Phylogenomic analysis, reclassification, and evolution of South American nemesioid burrowing mygalomorph spiders.

The family Nemesiidae was once among the most species-rich of mygalomorph spider families. However, over the past few decades both morphological and molecular studies focusing on mygalomorph phylogeny have recovered the group as paraphyletic. Hence, the systematics of the family Nemesiidae has more recently been controversial, with numerous changes at the family-group level and the recognition of the supra-familial clade Nemesioidina. Indeed, in a recent study by Opatova and collaborators, six nemesiid genera were transferred to the newly re-established family Pycnothelidae. Despite these changes, 12 South American nemesiid genera remained unplaced, and classified as incertae sedis due to shortcomings in taxon sampling. Accordingly, we evaluate the phylogenetic relationships of South American nemesioid species and genera with the principle aim of resolving their family level placement. Our work represents the most exhaustive phylogenomic sampling for South American Nemesiidae by including nine of the 12 genera described for the continent. Phylogenetic relationships were reconstructed using 457 loci obtained using the spider Anchored Hybrid Enrichment probe set. Based on these results Nemesiidae, Pycnothelidae, Microstigmatidae and Cyrtaucheniidae are not considered monophyletic. Our study also indicates that the lineage including the genus Fufius requires elevation to the family level (Rhytidicolidae Simon, 1903 (NEW RANK)). In Pycnothelidae, we recognize/delimit five subfamilies (Diplothelopsinae, Pionothelinae (NEW SUBFAMILY), Prorachiinae (NEW SUBFAMILY), Pselligminae (NEW RANK), Pycnothelinae). We also transfer all the 12 South American nemesiid genera to Pycnothelidae: Chaco, Chilelopsis, Diplothelopsis, Flamencopsis, Hermachura, Longistylus, Lycinus, Neostothis, Prorachias, Psalistopoides, Pselligmus, Rachias. Additionally, we transferred the microstigmatid genus Xenonemesia to Pycnothelidae, and we propose the following generic synonymies and species transfers: Neostothis and Bayana are junior synonyms of Pycnothele (NEW SYNONYMY), as P. gigas and P. labordai, respectively (NEW COMBINATIONS); Hermachura is a junior synonym of Stenoterommata (NEW SYNONYMY), as S. luederwaldti (NEW COMBINATION); Flamencopsis is a junior synonym of Chilelopsis (NEW SYNONYMY), as C. minima (NEW COMBINATION); and Diplothelopsis is a junior synonym of Lycinus (NEW SYNONYMY), as L. ornatus and L. bonariensis (NEW COMBINATIONS). Considering the transferred genera and synonymies, Pycnothelidae now includes 15 described genera and 137 species. Finally, these results provide a robust phylogenetic framework that includes enhanced taxonomic sampling, for further resolving the biogeography and evolutionary time scale for the family Pycnothelidae.

Phylogenomic analysis, reclassification, and evolution of South American nemesioid burrowing mygalomorph spiders

The family Nemesiidae was once among the most species-rich of mygalomorph spider families. However, over the past few decades both morphological and molecular studies focusing on mygalomorph phylogeny have recovered the group as paraphyletic. Hence, the systematics of the family Nemesiidae has more recently been controversial, with numerous changes at the family-group level and the recognition of the supra-familial clade Nemesioidina. Indeed, in a recent study by Opatova and collaborators, six nemesiid genera were transferred to the newly re-established family Pycnothelidae. Despite these changes, 12 South American nemesiid genera remained unplaced, and classified as incertae sedis due to shortcomings in taxon sampling. Accordingly, we evaluate the phylogenetic relationships of South American nemesioid species and genera with the principle aim of resolving their family level placement. Our work represents the most exhaustive phylogenomic sampling for South American Nemesiidae by including nine of the 12 genera described for the continent. Phylogenetic relationships were reconstructed using 457 loci obtained using the spider Anchored Hybrid Enrichment probe set. Based on these results Nemesiidae, Pycnothelidae, Microstigmatidae and Cyrtaucheniidae are not considered monophyletic. Our study also indicates that the lineage including the genus Fufius requires elevation to the family level (Rhytidicolidae Simon, 1903 (NEW RANK)). In Pycnothelidae, we recognize/delimit five subfamilies (Diplothelopsinae, Pionothelinae NEW SUBFAMILY, Prorachiinae NEW SUBFAMILY, Pselligminae NEW RANK, Pycnothelinae). We also transfer all the 12 South American nemesiid genera to Pycnothelidae: Chaco, Chilelopsis, Diplothelopsis, Flamencopsis, Hermachura, Longistylus, Lycinus, Neostothis, Prorachias, Psalistopoides, Pselligmus, Rachias. Additionally, we transferred the microstigmatid genus Xenonemesia to Pycnothelidae, and we propose the following generic synonymies and species transfers: Neostothis and Bayana are junior synonyms of Pycnothele (NEW SYNONYMY), as P. gigas and P. labordai, respectively (NEW COMBINATIONS); Hermachura is a junior synonym of Stenoterommata (NEW SYNONYMY), as S. luederwaldti (NEW COMBINATION); Flamencopsis is a junior synonym of Chilelopsis (NEW SYNONYMY), as C. minima (NEW COMBINATION); and Diplothelopsis is a junior synonym of Lycinus (NEW SYNONYMY), as L. ornatus and L. bonariensis (NEW COMBINATIONS). Considering the transferred genera and synonymies, Pycnothelidae now includes 15 described genera and 137 species. Finally, these results provide a robust phylogenetic framework that includes enhanced taxonomic sampling, for further resolving the biogeography and evolutionary time scale for the family Pycnothelidae.

