42, P>0.75, df=2). Thelastoma spp. females were more abundant than the other pinworm species in all four host classes; this was particularly striking in adult female hosts where as many as 140 Thelastoma spp. females were found in a single host (Fig.9). This is compared to 26 Figure 7. Mean intensity of infection with adult male pinworms. For convenience the Y axis is represented as Log1 0, but the actual mean values are shown above each bar. Mean intensity refers to the mean number of pinworms in infected hosts, discounting uninfected hosts. AF-Adult female hosts AM-Adult male hosts Li-Late instar hosts El-Early instar hosts AF IZZ AM El Lappendiculatum T.bulhoesi 28 Figure 8. Prevalence of infection with adult female pinworms, showing the percentage of each host class infected with the specified species of female pinworm. For example approximately 90% of adult female hosts contained adult female Thelastoma AF-Adult female hosts AM-Adult male hosts LI-Late instar hosts El-Early instar hosts 100 Thelastoma spp. 30 Figure 9. Distribution of adult female pinworms in each host class. The vertical line represents the range of abundances (pinworms per host). The thick horizontal line represents the mean, and the two thin horizontal lines the standard error. AF-Adult female hosts AM-Adult male hosts LI-Late instar hosts El-Early instar hosts AVThelastoma spp. B) -R diesingi C) -L. appendiculatum Number of Females o OI o o! S 32 a maximum of 26 pinworms per single host for R diesingi and 23 for L. appendiculatum. both of which occurred in adult female hosts (Fig.9). There were significantly more female Thelastoma spp. in adult female hosts than in the other host classes (Fig.9a, Q=17.82, P<0.001 for adult female versus late instar). Adult male and late instar hosts did not differ with respect to Thelastoma spp. burden (Fig.9a, Q=1.86, P>0.5), but both contained significantly more female Thelastoma spp. than early instars did (Fig.9a, Q=3.81, P<0.001 for adult male versus early instar hosts). H. diesingi Males 49% of hosts contained male IL diesingi. Significantly more adult female and late instar hosts were infected than were adult males or early instars ( Fig.5, chi2 = 3.9, P<0.05, df=l for adult female versus adult male hosts). H. diesingi lay between T. periplaneticola and T. bulhoesi/L. appendiculatum in abundance range (0-10) and mean intensity (3.5.SE 0.25). Mean intensity was not significantly different in any of the host classes for male IL diesingi (Fig.7, Hc=7.60, P>0.05) Females Overall prevalence of H. diesingi was 53% and did not differ significantly amoung adult female, adult male or late instar hosts (Fig.8, chi2=2.87, P>0.1, df=2). Adult male and early instar hosts did not differ significantly (Fig.8, chi2=3.03, P>0.05, df=l) but prevalence was significantly lower in early instar as compared to late instar or adult female hosts (Fig.8, chi2=16.0, P<0.001, df=l for early instar versus late instar). Highest abundance occurred in adult female hosts, but abundances of adult female and adult male/late instar were not significantly different (Fig.9b, Q=0.65, P>0.5). Adult male and late instar hosts were not different with respect to H. diesingi burden, 33 but both were significantly higher than early instar (Q=2.79, P<0.05 for late instar versus early instar). L. appendiculatum Males 24% of hosts contained male L. appendiculatum. Prevalence did not differ significantly amoung the four host groups (Fig.5, chi2 = 6.68 P>0.05, df=3). Male L. appendiculatum were modally distributed at 1 male per host (SE 0.03) (Fig.6).Mean intensity was not significantly different in any of the host classes (Fig.7, Hc=6.25 P>0.1). The variance to mean ratio of 0.88 was significantly less than 1 (P<0.05) indicating underdispersion. Females Overall prevalence of L,. appendiculatum was 48% and was not significantly different amoung host classes (Fig.5, chi2=1.47, P>0.5, df=3). JL appendiculatum was unique in that highest prevalence occurred in early instar hosts (Fig.5). There were no significant differences in abundance amoung the host classes (Fig.9c, Hc=4.23, P>0.1). As with FL diesingi and Thelastoma spp.. L appendiculatum was found over the greatest abundance range in adult female hosts, and these hosts had the highest mean number of females 'per host (Fig.9). Species combinations Hosts were further categorized according to the species of female pinworms present in each. Of eight possible pinworm combinations the same three were most prevalent within adult-female, adult-male and late-instar hosts, although the relative order was different. These three host categories were Thelastoma spp. only, the combination of Thelastoma spp. with H. diesingi and the combination of all three species (Fig.10). This is consistent with the condition in mid-instar hosts (Fig.3). 34 Figure 10. The relative proportion of the various combinations of adult female pinworms within each host class. The!-Thelastoma spp. H.dies.-IL diesingi L.append.-L appendiculatum A) -Adult female hosts B) -Adult male hosts C) -Late instar hosts D) -Early instar hosts 35 36 Single-species infections were more common in early instar hosts than in other host classes. 27%, 22%, and 23% of adult female, adult male and late instar hosts respectively contained one pinworm species only, while 64% of early instar infections were single species (Fig.10). Early instar hosts differed further in that they commonly contained L. appendiculatum alone, and rarely contained the Thelastoma spp./R. diesingi combination or all three species (Fig.10). The four host classes were not significantly different with respect to the proportion infected with Thelastoma spp. alone (Fig.10, chr=5.2, P>0.1, df=3) or IL diesingi alone (Fig.10, chi2=1.73, P>0.5, df=3) but a significantly greater proportion of L. appendiculatum-only infections were found in early instars as compared to the other three host classes (Fig.10, chi2=14.5, P<0.005, df=3). Similarly, adult female, adult male and late instar hosts did not differ with respect to the proportion infected with the combination of Thelastoma spp./K diesingi or the combination of three species (Fig.10, chi2=5.29 and chi2=4.71 respectively, P>0.1 for both) while there were significantly less early instar hosts containing these two combinations of pinworms (chi2=8.49, P<0.05 and chr=11.73, P<0.01 respectively). The relative order of prevalence observed in mid-instar hosts (Thelastoma spp. > H. diesingi > L. appendiculatum-) was conserved within all host classes except early-instar, where more hosts contained appendiculatum than contained IL diesingi or Thelastoma spp. (Fig.8). Effect of Host Moult There was no significant difference in mean adult female pinworm burden between recently moulted and control hosts in any of the species (Fig.ll, Paired T-test: Thelastoma spp.: T=0.23, P>0.25; £L diesingi: T=0.6, P>0.25; JL appendiculatum: T=0, P>0.25). 