Are individuals from thelytokous and arrhenotokous populations equally adept as biocontrol agents? Orientation and host searching behavior of a fruit fly parasitoid

Hymenopteran parasitoids generally reproduce by arrhenotoky, in which males develop from unfertilized eggs and females from fertilized eggs. A minority reproduce by thelytoky, in which all-female broods are derived from unfertilized eggs. Thelytokous populations are potentially of interest for augmentative biological control programs since the exclusive production of females could significantly lower the costs of mass rearing. Behavioral traits are a major component of parasitoid efficacy. Here, we examined orientation and host searching behavior in thelytokous and arrhenotokous populations of the fruit fly parasitoid Odontosema anastrephae Borgmeier (Hymenoptera: Figitidae). Orientation behavior to various odorant sources was studied in a two-choice olfactometer. No major differences were found between thelytokous and arrhenotokous wasps for this behavior. However, when host-searching behaviors were analyzed, some differences were found. Thelytokous females arrived sooner, foraged longer, and remained longer on non-infested guavas than arrhenotokous females. Individuals of both forms exhibited similar stereotyped behavioral sequences vis-à-vis guava treatments, with only slight deviations detected. Our results suggest that individuals from selected thelytokous and arrhenotokous O. anastrephae populations have similar abilities to search for tephritid larvae, supporting the use of thelytokous strains for augmentative releases.     

Biological Control Potential of Wolbachia -infected Versus Uninfected Wasps: Laboratory and Greenhouse Evaluation of Trichogramma cordubensis and T. deion Strains

The effects of thelytoky-inducing Wolbachia ( &#102 -proteobacteria) on Trichogramma cordubensis and T. deion (Hymenoptera, Trichogrammatidae) were studied in laboratory and greenhouse conditions. One infected (thelytokous, all female) line of each wasp species was compared with its conspecific uninfected (arrhenotokous, sexual) counterpart for several fecundity and dispersal traits. Arrhenotokous lines had a higher fecundity than their thelytokous counterparts, which suggests that Wolbachia negatively affect the fecundity of Trichogramma females. The arrhenotokous females dispersed more in the laboratory than their thelytokous counterparts. In the greenhouse, the opposite effect or no difference between lines was found, indicating that the laboratory set-up used to measure dispersal is not useful to predict relative dispersal of the females in the greenhouse. Calculations show that by releasing 100 adult wasps of both lines, thus including arrhenotokous males in the sexual line, more eggs are parasitized by the thelytokous wasps. Therefore, in spite of their lower individual female fecundity, thelytokous lines have a better potential for biological control than their arrhenotokous counterparts.     

Selection in the absence of males does not affect male-female conflict in the parasitoid wasp Leptopilina clavipes (Hymenoptera: Figitidae)

Divergent reproductive interests of males and females can lead to sexually antagonistic coevolution (SAC). In the absence of males, adaptations evolved under SAC are released from selection and expected to deteriorate. In this study, we investigated this prediction using two populations of the parasitoid wasp Leptopilina clavipes, one arrhenotokous and one thelytokous. Thelytokous females were induced to produce sons by curing them of their Wolbachia-infection. We examined whether thelytokous males were less able to inhibit female remating than arrhenotokous males and whether thelytokous females were more susceptible to male-induced longevity reduction than arrhenotokous females. The results showed that females were monandrous, regardless of whether mated with an arrhenotokous or thelytokous male. While ongoing courtship of males reduced female life span, there was no longevity cost of mating for either arrhenotokous or thelytokous females. Our results therefore do not support the idea that adaptations evolved under SAC deteriorate under prolonged female-only selection.     

Flight initiation in the egg parasitoid Trichogramma minutum: Effects of ambient temperature, mates, food, and host eggs

Emergence, preening, and flight initiation were studied in laboratory-reared Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae). Male parasitoids emerged first and flew before females. When both sexes were present in flight cylinders, female parasitoids flew before males. Flight propensity in males was negatively related to the number of emerging females, while flight propensity in females was independent of the number of males present. Ambient temperature significantly affected the propensity and timing of flight; between 70–80% of the parasitoids flew at 25 and 30 °C while less than 4%, mostly males, flew at 20 °C. No flights were observed at 15 °C. The presence of fresh host eggs caused a reduction in the proportion of female parasitoids that flew and a delay in the time to flight for the females that did fly. The presence of food increased the flight propensity of female parasitoids, but did not affect the timing of flight. The relationship between flight behaviour and the efficiency of mass-reared Trichogramma is discussed in terms of its importance for inundative release programmes.     