Giant steps: adhesion and locomotion in theraphosid tarantulas.

Theraphosid tarantulas are large spiders that bear dense hairy adhesive pads on the distal parts of their legs: scopula and claw tufts. These structures allow them to climb on vertical smooth surfaces and contribute to prey capture. While adult females and juveniles remain most of the time in their burrows, adult males actively walk searching for females during the reproductive period. Adhesion and locomotion thus play important roles in the ecology and reproduction of these animals. In this paper, we review the current state of the knowledge on adhesion and locomotion in tarantulas, focusing on functional and evolutionary morphology.

The reproductive period of tarantulas is constrained by their thermal preferences (Araneae, Theraphosidae).

Locomotor and physiological performance of ectotherms are affected by temperature. Thermoregulation is achieved by changes in behavior and the selection of micro-habitats with adequate temperatures to maintain the body temperature (Tb) within a range of preference. Apart from this temperature dependence at spatial scales, ectotherms are also affected by temperature at temporal scale. For instance, ectotherms can only be active some months of the year, particularly in temperate environments. Tarantulas are ectotherms that live in burrows most of their life. Nevertheless, after the sexual maturation molt, males leave their refugia and start a wandering life searching for females to mate. The reproductive period varies among species. In some species walking males are seen in late spring or early summer, while in other species males are only seen during fall or winter. Apart from the differences in lifestyles after maturation, tarantulas exhibit sexual dimorphisms in longevity and body mass, having smaller, shorter-lived males. Thus, to optimize energetic budgets, decreasing thermoregulation costs, we hypothesize and examine a putative correlation between an individual's preferred body temperature (Tpref) and the environmental temperature during the reproductive period. Hence, we characterize Tpref in seven tarantula species and analyze which factors (i.e., time of day, body mass, and sex) correlated with it. Furthermore, we assess putative correlated evolution of Tpref with ambient temperature (minima, mean, and maxima) during the reproductive period by means of phylogenetic independent contrasts. We did not find differences in thermal preferences between sexes; and only one species, Acanthoscurria suina, exhibited diel differences in Tpref. We found evidence of correlated evolution between Tpref and minimum temperature during the reproductive period among all seven species studied herein. Our results show that the reproductive period is constrained by thermal preferences, dictating when males can start their wandering life to mate.

Metabolism and water loss are not related to environmental heterogeneity in two mygalomorph spiders.

Regulation of energy and water balance are primary components of homeostasis in all organisms. But the processes associated with such homeostasis can be costly and affect the fitness of individuals. As a result, individuals from variable environments are expected to invest more in compensatory mechanisms to maintain homeostasis than individuals from stable environments. Grammostola quirogai and Grammostola anthracina are two spiders of the Theraphosidae family that live in rocky-hill habitats in the Pampas ecoregion. Both species inhabit mesic environments with moderate rainy weather but different precipitation variability. G. quirogai only exists in rocky-hill habitats surrounded by prairies, far from oceanic and estuarine coasts. These habitats are exposed to higher and more variable precipitation rates due to a stronger influence of El Niño. In contrast, G. anthracina lives in areas with less heterogeneous precipitation rates, and its populations expand up to the Atlantic coast. In this study, we used these two species to explore the impacts of water deprivation on their metabolic rate and water loss. We did not detect changes in metabolic rate or differences in water loss as a result of a water restriction treatment in any of the species. However, the mean total values of evaporative water loss for our studied species were lower than that of xeric species. These results provide evidence that the total evaporative water loss in tarantulas may not be related to environmental characteristics, as it has been widely reported among insect species.