37 Figure 11. Comparison of mean adult female pinworm burden of newly moulted versus control hosts. Vertical lines represent standard error. Moulted roaches were selected based on their white colour and held with controls for three days before dissecting. Controls were selected based on their dark colour, thus controls were at least six days post moult. Molted (ZZ Control 39 In hosts that were completely white (i.e. hosts that had recently moulted) pinworms were found collected together into a tight mass located more posteriorly in the hindgut than usual. Flagellate protozoa coinhabiting the anterior hindgut also congregated and moved down the hindgut with the nematodes. Hosts that were two days post-moult contained a mixed mass of pinworms and protozoa located at the colon-rectal sphincter. Pinworms and protists were located in their usual position in the anterior portion of the hindgut in hosts that were three or more days post-moult. Pinworm Fecundity Thelastoma spp. females contained considerably fewer eggs than did H. diesingi or L. appendiculatum (Table 2). The relative order of mean fecundity was H. diesingi > JL appendiculatum » Thelastoma spp. In all pinworm species there was a significant negative correlation between the number of conspecific females in the host and the mean number of eggs per female pinworm (Fig. 12, P<0.05 for all pinworm species). The rate of decrease of fecundity with increasing female pinworm density, as indicated by the slope (b) of the best fit regression line, was greater in R diesingi and L appendiculatum (Fig.l2,b=-0.034 and b=-0.037 respectively) than it was for Thelastoma spp. (b=-0.007). This difference is partly accounted for by the fact that Thelastoma spp. occurs over a greater abundance range than the other two species. When the analysis was restricted to Thelastoma spp. densities approximating those found for the other two species, the rate of decrease of Thelastoma spp. fecundity approached that found in EL diesingi and JL appendiculatum (Fig.l2d,b=0.018), Although it was still lower. The effect of pinworm density on fecundity was not apparent between species. No significant negative correlations were found between the number of non-conspecific females in the host and the mean fecundity of the pinworm species in question (Table 3). 40 41 Table 2. Mean eggs per female pinworm Species  Thelastoma spp. H, diesingi L. appendicular Mean Eggs per Female (n,SE) 32 (62, 2.1) 172 (49, 10.3) 136 (47, 9.2) 42 Figure 12. The effect of intraspecific adult female pinworm density on mean fecundity. Each point represents the mean fecundity for pinworms from a single host. The straight line is the best fit regression of female pinworm numbers on Log,0 mean fecundity. AVThelastoma spp.. Regression equation: Y= -0.007x + 1.58 (T=8.64, P<0.0001) BVThelastoma spp.. excluding those hosts which contained more than 14 adult female Thelastoma spp. females. Y= -0.018x + 1.68 (T=7.57, P<0.0001) Q-EL diesingi. Y= -0.034x + 2.22 (T=6.23, P<0.0001) D)-L. appendiculatum Y= -0.037x + 2.33 (T=5.59, P<0.0001) 43 5 10 15 2 4 6 8 10 12 Female Worms per Host 44 45 Table 3. The effect of interspecific adult female pinworm density on mean fecundity. Each predicted variable (Y) represents the number of eggs of the species in question, xl, x2 and x3 are the numbers of female PL disingi. L. appendiculatum and Thelastoma spp. respectively. Thelastoma spp.: Y= -1.20x1 + 0.78x2 + 33.77 xl=H diesingi (T=1.47, P>0.1) x2=Li appendiculatum (T=1.52, P>0.1) iL diesingi: Y= -0.65x3 - 1.35x2 + 183.70 x3=Thelastoma spp. (T=1.83, P>0.07) x2=Li appendiculatum (T=0.73, P>0.4) JL appendiculatum: Y= 0.70x3 - 2.92x1 + 136.00 x3=Thelastoma spp. (T=1.14, P>0.2) xl=lL diesingi (T=0.62, P>0.5) 46 Comparison of JL appendiculatum in Isolation with Lj. appendiculatum from the Multi-species Colony As in previous data sets, hosts were divided into four classes (adult female, adult male, late instar and early instar) Male L appendiculatum were more prevalent in the isolated colony (83%) than in the mixed colony (24%). In all four host classes prevalence was significantly higher in the isolated colony than in the mixed colonies (Fig. 13, adult female: chi2=5.5, P<0.025, df=l; adult male: chi2=20.2, P<0.001, df=l; late instar: chi2=22.0, P<0.001, df=l; early instar: chi2=3.95, P<0.05, df=l). Prevalence of females was 100% in all four host classes in the L appendiculatum-only colony. This compares with the 40% prevalence found in mid instar hosts from the mixed colonies and the 48% found overall in the four host classes (adult female, adult male, late instar and early instar) from the mixed Colonies. As in the mixed colonies, male L. appendiculatum were modally distributed at 1 per infected host in all host classes. In contrast to the condition found for L appendiculatum in the multi-species colony, differences did exist amoung the host classes with respect to pinworm abundance in the L. appendiculatum-only colony. There were significantly more adult female L. appendiculatum in adult female hosts than in the other three host classes in the L. appendiculatum-only colony (Fig. 14, Q=3.19, P<0.01 for adult female versus late instar). Adult male and late instar hosts did not differ with respect to pinworm burden (Fig.14, Q=1.61, P>0.5) but both contained significantly more pinworms than early instar hosts (Fig.14, Q=3.26, P<0.01 for adult male versusearly instar). L appendiculatum from the isolate colony showed no significant differences in mean intensity when compared to L appendiculatum from the mixed colonies in any of the host classes (Fig.14, adult female: T=1.19, P>0.05; adult male: T=1.04, P>0.2; late instar: T=2.01, P>0.05; early instar:, T=0.19, P>0.5), although mean intensity was 47 Figure 13. Prevalence of infection with adult male L,. appendiculatum in the colony containing hosts infected with four species of pinworm as compared to the colony of hosts infected with L. appendiculatum only. AF-Adult female hosts AM-Adult male hosts LI-Late instar hosts El-Early instar hosts - 49 Figure 14. Distribution of adult female L appendiculatum from the colony containing hosts infected with L,. appendiculatum only compared to the colony containing hosts infected with three species of pinworm. For comparative purposes data for L. appendiculatum from the mixed colony (Figure 9c) is here reproduced next to the corresponding data from the L. appendiculatum-onlv colony. The vertical line represents the range of abundances (pinworms per host). The thick horizontal line represents the mean, and the two thin horizontal lines the standard error. AF-Adult female hosts AM-Adult male hosts LI-Late instar hosts El-Early instar hosts I-Data from the appendiculatum-only colony M-Data from the mixed (four pinworm species) colony (Fig.9c). Number of Females o O l o O l ro o ro 01 CO o > m OS 51 higher in the isolate colony in three (adult female, adult male and late instars) of