Odor-mediated patch choice in the parasitoid Venturia canescens: temporal decision dynamics

Parasitoids foraging for hosts in a heterogeneous environment would greatly benefit if they could decide already from a distance in which areas search for resources would be most profitable and to avoid areas of low fitness returns. Interestingly, the temporal dynamics of the decision process in parasitoid patch choice have rarely been investigated. In a Y-tube olfactometer, we tested whether thelytokous and arrhenotokous females of the parasitoid wasp Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae) respond to differences in cues indicating the quality of a host-containing patch and choose more profitable patches. Special attention was given to the time it took females to make their choices (patch choice time) when differences in patch quality were either qualitative (absence vs. presence of hosts and kairomone) or quantitative (various concentrations of hosts and kairomone, and presence of competitors). We found that both thelytokous and arrhenotokous wasps only chose the higher-quality patch based on odor cues when the difference was qualitative. When patches differed only with respect to the number of hosts, or the presence or absence of competing female parasitoids, no significant preference could be found in females of either strain of the parasitoid. In contrast, both the time until females reached the junction of the Y-tube olfactometer (response time) and the time until females decided for either patch (decision time) varied with parasitoid strain and odor treatment. Thelytokous wasps were faster than arrhenotokous wasps in their response time and in their decision time. However, females of both strains responded faster with increasing number of total hosts releasing kairomone. Yet, decision time for patches did not significantly vary as a function of patch quality offered to Venturia wasps.     

Geographical distribution and genetic relatedness of sympatrical thelytokous and arrhenotokous populations of the parasitoid Venturia canescens (Hymenoptera)

Theory predicts that asexual reproduction has a competitive advantage over sexual reproduction because of the twofold cost of producing males. Few systems are suitable for directly testing this prediction. In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (asexual) reproducing individuals occur sympatrically. We sampled 922 wasps from 22 localities along the coast of south‐eastern France. Thelytokous wasps were less abundant (23%) than arrhenotokous wasps and were almost always found in sympatry with arrhenotokous ones. An analysis of genetic relatedness using amplified fragment length polymorphism (AFLP) markers showed the existence of a widespread thelytokous clone. In addition, a few thelytokous individuals were found to be closely related to arrhenotokous ones and vice versa. These data suggest the occurrence of occasional gene flow between both reproductive modes and/or recurrent origin of thelytokous clones from coexisting arrhenotokous populations in the area. The results are discussed in the context of the paradox of sex.     

不同温度下松毛虫赤眼蜂孤雌产雌品系和两性生殖品系对米蛾卵的寄生功能反应

赤眼蜂部分蜂种或品系受Wolbachia侵染营孤雌产雌生殖。 通过室内试验分析了在4个恒温（20, 25, 30和35℃）下松毛虫赤眼蜂Trichogramma dendrolimi两性生殖品系和孤雌产雌品系对米蛾Corcyra cephalonica卵的寄生功能反应, 旨在比较不同温度两品系的寄生功能反应差异, 评价孤雌产雌品系在生物防治中的应用潜力。 结果表明： 松毛虫赤眼蜂两个品系对米蛾卵寄生作用均随寄主密度的增加而增大;随温度的升高松毛虫赤眼蜂两品系的功能反应类型由Ⅲ型改变为Ⅱ型。孤雌产雌品系以30℃的处置时间最短(0.0207 d), 最大日寄生量为48.31粒卵, 其次是25℃, 35℃最小;两性生殖品系以25℃的处置时间最短(0.0188 d), 最大日寄生量为53.08粒卵, 其次是30℃, 20℃最小; 松毛虫赤眼蜂两品系的寄生功能反应存在显著差异, 30℃下孤雌产雌品系为Ⅱ型功能反应而两性生殖品系为Ⅲ型。 从处置时间来看, 20℃时两品系无显著性差异(P≥0.05), 在25℃和35℃孤雌产雌品系寄生米蛾卵时花费的时间显著长于两性生殖品系(P<0.05), 而30℃却相反。 可见, 寄主密度、 温度和Wolbachia影响松毛虫赤眼蜂功能反应。     