Kinematics of male Eupalaestrus weijenberghi (Araneae, Theraphosidae) locomotion on different substrates and inclines.

BACKGROUND: The mechanics and energetics of spider locomotion have not been deeply investigated, despite their importance in the life of a spider. For example, the reproductive success of males of several species is dependent upon their ability to move from one area to another. The aim of this work was to describe gait patterns and analyze the gait parameters of Eupalaestrus weijenberghi (Araneae, Theraphosidae) in order to investigate the mechanics of their locomotion and the mechanisms by which they conserve energy while traversing different inclinations and surfaces. METHODS: Tarantulas were collected and marked for kinematic analysis. Free displacements, both level and on an incline, were recorded using glass and Teflon as experimental surfaces. Body segments of the experimental animals were measured, weighed, and their center of mass was experimentally determined. Through reconstruction of the trajectories of the body segments, we were able to estimate their internal and external mechanical work and analyze their gait patterns. RESULTS: Spiders mainly employed a walk-trot gait. Significant differences between the first two pairs and the second two pairs were detected. No significant differences were detected regarding the different planes or surfaces with respect to duty factor, time lags, stride frequency, and stride length. However, postural changes were observed on slippery surfaces. The mechanical work required for traversing a level plane was lower than expected. In all conditions, the external work, and within it the vertical work, accounted for almost all of the total mechanical work. The internal work was extremely low and did not rise as the gradient increased. DISCUSSION: Our results support the idea of considering the eight limbs functionally divided into two quadrupeds in series. The anterior was composed of the first two pairs of limbs, which have an explorative and steering purpose and the posterior was more involved in supporting the weight of the body. The mechanical work to move one unit of mass a unit distance is almost constant among the different species tested. However, spiders showed lower values than expected. Minimizing the mechanical work could help to limit metabolic energy expenditure that, in small animals, is relatively very high. However, energy recovery due to inverted pendulum mechanics only accounts for only a small fraction of the energy saved. Adhesive setae present in the tarsal, scopulae, and claw tufts could contribute in different ways during different moments of the step cycle, compensating for part of the energetic cost on gradients which could also help to maintain constant gait parameters.

The first Paratropididae (Araneae, Mygalomorphae) from Colombia: new genus, species and records.

The family of mygalomorph spiders Paratropididae Simon, 1889 is here reported for the first time for Colombia, where it is represented by three genera (Anisaspis, Paratropis, Stormtropis gen. n.) and eight species. One genus, Stormtropis, and six species constitute new taxa that are here diagnosed, described and illustrated. The geographical distribution of Paratropispapilligera FO Pickard-Cambridge, 1896 and Paratropiselicioi Dupérré, 2015 are also redescribed and expanded on the basis of new material examined. The diagnosis of the subfamily Paratropidinae, Paratropis Simon, 1889 and Anisaspis Simon, 1892 are emended including the variations of the new species. Likewise, a geographic distribution map for the entire family and a taxonomic key for the males of Paratropidinae are included. Other biogeographic, morphological, and taxonomic aspects are discussed.

The spider fauna from Uruguay River islands: understanding its role in a biological corridor.

Biological corridors are connections which link habitats in a regional scale, allowing the gene flow between populations. The Uruguay River comprises riverside and insular riparian forests along subtropical to temperate zones passing through different biogeographic provinces. The aim of this study was to characterise the spider fauna from the Uruguay River islands highlighting their connection role for the spider community of riparian forest. Spiders were studied from surveys in a fluvial island of the southern course of the river with nine campaigns being carried out from September 2007 to September 2009. Three complementary collecting methods were used: G-Vac, night hand collecting and pitfall traps. A total of 58 samples were taken in each campaign. A total of 33 families, 145 species/morphospecies and 8 guilds were registered. Theridiidae and space web weavers showed the highest abundance and species richness. Web weavers were predominant in the spider community evidencing the importance of the forest vegetation heterogeneity in spider diversity. Fifteen species have been recorded for the first time for Uruguay. Additional data of previous surveys in the northern islands of the river were analysed and compared. Several species confirm the role of the Uruguay River as a biological corridor from the upper to lower course of the river. The riparian forests from the islands constitute a southernmost intromission of Paranaense biota between Chaco and Pampa regions along the river. The results obtained are an important input for the conservation of these areas. Knowing the biodiversity, as well as its dynamics and the flow of biota that exists in these environments, would allow planning the management from a regional point of view.