the four host classes (Fig. 14). Female appendiculatum in the isolate colony contained an average of 92 eggs each (SE 4.9, range 28-154). This was significantly less than the average of 119 (SE 9.9, range 30-434) eggs per female L. appendiculatum found in the mixed colonies (F=4.05, P<0.001). The negative effect of female pinworm density on mean fecundity observed in the mixed colonies was also observed in L. appendiculatum in the isolate colony (Fig. 15). The correlation coefficient for the effect of density on fecundity in the isolate colony was not significantly different from that in the mixed colony (r2=0.33 and 0.35 respectively), and the common correlation coefficient for L. appendiculatum in mixed and isolated colonies was calculated at 0.35. Experimental Infections Figure 16 shows the average number of L. appendiculatum recovered from experimentally infected roaches dissected at various periods post-infection. Third and fourth stage larval males were found on days one and two in numbers ranging from two to twelve per host. By three to four days post-infection the males reached adulthood, and the number of males in a host declined to one by day five. A similar phenomenon was observed in female L. appendiculatum. where no more than four adults were recovered from a single host, the number recovered usually being one or two. Juvenile females were found in numbers up to twelve per host. There were no noticeable changes in number or development of females when males reached adulthood. Infections were performed using eggs from female Thelastoma spp. that had been collected from adult female hosts and early instar hosts. The hypothesis was that the two species of female Thelastoma might mimic the males. As shown earlier, T. bulhoesi 52 Figure 15. The effect of adult female L. appendiculatum density on mean fecundity in the colony containing hosts infected with L. appendiculatum only. Each point represents the mean fecundity for pinworms from a single host. The straight line is the best fit regression of female pinworm numbers on Log10 mean fecundity. Regression equation: Y= -0.026x + 2.1 (T=5.79, P<0.001) 2.5 r E CD UL k-CD CL to O) CD LU 1.5 CD CD O 0.5 0 I • 10 Number of Females 54 Figure 16. Number of L appendiculatum recovered from host hindgut on various days after experimental infection. Data points are means for repeated infections using varied initial doses of infective stage larvae. Vertical bars represent standard error, only half of which is shown for clarity. Dark triangles-Juvenile female L. appendiculatum Dark squares-Juvenile male JL appendiculatum Open triangles-Adult female L. appendiculatum Open squares-Adult male L. appendiculatum 12 10 8 0 T 9 A A 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Days Post Infection 56 preferred early instar hosts while 31 periplaneticola predominated in adult female hosts. Thus eggs collected from female Thelastoma spp. occurring in adult female hosts should produce 31 periplaneticola males, while eggs from females occurring in early instars should produce 31 bulhoesi males. Results of experimental infections are shown in Table 4. Only 31 bulhoesi males were recovered, regardless of the host from that the pinworm eggs were collected or the type of host infected. Thelastoma spp. females develop more slowly than female L, appendiculatum. Adult Thelastoma spp. females were recovered only after 29 days at 29 C. (Fig. 17). Male 31 bulhoesi develop similarly to male L appendiculatum in that development from hatching to adulthood took approximately three days at 29 C and the climax infrapopulation consisted of a single adult male per host (Fig.17). As adult females were only recovered from one host it is difficult to assess the significance of the decline in pinworm numbers occurring over time, however only four adult female Thelastoma spp. were recovered from this host (Fig.17), and this is less than the mean of 9.0 (SE 0.62) larval Thelastoma spp. females recovered per experimentally infected host. 57 58 Table 4. Species of male Thelastoma spp. produced from eggs collected from female pinworms occurring in adult female and early instar hosts Source of Eggs (Host) Adult Female Adult Female Adult Female Early Instar Early Instar Early Instar Host Infected (n) Adult Female (3) Adult Male (3) Early Instar (4) Adult Female (2) Adult Male (2) Early Instar (3) Males Produced 31 bulhoesi 31 bulhoesi 31 bulhoesi 31 bulhoesi 31 bulhoesi 31 bulhoesi 59 Figure 17. Number of Thelastoma spp. recovered from host hindgut on various days after experimental infection. Each point represents a single host. Dark triangles-Juvenile female Thelastoma spp. Dark squares -Juvenile male Thelastoma spp. Open triangle-Adult female Thelastoma spp. Open squares-Adult male L. bulhoesi • • • n 5 10 15 Days Post Infection • o A • • 20 25 30 61 Discussion Ecological theory holds that complete competitors can not coexist in the same niche, as over time the more efficient of the competing species will take over all niche space to the exclusion of others (Begon etal. 1986). The occurrence of several pinworm species parasitizing the same host organ thus leads one to speculate as to the factors permitting their coexistence. The explanation most commonly proposed for such a condition is that resource partitioning has occurred between species such that the species today occupy several "sub"-niches, and that by occupying these different niches the pinworm species avoid competition and are able to coexist within the same individual host. An assumption of this model is that the organisms are existing at densities that approximate the carrying capacity of the host hindgut, and thus niche segregation results from competition between species for limited resources. It is the contention of this paper that, for most hosts in the present study, this assumption does not apply, and thus the coexistence of several species in the same host is permitted by the overabundance of resources relative to the pinworm burden. Only in very small hosts (early instar), where resources are limiting to individual pinworms, is competition important. The severe resource limitation of these small hosts, in fact the relative lack of space itself, has resulted in the development of intense intraspecific population limitation. A side effect of this is that in larger hosts infrapopulations are maintained well below the carrying capacity of the host hindgut. The nematode guild was constant over a wide range of host sizes and over time. Thelastoma spp. was the dominant worm in both number of hosts infected (prevalence) and number of pinworms in infected hosts (intensity), particularly the latter. L. appendiculatum consistently placed lowest in these categories, while H. diesingi was in an iritermediate position, skewed toward L. appendiculatum. There were e^xplainable exceptions which appeared sporadically over time, particularly from 