EFFECTS OF PARTHENOGENESIS AND GEOGRAPHIC ISOLATION ON FEMALE SEXUAL TRAITS IN A PARASITOID WASP

Population divergence in sexual traits is affected by different selection pressures, depending on the mode of reproduction. In allopatric sexual populations, aspects of sexual behavior may diverge due to sexual selection. In parthenogenetic populations, loss‐of‐function mutations in genes involved in sexual functionality may be selectively neutral or favored by selection. We assess to what extent these processes have contributed to divergence in female sexual traits in the parasitoid wasp Leptopilina clavipes in which some populations are infected with parthenogenesis‐inducing Wolbachia bacteria. We find evidence consistent with both hypotheses. Both arrhenotokous males and males derived from thelytokous strains preferred to court females from their own population. This suggests that these populations had already evolved population‐specific mating preferences when the latter became parthenogenetic. Thelytokous females did not store sperm efficiently and fertilized very few of their eggs. The nonfertility of thelytokous females was due to mutations in the wasp genome, which must be an effect of mutation accumulation under thelytoky. Divergence in female sexual traits of these two allopatric populations has thus been molded by different forces: independent male/female coevolution while both populations were still sexual, followed by female‐only evolution after one population switched to parthenogenesis.     

The biosystematic relations between a thelytokous and an arrhenotokous form ofAphytis mytilaspidis (Le Baron) [Hymenoptera: Aphelinidae]

Biological and morphological characters of the thelytokous and arrhenotokous forms ofA. mytilaspidis and their hybrids were investigated. Some biological differences between the thelytokous and arrhenotokous forms were found. The former is adapted to warmer condition and will parasitizeHemiberlesia lataniae as well asDiaspis echinocacti, while the latter will not parasitizeH. lataniae. The overall fecundity of the thelytokous form is lower than that of the arrhenotokous form, whereas the “effective fecundity”, female progeny per female parent, is higher in the thelytokous form. Apart from the no. of setae in the wings delta, there are no distinguishing morphological characters between the arrhenotokous and thelytokous forms.     

Reduced sexual functionality of PI‐Wolbachia‐infected females of Tetrastichus coeruleus

Wolbachia are endosymbiotic bacteria known to manipulate the reproduction of their hosts by, for example, inducing parthenogenesis. In most cases of Wolbachia‐induced parthenogenesis, the infection is fixed and the entire host population consists of females. In the absence of males and sexual reproduction, genes involved in sexual reproduction are not actively maintained by selection. Accumulation of neutral mutations or selection against maintenance of sexual traits may lead to their loss or deterioration. In addition, females may lose the ability to reproduce sexually due to ‘functional virginity mutations’ that may spread concomitantly with the Wolbachia infection through a population. The parasitoid wasp Tetrastichus coeruleus (Nees) (Hymenoptera: Eulophidae) forms an ideal model to study the decay of sexual functionality, because it has both Wolbachia‐infected, parthenogenetic populations and uninfected, sexual populations. We compared several components of sexual functionality of arrhenotokous (sexual) and thelytokous (parthenogenetic) T. coeruleus females. First, we tested whether arrhenotokous and thelytokous females were equally attractive and receptive to males. Second, we examined whether mating is costly to females by measuring the life span of mated and virgin females. Last, we studied the morphology of the spermathecae of arrhenotokous and thelytokous females. Mated females had shorter life spans than virgin females, showing that mating carried a fitness cost. Two sexual traits of thelytokous females have degraded compared to arrhenotokous females. Arrhenotokous and thelytokous females were equally attractive to males, but thelytokous females were unreceptive to males. Furthermore, there was a clear difference in spermathecal morphology between arrhenotokous and thelytokous females. Our data do not allow distinction between the various potential causes of such degradation. Although the longevity cost of mating may indicate selection against the maintenance of costly sexual traits, accumulation of neutral mutations, functional virginity mutations, manipulation by Wolbachia, and/or the genetic distance between the two populations may all have contributed to the decay of sexual traits in thelytokous females.     