Kankuamo, a new theraphosid genus from Colombia (Araneae, Mygalomorphae), with a new type of urticating setae and divergent male genitalia.

A new monotypic Theraphosidae genus, Kankuamo Perafán, Galvis & Pérez-Miles, gen. n., is described from Colombia, with a new type of urticating setae. These setae differ from others principally by having a small distal oval patch of lanceolate reversed barbs. Males of Kankuamo gen. n. additionally differ by having a palpal bulb organ very divergent from all known species, with many conspicuous keels dispersed across the median tegulum to the tip, mostly with serrated edges. Females differ by having spermathecae with a single notched receptacle, with two granulated lobes and several irregular sclerotized longitudinal striations. The new urticating setae, type VII, is characterized, illustrated and its releasing mechanism is discussed. It is hypothesized that these setae are the first in Theraphosinae subfamily whose release mechanism is by direct contact. Kankuamo gen. n. is described and illustrated on the basis of the type species Kankuamo marquezi Perafán, Galvis & Gutiérrez, sp. n., and their remarkable characteristics, morphological affinities and cladistic relationship are analyzed.

Tarantulas (Araneae: Theraphosidae) use different adhesive pads complementarily during climbing on smooth surfaces: experimental approach in eight arboreal and burrower species.

Tarantulas are large spiders with adhesive setae on their legs, which enable them to climb on smooth vertical surfaces. The mechanism proposed to explain adhesion in tarantulas is anisotropic friction, where friction is higher when the leg pushes than when it pulls. However, previous studies and measurements of adhesion in theraphosids were performed using dead specimens. To test their ability to climb, we studied static friction of live theraphosid spiders on different surfaces and at different inclines. We compared burrower with arboreal species to test the hypothesis of higher friction in arboreal tarantulas. We found a complementary participation of claw tufts and scopula of anterior and posterior legs when the tarantula climbs. The mechanics of climbing in association with the biological characteristics of the species are discussed.

Bumba, a replacement name for Maraca Pérez-Miles, 2005 and Bumbalennoni, a new tarantula species from western Amazonia (Araneae, Theraphosidae, Theraphosinae).

We propose the name Bumba as a new name for Maraca, preoccupied by Maraca Hebard, 1926 (Orthoptera). We describe and illustrate Bumbalennoni, a new theraphosid species from Caxiuanã, Pará, Brazil. This species differs from the other species of the genus in the extremely reduced keel on male palpal organ and in the higher number of labial and maxillary cuspules. Females additionally differ in the spermathecal morphology. As a consequence of the name replacement, three new combinations are established.

Three new species of Melloleitaoina Gerschman & Schiapelli, 1960 (Araneae, Mygalomorphae, Theraphosidae) from northern Argentina.

Three new species of the monotypic genus Melloleitaoina Gerschman & Schiapelli, 1960 are described from northern Argentina: M. mutquina sp. n., M. uru sp. n. and M. yupanqui sp. n. The female specimen originally described as M. crassifemur is not conspecific with the male holotype and thus is removed from this species and described as M. uru sp. n.; M. crassifemur is redescribed. All species are diagnosed, illustrated and a key to species is provided.

Two new species of Chaco Tullgren from the Atlantic coast of Uruguay (Araneae, Mygalomorphae, Nemesiidae).

We describe two new species of the nemesiid spider genus Chaco from Rocha Province, Uruguay. These new species are diagnosed based on genital morphology, male tibial apophysis spination, and burrow entrance. We test cospecificity of one species, Chaco costai,via laboratory mating experiments. The new species are diagnosed and illustrated and habitat characteristics, and capture behavior are described. We conduct a cladistic analysis based on a previously published morphological character matrix that now includes the newly described species.

ResumenEn este trabajo se describen dos nuevas especies de arañas, nemésidas del género Chaco recolectadas en el departamento de Rocha, Uruguay. Las nuevas especies se distinguen de las conocidas por caracteres tomados de la morfología de la genitalia, número de espinas en la apófisis tibial de los machos y la tapa-cortina en la cueva. Se realizó un análisis filogenético de las especies del género Chaco incluyendo las nuevas especies en una matriz previa.