62 September 1988 to January 1989 when numbers of Thelastoma spp. females declined, but the overall pattern was clear: Thelastoma spp. was the most abundant pinworm followed by IL diesingi then L,. appendiculatum. The absolute number of pinworms per host was also fairly constant for all species over a wide range of host size classes. Prevalence and intensity reported in this study are typical of those reported from other locales (Gordon 1963, Hominick and Davey 1972a, 1972b, 1973, KJiairul Anuar 1977, 1987, McCallister and Schmidt 1981, 1988, Perigrine 1974a, Todd 1943). While there are few specific accounts of 31 periplaneticola or 31 bulhoesi. there are many reports of other Thelastoma spp., as well as EL diesingi and L. appendiculatum. from R americana and other cockroach hosts. These reports usually describe morphology only and omit the number of worms found. However, where prevalence and intensity-are reported they are consistent with the numbers observed in our colony. Khairul Anuar (1987) gives a break down of the numbers and species combinations of R diesingi. L appendiculatum and Thelastoma malaysiense in adult P, americana from various locals in Penang, Malaysia and reports mean intensities similar to those found in our study in both absolute and relative terms (approximately 9.3, 4.1 and 3.8 adult females per host for 31 malaysiense. R diesingi and L appendiculatum respectively). Khairul Anuar (1987) also reports that the combination of 31 malaysiense with L appendiculatum is quite rare, occurring in only 8 of 155 hosts. This agrees with the negative correlation found between Thelastoma spp. and L,. appendiculatum in the present study;! Khairul Anuar (1987) did not observe the high intensity of Thelastoma spp. found in adult female hosts in our colony, but other reports of high numbers of other Thelastoma spp. do exist. Leibersperger (1960) reports finding 168 31 periplaneticola in a single P. americanus. but does not differentiate between pinworm sexes or host age or sex; however his report of total pinworms recovered suggests that there were many male and female pinworms in some individual hosts. T. periplaneticola thus appears • 1 ' ' II 63 unusual, as it occurs in relatively large numbers (100 and more) in a single host, a phenomenon not found in T. bulhoesi. H. diesingi or L. appendiculatum. Hosts in this study were held in sealed containers, and this leads to the possibility of artefact in the pinworm distribution produced by this artificially high host density. This may be partially responsible for the high overall prevalence. However, studies from "wild" cockroach populations (often in and around human domiciles) report that prevalence of pinworm infection usually reaches 85% or more, indicating that our laboratory conditions were a fair approximation of the natural i condition. "Wild" cockroaches are gregarious, be it in a rotting log or a student's apartment, and the restrictions imposed by the containers in this study do not appear to alter the pinworm distribution. Host moulting had no affect on the number of adult female pinworms for any of the species. Previous workers (Dobrovolny and Ackert 1934, Todd 1944) report slower development times for L. appendiculatum than were found in this study (30 days for females to attain adulthood at 30 C. as compared to 12 days at 29 C. reported here) and conclude that the pinworms were more likely to be found in older hosts, where the time between host moults was sufficient for the pinworms to reach maturity, because the larval pinworms did not survive thei host moult. Larval worms were not counted in this study, but they were usually present in high numbers in hosts that had recently moulted, and the more rapid rate of development found for L. appendiculatum in our study casts doubt on this hypothesis. The worms are displaced to the posterior hindgut for a short time during the host moult, then migrate back up to the anterior hindgut after the moult is completed. The hindgut protozoa show a similar response to the moult, indicating that the initial posterior displacement of the parasites is likely due to the increases in host hindgut peristalsis during moulting and the constrictions imposed by the shed hindgut lining. Ability to cope with the host moult is not surprising, as it would be fundamental to the 64 establishment and continuation of the parasitic association. Thus it appears that each pinworm species maintains a relatively low and consistent intensity in hosts worldwide, despite probable differences in host diet and environment. In attempting to explain such stability in guild structure, the effects interspecific competition are often invoked. Niche diversification in response to interspecific competition has been assigned a major role in shaping parasite communities (Holmes 1986, Schad 1963). This parasitic association is likely quite ancient (Morris, 1981) and thus past interspecific competition may have resulted in niche diversification that is today reflected in the pinworm species occupying distinct niches within the host hindgut at abundances which are determined by the carrying capacity of each individual niche. The resultant decrease in interspecific competition would then allow the species to coexist. Previous studies suggest the presence of niche diversification in pinworms of turtles (Schad 1963) and of £, americana (Hominick and Davey 1972a). I was unable to find any evidence for niche segregation in our colony. All species seem to prefer the anterior portion of the hindgut. There is evidence that the pinworms location in the hindgut is affected by the composition of the host diet (Hominick and Davey 1972b, Hominick and Davey 1973, Peregrine 1974a, Peregrine 1974b). The apparent preference of both the nematode and protozoan parasites for the anterior portion of the hindgut probably means that this area contains more or better quality nutrients than the depauperate posterior hindgut. This seems reasonable as this area is nearest the source of mcoming nutrients. The reported preference of IL diesingi for a more posterior position in FL diesingi-only infections (Hominick and Davey 1973) was not apparent in this study. Radial distribution was not measured in this study but, in the smaller hosts particularly, radial distribution is probably unimportant as the pinworms occupy such a substantial proportion of the hindgut. Microscopic examination 65 revealed no differences in intestinal' contents between worm species. Rather than suggesting niche segregation, these data indicate that there is a high degree of niche overlap, or that there is a single niche within which the species coexist. The existence of a single niche is supported by the negative effects that were observed between species. The best measure of the importance of these interspecific effects was obtained from the comparison of Lx appendiculatum in isolation with L. appendiculatum