Biology of the shore fly Scatella stagnalis in rockwool under greenhouse conditions

The development times and survival of immature stages in rockwool and the fecundity and longevity of adult Scatella stagnalis were determined and stage-specific life-tables constructed for the species at constant 20 and 25 °C and at a fluctuating temperature (23–34 °C, mean 28.5 °C). Development time from egg to adult decreased with temperature, being 15.9±0.1 days at 20 °C, 11.4±0.1 days at 25 °C and 10.1±0.2 days at fluctuating temperature with mean of 28.5 °C. The lower threshold for egg-to-adult development was 6.4±2.7 °C and the total quantity of thermal energy required to complete development was 212.8±.0 °C. The proportion of females in two populations studied was 0.521. High temperature increased the mortality of pupae from 7% (20 °C) and 10% (25 °C) to 29% at 28.5 °C. At 25 °C, female longevity was 15.5±0.7 days and fecundity 315±19 eggs/female (20.4 eggs/female/day). Males lived for 22.0±1.1 days. At constant 25 °C, the net reproductive rate was 126.1 female eggs/female, generation time was 18.4 days, the doubling time of the population 5.3 days, and the intrinsic rate of increase (r _m) 0.263 day^–1.     

The influence of temperature and host availability on the host exploitation strategies of sexual and asexual parasitic wasps of the same species

In the hymenopteran parasitoid Venturia canescens, asexual (obligate thelytoky not induced by Wolbachia bacteria) and sexual (arrhenotokous) wasps coexist in field conditions despite the demographic cost incurred due to the production of males by sexual females. Arrhenotoky predominates in field conditions, whereas populations in indoor conditions (mills, granaries) are exclusively thelytokous. These differences in the relative abundance of the two modes of reproduction between environments suggest that the individuals of each reproductive mode may have developed strategies adapted to the conditions prevailing in each kind of habitat. The two environments contrast in temperature variability and in the spatial heterogeneity of host availability. In this study, we considered the combined effect of temperature and host availability on host patch exploitation by thelytokous and arrhenotokous V. canescens. As expected, arrhenotokous females were more sensitive to temperature changes. If the temperature decreased before foraging, they remained longer and exploited patches more thoroughly. This is consistent with the expected behaviour of parasitoids in response to signs of unfavourable conditions that entail increasing risk of time limitation or a reduced probability of attaining further patches. Both arrhenotokous and thelytokous females increased patch exploitation with host availability. However, unexpectedly, we found no difference in the way the two types of wasp responded to differences in host availability. Differences in the strategies adopted under different environmental conditions may indicate divergence of niche-specific life history traits between the two modes of reproduction. Niche displacement may partly account for the coexistence of these two modes of reproduction at a geographical scale.     

Diploidy restoration in Wolbachia-infected Muscidifurax uniraptor (Hymenoptera: Pteromalidae)

Thelytokous reproduction, where females produce diploid female offspring without fertilization, can be found in many insects. In some Hymenoptera species, thelytoky is induced by Wolbachia, a group of cytoplasmically inherited bacteria. We compare and contrast early embryonic development in the thelytokous parthenogenetic species Muscidifurax uniraptor with the development of unfertilized eggs of the closely related arrhenotokous species, Muscidifurax raptorellus. In the Wolbachia-infected parasitic wasp M. uniraptor, meiosis and the first mitotic division occur normally. Diploidy restoration is achieved following the completion of the first mitosis. This pattern differs in the timing of diploidy restoration from previously described cases of Wolbachia-associated thelytoky. Results presented here suggest that different cytogenetic mechanisms of diploidy restoration may occur in different species with Wolbachia-induced thelytoky.     

Searching Speed of Trichogramma minutum and Its Potential as a Measure of Parasitoid Quality

Searching speed has been proposed as a quality criterion for the prediction of field efficacy of mass-reared Trichogramma spp. The searching speed, walking speed, and activity of female T. minutum Riley were measured using an automated video tracking system. Two- and 3-day-old thelytokous females searched twice as fast as 1-day-old females, whereas the searching speed of a strain of arrhenotokous females was similar at all ages. When reared on Actebia fennica Tauscher, the resulting wide variation in wasp size was not related to searching speed or walking speed. Both strains of T. minutum searched faster later in the day. In a second laboratory experiment, the proportion of hosts parasitized, the proportion of hosts accepted, and searching speeds of parasitoids were measured simultaneously for groups of females of both strains. Percentage parasitism was related to host acceptance but not searching speed. Searching speed was unrelated to the estimated number of host contacts. These results suggest that searching speed is not a useful measure of parasitoid efficacy for T. minutum.     