Mygalomorph spider community of a natural reserve in a hilly system in central Argentina.

The diversity, abundance, spatial distribution, and phenology of the mygalomorph spider community in the "Ernesto Tornquist" Strict Nature Reserve were analyzed in this study. Located in southwestern Buenos Aires, Argentina, the Reserve is representative of the Ventania system, which is a sigmoidal mountain belt 180 km in length. This exceptional hilly ecosystem is home for many endemic species and rich native fauna and flora. Spider abundance was sampled monthly from October 2009 to October 2010 by hand capture and pitfall traps on grassland slopes. The species recorded in the study area were: Actinopus sp.1 (Actinopodidae); Grammostola vachoni and Plesiopelma longisternale (Theraphosidae); Acanthogonatus centralis (Nemesiidae); and Mecicobothrium thorelli (Mecicobothriidae). Grammostola vachoni and Acanthogonatus centralis were the dominant species in hand capture and pitfall traps, respectively. The seasonal variation, diversity, and abundance of the mygalomorph community are analyzed and discussed here. The Mygalomorphae of the Ventania system comprises an important group of sedentary and cryptozoic spiders that seem to be highly dependent on habitat type and environmental factors.

Tarantulas do not shoot silk from their legs: experimental evidence in four species of New World tarantulas.

Theraphosid tarantulas, like all other spiders, secrete silk from spigots on the abdominal spinnerets. A few years ago, it was proposed that the large tarantula Aphonopelma seemanni could extrude silk from specialized spigots on the tarsi to help adhesion to vertical surfaces. This suggestion was later questioned because silk was not observed after the spinnerets had been sealed. Recently, experiments with the tarantula Grammostola rosea again suggested tarsal silk secretion. All observations of the supposed tarsal silk were made in spiders with functional spinnerets, thus contamination with silk coming from the spinnerets could not be excluded. Recent morphological arguments also questioned putative tarsal spigots and proposed that they are actually contact chemoreceptors. We here test the supposed tarsal silk secretion in Aphonopelma seemanni, Avicularia avicularia, Brachypelma vagans and Grammostola mollicoma using similar experimental conditions as the previous authors, but with sealed spinnerets. Our results clearly demonstrate that when spinnerets are sealed, tarantulas do not show any tarsal silk secretion. We reinterpret those putative tarsal spigots and discuss possible evolutionary implications of these findings.

Intraspecific non-sexual interactions of Grammostola schulzei (Araneae: Theraphosidae) under laboratory conditions.

Intraspecific interactions of araneomorph spiders have received considerable attention, but there are few detailed studies on intraspecific interactions of mygalomorph spiders. Moreover, a thorough understanding of theraphosid biology and ecology is necessary from a conservation standpoint because natural populations may be threatened by habitat disturbances and captures for pet commerce. We described the behavior of conspecific individuals of Grammostola schulzei during non-sexual interactions, under laboratory conditions. Pairs of individuals involving adult males, adult females and juveniles were confronted and observed in resident and intruder conditions, totalizing 115 trials. When confronted two adult females, they retreated or grappled, and performed gaping display with bite attempts, usually resulted in severe injury of the intruder spiders. When confronted females with large juveniles, we frequently observed cannibalism on juveniles. Juveniles exposed to females or to other juveniles retreated or made leg tapping with forelegs and palpal drumming, which are common displays of courting adult males. Adult males courted and clasped some juveniles, but juveniles avoided or reject clasping. The behaviors observed during intraspecific interactions could play an important role determining spatial distribution and could lead to behavioral adaptations of territoriality.

Intraspecific non-sexual interactions of Grammostola schulzei (Araneae: Theraphosidae) under laboratory conditions

Intraspecific interactions of araneomorph spiders have received considerable attention, but there are few detailed studies on intraspecific interactions of mygalomorph spiders. Moreover, a thorough understanding of theraphosid biology and ecology is necessary from a conservation standpoint because natural populations may be threatened by habitat disturbances and captures for pet commerce. We described the behavior of conspecific individuals of Grammostola schulzei during non-sexual interactions, under laboratory conditions. Pairs of individuals involving adult males, adult females and juveniles were confronted and observed in resident and intruder conditions, totalizing 115 trials. When confronted two adult females, they retreated or grappled, and performed gaping display with bite attempts, usually resulted in severe injury of the intruder spiders. When confronted females with large juveniles, we frequently observed cannibalism on juveniles. Juveniles exposed to females or to other juveniles retreated or made leg tapping with forelegs and palpal drumming, which are common displays of courting adult males. Adult males courted and clasped some juveniles, but juveniles avoided or reject clasping. The behaviors observed during intraspecific interactions could play an important role determining spatial distribution and could lead to behavioral adaptations of territoriality. Rev. Biol. Trop. 59 (3): 1173-1182. Epub 2011 September 01.