in the multi-species colony. Results of this comparison show that while L. appendiculatum prevalence was much lower in the mixed colony, the mean intensity of infection with L. appendiculatum was not significantly different in the mixed versus the isolate colony. This was true for both sexes of appendiculatum over all host size classes examined.! These data suggest that the other pinworm species have a inhibitory effect on the initial establishment of L. appendiculatum. but that there are no important interspecies effects on the resultant number of L. appendiculatum once infection of the host is established. Thus EL diesingi and/or Thelastoma spp. appear to confer some heterogeneity on the host population with regards to infectability with Lu appendiculatum. This is analogous to the host heterogeneity provided by varying host immunocompetence in other parasitic associations. In this case it is the presence of other worm species which is inhibiting the establishment of JL appendiculatum. The negative correlation observed between female Thelastoma spp. anil L. appendiculatum in the mixed colony may then be due to an inhibitory effect of the former on the establishment of the latter. This would be especially important in adult female hosts where Thelastoma spp. can reach very high numbers and perhaps pose more of a problem to the establishment of L. appendiculatum. Adult female hosts eat and presumably defecate more than other host classes, and thus represent the most prodigious source of pinworm generation. Denial of these hosts to L. appendiculatum would further explain their scarcity in the mixed colonies. •if,: 66 The fact that L. appendiculatum in the mixed colony was unique in having a higher prevalence in early instar as opposed to the larger hosts may thus be a result of its being more likely to be able to secure sole possession of the hindgut in smaller hosts and thus exclude other pinworm species. This is supported by the disproportionately high number of early instars infected with L. appendiculatum only. Thus interspecific competition for occupancy of the anterior hindgut may occur in early instars, but the evidence suggests that most worms are able to maintain themselves in larger hosts at a slightly more posterior position without any noticeable detrimental effects to their numbers or fecundity. These exclusion effects between pinworm species add support to there being one niche within which all the pinworm species coexist. Interspecific competition is important in determining the combination of pinworm species present in a host, but its effects on the actual number of pinworms in the host appear for the most part negligible. There was a negative effect observed in the multi-species colony between L; appendiculatum and Thelastoma spp., but data from the L appendiculatum-only colony indicates that L. appendiculatum prevalence is suppressed by Thelastoma spp., while intensity remains essentially unaffected. The effects of intraspecific competition must be examined to discern the factors governing pinworm density. Experimental infections of mid to late instar hosts with L. appendiculatum and T\ bulhoesi indicate that there is intense intraspecific population limitation such that the resultant infrapopulation consists of a single adult male and one or a few adult females. The occurrence of a single male L. appendiculatum or T\ bulhoesi per infected host in the multi-species colony indicates that this process is ongoing in this colony also. Interspecific effects are not important in determining this mean intensity, as male L. appendiculatum maintained this modal distribution in the L. appendiculatum-only colony and experimental infections produced a single male per 67 infected host in the absence of other species. There is a strong intraspecific effect such that male L. appendiculatum and Ji bulhoesi limit their infrapopulation densities to a single male per host. Experimental infections were performed using mid to late instar hosts, where resources such as food and space are probably not limiting for the relatively minute males, and it is difficult to envision a resource which is in such abundance so as to consistently support exactly one male worm per host. Despite this, male infrapopulations are severely limited to one male worm per host in T\ bulhoesi and appendiculatum. and three or four males per host in HL diesingi. The exception to this, T periplaneticola in adult female hosts, appears to be an anomaly and is discussed later. t The resource competed for by males is likely female worms. The confines of the host gut limit the number of females that an individual male pinworm has access to, and this has probably favored development of intense interference competition in male worms. Experimental infections indicate that L,. appendiculatum and T\ bulhoesi begin elinimating conspecifics only after sexual maturity is reached. Juvenile males tolerate each other such that several are found in a single host, whereas only a single adult male (T. bulhoesi and L. appendiculatum') or a very few (H. diesingi) coexist within an individual host. There may be a "race" between male worms to reach adulthood first, kill conspecifics and thus procure all females. This may explain the males small size and rapid rate of development i in comparison to females. Other pinworm species parasitic in the cockroach hindgut, such as Blatticola  monandros in Parellipsidion pachycercum (Zervos 1988a), Protrellis dixoni in Drymaplaneta variegata (Zervos 1988b) and Blatticola blattae in Blatella germanica (Van Luc and Spiridonov 1990, personal observations) also severely limit their own infrapopulations such that most cockroaches contain one male and one female worm only. 68 Zervos (1988a) reviews the possible mechanisms that might generate such a distribution and suggests that this limitation is mediated via a sex-specific chemical toxin released by the pinworms which kills conspecifics. Active killing of conspecifics, resulting in very small infrapopulations, has been reported in other parasitic species including mites parasitic in marine mussels (Mitchell 1965) and hymenopterans parasitic in caterpillars (Salt 1959). Chemical mediation of this killing has been suggested in Gyrocotyle spp. (Platyhelrninthes) parasitizing the gut of raffish (Halvorsen and Williams 1968). Chemically mediated interference competition is responsible for allelopathy in many species of plant and microorganism (Rice 1974). What these organisms share with the pinworm is a physically confining environment where there is little opportunity for dispersal away from conspecific competitors, or the parasites themselves occupy a substantial volume of the parasitized organ, such that density dependent intraspecific competition is intensified. There is undoubtedly a common selective significance to a sedentary lifestyle which promotes strong intraspecific competition