Modulation of Flight Activity in Lobesia botrana Den. & Schiff. (Lepidoptera: Tortricidae) Females Studied in a Wind Tunnel

We studied modulations of flight activity in European grapevine moth females (EGVM) by individual observations in a wind tunnel. The effect of different factors was analyzed: variation in atmospheric pressure prior to the experiments, time of day, first experience of flight, age, mating, and odor of tansy, which attracts females. The circadian flight activity showed a peak the hour preceding the onset of scotophase and sustained activity occurred during the 6 h around this peak. Females with a flight experience in the tunnel took off more quickly than naive ones (3.9 ± 7.4 vs 20.3 ± 22.8 s). Three-day-old unmated females subjected to negative variations of atmospheric pressure (10 hPa) during the 4 h prior to the experiments increased their duration of flight (12.1 ± 8.7 vs 5.3 ± 3.4 s) compared to those not subjected to variation. One-day-old females were less active than older ones; flight was shorter than in 2-day-old females (2.7 ± 6.7 vs 5.1 ± 9.5 s) and fewer of them took off (28 vs 63%). Mating also affected the flight activity of 2-day-old females; mated females flew longer than virgins (12 ± 16.8 vs 5.1 ± 9.5 s) and took off more quickly (6.5 ± 14.4 vs 19.3 ± 20.1 s). Tansy odor in the tunnel did not significantly affect the flight behavior of virgin females, but it increased the proportion of mated females that initiated flight (87 vs 70%) and duration of flight (11.2 ± 24.4 vs 7.2 ± 13.7 s), and it reduced the latency to takeoff (2.1 ± 7.4 vs 8.1 ± 19.1 s). Flight duration in tansy odor was inversely correlated with the total number of eggs laid during the female's whole life. Our experimental settings did not allow observation of movements directed toward the odor source.     

Biotic performances of thelytokous and arrhenotokous strains of <Emphasis Type="Italic">Thrips tabaci</Emphasis> (Thysanoptera: Thripidae) showing resistance to cypermethrin

Two distinct reproductive types of Thrips tabaci Lindeman, one thelytokous and one arrhenotokous, have been shown to develop target-insensitive cypermethrin resistance conferred by sodium channel mutation (T929I). In this study, we examined the relation between cypermethrin resistance and biotic performances of these reproductive types. Among the T. tabaci strains examined, resistant arrhenotokous strains became adults more quickly after hatching than resistant thelytokous strains. Female adults of resistant thelytokous strains exhibited shorter longevity and oviposited fewer eggs than those of susceptible strains of the same reproductive type. Resistant arrhenotokous strains exhibited similar longevity and fecundity to susceptible thelytokous strains. We further examined the relation between reproductive types and T929I using a total of 85 strains. All 53 arrhenotokous strains encoding T929I were judged to be cypermethrin resistant. Among 32 thelytokous strains with T929I, only four were regarded as cypermethrin resistant. Based on the results, we discuss possible mechanisms accounting for the recent prominence of arrhenotoky T. tabaci in Japan.     

Egg production in response to combined alternative foods by the predator Coccinella transversalis

The peformance of the parasitoid Anagyrus kamali Moursi [Hymenoptera: Encyrtidae], as a function of host density, temperature, and photoperiod was investigated with the objective to optimize a mass-rearing system in the context of a biological control program. The number of hosts parasitized at densities varying from 2–100 hibiscus mealybug (HMB), Maconellicoccus hirsutus Green [Homoptera: Pseudococcidae], corresponded to a type II-III functional response in fixed-time conditions and a type III in variable-time conditions. Twenty-six percent of the oviposited eggs led to progeny emergence with a sex ratio of 0.49±0.102 (M/F), regardless of host density. Fecundity and oviposition period under six abiotic combinations (i.e., two temperatures (26±2 °C and 32±2 °C) and three photoperiods (L0:D24, L12:D12, L24:D0)) were measured. Lifetime fecundity and reproductive life were significantly affected by temperature and photoperiod conditions. Optimum female parasitoid lifetime fecundity was attained at 26±2 °C, L0:D24 with an average of 116.1±17.43 eggs. At 32±2 °C, L24:D0 and L12:D12, an average of 79.4±34.57 and 85.8±35.81 eggs were laid, respectively. Reproductive longevity was maximal at 26±2 °C, L0:D24 with 12±4.85 days of oviposition. Because the parasite A. kamali can be reared optimally without light, this may save tremendous energy costs.     