by virtue of the inescapable proximity of conspecifics to each other and the subsequent value of local resources. Conspecifics usually represent the strongest potential competitor to an organism. By eliminating conspecific rivals, an individual secures all of the available niche space and all potential mates. Furthermore, such an individual gains increased proportional representation in subsequent generations. An important determinant as to whether an organism will actively kill conspecifics is likely the ease with which such action can be accomplished. Most organisms are fairly well equipped to deal with or escape from physical challenges from conspecifics as they are identical in structure to such a challenger; thus the cost of killing conspecifics, in terms of search time and reciprocal damage incurred, may outweigh the benefits for many organisms. ••' '!; 69 The confines of the cockroach hindgut facilitate intraspecific competition in these pinworms. Search time is rninimal and the cost of this interaction to the killer also appears to be minimal, as it is probably mediated via a chemical which is mcUscriminantly released into the hindgut. The hindgut also provides a vessel in which such a chemical can be concentrated. Free living organisms are not likely to employ this system, as the chemical would simply diffuse into the surrounding medium. Plants are the free living exception, their lack of mobility enhancing the effectiveness of a local toxin. This system could enhance territoriality in the free living environment, as chemicals could be released which would demarcate a territory and discourage conspecific competitors. It would be interesting to investigate territoriality in free living nematodes to see if this parasitic population limitation is an intensification of such a system. The relative order of abundance of male pinworms is thus an inverse reflection of the severity of intraspecific population limitation. The occurrence in females of an identical relative abundance order (Thelastoma spp.» H. diesingi > L appendiculatum1) suggests that a similar process may be determining their numbers. Experimental infections indicate that infrapopulation limitation in females is similar, though less severe, to that seen in male worms. L, appendiculatum reduces its numbers to one or a few female worms per host after infection with twenty or thirty infective eggs. The occurrence of more than one adult female in the post-infective infrapopulation may in part be due to dilution of the responsible chemical agent in larger hosts, as smaller hosts from the infected colonies typically contained a single female worm. The occurrence of many (twenty and more) female worms in larger hosts from the infected colonies is likely the result of overlapping infections. This indicates that while the males limitation of the number of conspecifics in the host is ongoing for the duration of the male worm's life, the females active killing of conspecifics may have a i i 70 shorter duration, a "window" that remains open long enough for the female to kill conspecifics which were ingested at the same time that she was, and then shuts such that females resulting from subsequent infection events (i.e. host ingestion of infective-stage pinworms) are not affected. It may also be that as females mature they become resistant to the effects of conspecifics, or that the effects of such a chemical vermicide are less severe on the diploid females than on the haploid males. Whereas the male worms are probably competing for mates, it is not as clear what resources the female worms are competing for. Cockroaches such as early instar P. americana or adult R pachycercum are themselves so small that they are able to maintain only a limited number of pinworms due to the size limitations of their hindgut. Parasite size and gut carrying capacity has been deemed important in determining the mechanisms of population limitation in Ascaris lumbricoides infections of humans (Keymer 1982). The adult female pinworm occupies a substantial proportion of the cockroach hindgut, so that space itself may be the limiting factor (Fig. 18a). This is supported by the occurrence of modal distributions of one parasite per host in other nematodes infecting small insects (Adamson and Buck 1990, Zervos 1988a, Zervos 1988b). The actual space over which the worms are competing may be even smaller than the entire hindgut, for pinworms show a marked preference for the anterior part of the hindgut (Adamson and Buck 1990, Hominick and Davey 1973, McCallister and Schmidt 1981). Thus space itself, in particular the anterior section of the hindgut of early instar hosts, may be the limited resource over which the worms compete. However late instar cockroach nymphs and adults occasionally support more than a hundred worms (Fig. 18b). Despite this, the majority of these hosts harbour far fewer worms than the available space would permit (Fig. 19). The continuation of this active interference competition for space in larger hosts, where space is no longer limiting, would then seem to violate the tenet of limited resources1 being necessary for ongoing competition, as the result is populations that are apparently well below the carrying capacity of their 71 f i r r l S Figure 18. Relative position and size of adult male and female Thelastoma spp. in situ. A-First instar host B-Adult female host F-Female pinworm M-Male pinworm mt-Malphigian tubules il-JJeum r-Rectum 72 73 Figure 19. Adult female pinworm density (bars) and hindgut volume (squares with connecting line) in the various host classes. El-early instars Mi-mid instars Ii-late instars AM-adult male AF-adult female \ / X H.diesingi - B - Hindgut Volume El Ml LI A M A F Host Class 75 environment. Of course the tenet remains inviolate as resources are limited for a portion of the pinworm suprapopulation, namely those infecting early instar hosts. These pinworms occupy a niche which is distinctly separated into discrete patches of very constant dimension. Once "assigned" to a given patch (i.e. a particular instar) there is no opportunity for an individual pinworm to disperse out of that patch. One would predict that competition would be most severe in the patch that represents the most limited supply of resources. Resources (indeed space itself) are most lirniting in smaller hosts, and small hosts (be they small roach species or early instars of a larger species) probably represents the main focus of selective pressures which favour population limitation, providing the impetus that maintains this limitation in larger hosts where space is no longer limiting. All cockroaches go through early developmental stages which are quite small in relation to the pinworm. Furthermore, newly emerged first instar hosts represent the only host population which is guaranteed uninfected, and a pinworm which gains sole possession of such a host might be able to then exclude subsequent pinworms from establishing or at least avoid being itself excluded. The unusually high occurrence of L. appendiculatum-only infections in early instar hosts suggests that this is the case for this species. P. pachycercum is a relatively small cockroach, and IL monandros occupies a substantial portion its hindgut. This is analogous to Thelastoma spp., FL diesingi and L. appendiculatum that occur in early instar P. americana. This study demonstrates that this population limitation continues over a wide range of host sizes, the result being larger hosts which harbour far fewer nematodes than they could potentially. This availability of niche space in larger hosts has facilitated the co-occurrence of multi-species infections within a single host species. This is supported by the preponderance of single species infections within small insect species where all "patches" are of lirniting dimension. 76 Another manifestation of intraspecific competition was the negative effect of pinworm density on fecundity. Competition for limited nutrients would help explain the negative fecundity affects of female conspecifics on one another, as egg production is likely affected by nutrient uptake (Peregrine, 1974). The negative effect of increased worm burden on mean fecundity was species specific. Only conspecific females suppressed each others fecundity. The suppression of fecundity caused by the presence of even a single conspecific female was significant, whereas the presence of many worms of the other two species had no apparent effect. This was true for all three species. Thus if competition for nutrients is a causal factor in suppressing fecundity, it is necessary that these worm species feed on three absolutely distinct components of the hindgut microflora. As previously stated, there is no evidence for niche segregation in these pinworms, and thus exploitative competition for nutrients is probably not the cause of the density-dependent fecundity suppression. Furthermore, there was a significant difference in fecundity (approximately 30% for all species) in female pinworms from one versus two pinworm infections of large hosts. It is difficult to envision a nutrient supply which is capable of supporting 150 worms and yet is limiting when 'only two worms are present. A further point against nutrients as the factor governing the effect of crowding on fecundity is the exponential, as opposed to linear, decrease in mean fecundity as worm density increases. The geometric decrease in fecundity seen in all three species indicates that the relative effects of crowding on fecundity are relatively stronger at the lower worm densities and then become less marked at higher worm densities. This is not consistent with nutrient supply as the factor determining fecundity. A chemical factor released by females, which reduces conspecific fecundity, is consistent with these data. This implies a mechanism similar to that proposed for the active killing of conspecifics which occurs when adulthood is reached. The chemical 77 factor may be the same one responsible for the conspecific limitation but excreted in a diluted or modified form. This is supported by the co-occurrence of reduced population limitation with a reduced rate of conspecific density-dependent fecundity suppression in Thelastoma spp.. Thus classical exploitation competition for resources does not appear to be important in the majority of infrapopulations. It appears that the strong intraspecific population limitation keeps the pinworms below the carrying capacity of the hindgut of most hosts. Only in smaller hosts, where space is limiting, would exploitation competition be important, and it is probable that this population limitation is a response to conditions imposed by small hosts. The potential carrying capacity of the hindgut of larger cockroaches is exemplified in one anomalous case where population limitation is not apparent: adult female cockroaches infected with Thelastoma spp. females and T periplaneticola males. There were many more Thelastoma spp. recovered than were FL diesingi or L. appendiculatum (total worms recovered of each species: 5991, 1210 and 640 respectively), and the occurrence of large numbers of Thelastoma spp. in adult female hosts seems the best explanation for this numerical dominance. In adult female hosts there is apparently a break down in the intraspecific limiting affect of Thelastoma spp. such that a hundred or more can coexist in a single host. Male T. periplaneticola revert to a low intensity infection in other hosts, becoming modal at one worm per host in early instars. This indicates that T. periplaneticola retains its ability to limit its population size, but that this is somehow repressed in adult female hosts. These high numbers of pinworms probably approximate the potential carrying capacity of the host, as there appears to be little space for more than about 150 pinworms in a single adult female host (personal observation). Very high numbers of these worms were also found in a few late instar hosts. Late instars were not identified as to sex, and these few individuals may represent female roaches. No adult 78 male hosts were found to contain such high numbers of Thelastoma spp., indicating that it is some inherent difference between male and female roaches that allows this coexistence. Some affect of host hormones is possible, as the cockroach ovary is known to produce hormones, while cauterization of host neurosecretory cells has been shown to affect the population of FL diesingi parasitizing Blatta orientalis (Gordon, 1968). Perhaps female host hormone in the gut interacts with the pinworm antihelrrrinthic so as to make it inert. Alternatively, the environment of the adult female hindgut may alter the worms so as to stop them releasing the antmelminth or provide them with a resistance to its action. Thus the reduction of population limitation in adult female hosts provides a population "sink" from which Thelastoma spp. can numerically dominate the supraguild. i, Thelastoma spp. numerical dominance in our colony may seem attributable to the inclusion of two worm species CL periplaneticola and 3L. bulhoesi) in a single group (Thelastoma spp.). The subsequent comparisons with R diesingi and L appendiculatum would then seem unjustified as data from two worm species were summed and compared to single species data. To some extent this is true and unavoidable, as I am at present unable to distinguish two morphological types in female Thelastoma spp.. However, overall there were five and eight times respectively as many female Thelastoma spp. recovered than were R diesingi or L appendiculatum. and thus if it truly was two species then one or both of them was in some way peculiar from R diesingi or L appendiculatum. The evidence from male worms indicates it was one form (T\ periplaneticola) that may have been particularly