Diploid arrhenotoky and automictic thelytoky in soft scale insects (Lecaniidae: Coccoidea: Homoptera)

Parthenogenesis is reported in three soft scales with 2n=16. In the unfertilized eggs of all three, oogenesis is normal and diploidy is restored by the fusion of the division products of the haploid female pronucleus. In Lecanium putmani Phillips 12 of 13 uninseminated females collected in the wild produced only males. The 21 inseminated females produced 15% males. The males were diploid but contained one euchromatic (E) and one heterochromatic (H) chromosome set. Most of the eggs produced by the inseminated females contained sperm but a few did not. It was concluded, therefore, that females develop from fertilized eggs and males from unfertilized eggs and that the species was diploid arrhenotokous. In L. cerasifex Fitch only 18 of 56 females collected in the wild had been inseminated. The frequency of males among their embryos was 22%. The males were again diploid with one E and one H set of chromosomes. Among the 38 uninseminated females, 27 produced only males, and 10 produced only females. All the female producers contained needle-like bacterial symbionts. Most of the male producers, and most of the inseminated females contained no symbionts; the rest contained rod-like symbionts. It was concluded, therefore, that the females of L. cerasifex studied belonged to two races, a diploid arrhenotokous race and an obligate automictic thelytokous race. Eucalymantus tessellatus (Signoret) is obligate automictic thelytokous. All the females examined were uninseminated and produced only females.Supported by Grant GB 23665 from the National Science Foundation, Washington, D.C.     

The dynamics of energy allocation in adult arrhenotokous and thelytokous Venturia canescens

The ability to adjust resource allocation to the quality of the environment has broad implications for animal reproductive success. Organisms with complex life cycles that may experience various selection pressures during their lifetime are expected to evolve mechanisms to modulate the resource allocation strategies adopted during ontogeny to the conditions encountered by the adult. In the parasitoid Venturia canescens Gravenhorst (Hymenoptera: Ichneumonidae), thelytoky appears to have been selected for in anthropogenic habitats, where hosts are relatively numerous and food is absent, and arrhenotoky in natural habitats where hosts are more scarce and food is present. A previous study postulated that during their juvenile stage, females of both reproductive modes adopt strategies of energy allocation in accordance with these conditions, possibly providing a direct short‐term advantage to arrhenotokous forms, which partially co‐occur with thelytokous forms under natural conditions. To test this assumption, we provided daily adult thelytokous and arrhenotokous females with a small number of hosts together with food. To compare their lifetime resource allocation strategies, we recorded wasp longevities, egg loads, and carbohydrate reserves in wasps of different ages. Our analysis indicates that thelytokous females are able, to a certain extent, to cope with these conditions, because they reached the same longevity as arrhenotokous females. Nevertheless, thelytokous females suffered from a higher degree of time limitation compared with arrhenotokous ones, and arrhenotokous wasps appeared to maintain their energetic advantage over the adult stage. These results provide new insights, and point to the consideration of other activities, such as flight performance and/or ability to reach food and hosts, in the understanding of the role of resource allocation strategies in the maintenance of sex in this species.     

The cytological basis for reproductive variability in the Anoetidae (Sarcoptiformes: Acari)

Meiosis is described in a thelytokous strain of the anoetid mite Histiostoma feroniarum (Dufour) and in both sexes of the arrhenotokous strain of this species. Oogenesis in the thelytokous strain is accomplished by ameiotic mitosis with only one pseudo-maturation division. During this division one or more chromosomes may move to the poles precociously and while in this position can be mistaken for centrioles. Fourteen chromosomes are found at metaphase of the pseudo-maturation division and in cleaving eggs of this strain. In the arrhenotokous strain, male meiosis consists of a single mitotic division. Oogenesis is regular and 7 bivalents are observed at the first maturation division. Metaphases of the first cleavage division in fertilized eggs show 14 chromosomes and 7 chromosomes in unfertilized eggs.It is postulated that the thelytokous strain has arisen from the arrhenotokous strain. This assumption is in agreement with that suggested for several insect species previously reported. The evolution in the Acari and the variability in the modes of reproduction in this suborder are discussed in light of the findings in this paper on the Anoetidae.