responsible for the increased numbers of Thelastoma spp. pinworms. The frequency distribution for male T. periplaneticola is very similar to that found for female Thelastoma spp.. Male T\ bulhoesi. with its modal distribution of one, is most like L appendiculatum. and if T. bulhoesi females behave as R diesingi and L. appendiculatum females do (and mirror their respective male's 79 distribution) then female 31 bulhoesi may be rare and difficult to distinguish in a background of many female T. periplaneticola. Confounding this issue was the inability to produce male 31 periplaneticola from experimental infections, with all males produced being 31 bulhoesi. More infections with Thelastoma spp. eggs are necessary to satisfactorily explain this result, however it suggests a male polymorphism. This would mean that the 31 bulhoesi morph retains its strong self limiting effect while the 31 periplaneticola morph does not and coexists in high numbers. A similar phenomenum has been reported for two pinworm species infecting skinks and geckos (Ainsworth, 1990), where one morph maintains a mean intensity of one male per host while the other exists at numbers of up to seven per host. As both morphs would be competing for the same females, and there were eighteen times as many male 31 periplaneticola recovered as 31 bulhoesi. the T. periplaneticola morph would be at a distinct advantage in procuring a mate. Experimental infections indicate it is probably not an environmentally determined polymorphism, as male 31 bulhoesi developed in hosts of varying size and sex. Whether it is actually only one species of pinworm in Thelastoma spp. with a male polymorphism, or a combination of a more populous species (31 periplaneticola') and a rarer species (31 bulhoesi'). the comparison of Thelastoma spp. with H. diesingi and L appendiculatum is justified and does not in itself explain the dominance of Thelastoma spp. Rather the explanation presumably lies in the reduced population limitation occurring in adult female hosts. In conclusion the factor that determines and maintains the stability of the guild structure in our colony is intraspecific interference competition. This interference competition is likely mediated via a species and sex-specific chemical toxin released by the pinworms when adulthood is reached. The relative abundance of pinworms through time (Thelastoma! spp>> H. diesingi > L. appendiculatum') is thus an inverse reflection of the degree of intraspecific limitation within each species. 80 Interspecific effects are uriimportant in determining infrapopulations, but are important in determining prevalence. Thus while interspecific competition affects the supraguild by deterrnining the species combinations present in each infrapopulation, intraspecific population limitation dictates the number of individuals of each species present in each infrapopulation. There is some debate as to the relative importance of interspecific competition in structuring communities. The occurrence of site-specific parasites has been used as evidence of niche diversification due to interspecific effects (Holmes 1973, Schad 1963), while reviews of field experiments and studies on free living organisms differ in their assessment of the importance of interspecific effects in maintaining community structure (Brooks 1980, Connell 1983, Schoener 1983, Shorrocks et al. 1984). This study indicates that species may not compete even in the presence of substantial niche overlap. However, all systems are different and the debate over the importance of interspecific competition is somewhat artificial, as what is true for one group of organisms may not be for another. Pianka (1981) describes "a gradient in the intensity of (interspecific) competition, varying continuously between the endpoints of a complete competitive vacuum (no competition) to a fully saturated environment with demand equal to supply...". These pinworms then represent a system biased toward the former of these two extremes. The i continuing co-existence of these non-competing species is facilitated by intense intraspecific population limitation in small hosts that prevents parasite densities from reaching levels sufficient to cause resources to be limited in the majority of larger hosts. These effects are likely direct adaptations to the parasitic lifestyle, where the small gut size of early instar hosts relative to the size of the pinworm limits the availability of space for female pinworms and the availability of mates for male pinworms, resulting in intense intraspecific competition. 81 Each species maintains its own population without regard to the other species present. Populations are maintained well below the apparent carrying capacity of the majority of hosts, and thus competition for resources is probably of little importance, particularly in the larger hosts. However, the best nutrients probably occur most anteriorly in the host hindgut, so some exploitation competition (both inter- and intra-specific) may occur in this area. Thelastoma spp. (in particular T periplaneticola') appear to reduce this intraspecific interference competition in adult female hosts, thus providing a population "sink" from which Thelastoma spp. can numerically dominate the population despite its very low fecundity. This phenomenum may be the result of a peculiar interaction between the pinworm-produced antmelminth of T periplaneticola and some moiety, perhaps hormonal, in the hindgut of the adult female cockroach. REFERENCES Adamson, M.L. 1989. Evolutionary biology of the Oxyurida (Nematpda): bipfacies of a haplodiploid taxon. Advances in parasitology, 28: 175-228. Adamson, M.L. and A.E. Buck. 1990. Pinworms from water scavenger beetles (Coleoptera: Hydrophilidae) with a description of a new species, Zonothrix columbianus sp. n. 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Field experiments on interspecific competition. American Naturalist, 122(2): 240-285. Shorrocks, B., J. Rosewell and K. Edwards. 1984. Interspecific competition is not a major organizing force in many insect communities. Nature, 310: 310-312. Strand, M. and M. Brooks. 1977. Pathogens of Blattidae (Cockroaches). Bulletin of the World Health Organization, 55(1): 289-304 Todd, A.C. 1943. Thelastoma icemi (Schwenck), a nematode of cockroaches. J. Parasit., 29: 404-406. Todd, A.C. 1944. On the development and hatching of the eggs of Hammerschmidtiella  diesingi and Leidynema appendiculatum. nematodes of roaches. Transactions of the American microscopical society, 63: 54-67. Van Luc, P. and S. Spiridonov. 1990. Experimental evidence of arrhenotoky in the nematode Blatticola blattae (Oxyurida: Thelastomatidae). Helrninthologia, 27: ,67-70. Zar, J.H. 1984. Biostatistical Analysis (2nd ed.). Prentice-Hall Inc. Zervos, S. 1988a. Population dynamics of a thelastomatid nematode of cockroaches. 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