Hay pocos estudios detallados sobre las interacciones intraespecíficas de arañas migalomorfas. Por lo tanto, se describe el comportamiento de individuos conspecíficos de Grammostola schulzei durante interacciones nosexuales en condiciones de laboratorio. Se confrontaron y observaron pares de individuos involucrando machos adultos, hembras adultas y juveniles en condiciones de locatarios y visitantes, totalizando 115 encuentros. Cuando dos hembras adultas se enfrentaron, retrocedieron o lucharon adoptando elevaciones anteriores e intentos de mordeduras que usualmente resultaron en arañas visitantes heridas. Cuando se enfrentaron hembras con juveniles, frecuentemente se observó canibalismo sobre los juveniles. Los juveniles expuestos a otros juveniles o hembras retrocedieron o realizaron golpes con patas delanteras y tamborileos de palpos, los cuales constituyen unidades de comportamiento comunes durante el cortejo de los machos. Los machos adultos cortejaron y trabaron a algunos juveniles, mientras que los juveniles los evitaron o rechazaron el enganche. Los comportamientos observados durante las interacciones intraespecíficas podrían jugar un papel importante en la distribución espacial y podrían generar adaptaciones al territorialismo.

On the Avicularia (Araneae: Theraphosidae: Aviculariinae) species from Uruguay

The taxonomic status of four species of Avicularia Lamarck 1818 described from Uruguay: Aviculariaanthracina (C.L. Koch 1842), Avicularia alticeps (Keyserling 1878), Avicularia parva (Keyserling 1878) and Aviculariatigrina (Pocock 1903) is discussed. The holotypes and/or original descriptions of these species were examined, and twotaxonomic synonymies are needed, which are presented herein. Avicularia anthracina is transferred to Grammostola, resulting in Grammostola anthracina (C.L. Koch 1842) new combination and is considered a senior synonym of Grammostola mollicoma Ausserer 1875 new synonymy. Likewise, Avicularia parva is transferred to Catumiri Guadanucci 2004, where it is placed in the synonymy of Catumiri uruguayense Guadanucci 2004 new synonymy. Avicularia tigrina andAvicularia alticeps, originally described in the genera Ischnocolus Ausserer 1875 and Pterinopelma Pocock 1901,respectively, are herein considered nomina dubia since their types are presumed lost.

Silk production from tarantula feet questioned.

As with all spiders, tarantulas spin silk from specialized structures in the abdomen called spinnerets, which are key features unique to the group. Recently Gorb et al. reported that the zebra tarantula Aphonopelma seemanni also secretes silk from its feet, which might improve its ability to climb on vertical surfaces. Here we show that when the spinnerets are experimentally sealed, the zebra tarantula cannot secrete silk or similar threads, disagreeing with previous reports by Gorb et al.. Additional evidence also disagrees with leg secretion of silk.

Mating system in the tarantula spider Eupalaestrus weijenberghi (Thorell, 1894): evidences of monandry and polygyny.

The road tarantula Eupalaestrus weijenberghi shows a strongly female-biased sex ratio since adult females live several years while adult males live only for 2 months. In this scenario selective males could be expected. However, several factors such as the rates of reproduction of each sex, degree of sexual selectivity and synchronicity of female receptiveness determine the operational sexual ratio and mating system of the species. Our objective was to determine the mating rates and mating tactics for females and males of E. weijenberghi and their variation throughout the reproductive period. Four hundred sexual encounters among 20 females and 20 males in all possible pair-wise combinations were carried out during 29 days, a brief but intense experimental period, as it also occurs in the field. Mating success differed strongly between sexes. Females mated once: five females mated at the first attempt, eight initially rejected males and copulated in subsequent attempts. Half of the males did not copulate and the others copulated 1-3 times. Mated females actively rejected males. Results indicate a mating system with monogamous females and polygamous males. Not all the females were receptive in every reproductive season. We suggest that female monogamy drastically affects the operational sex ratio, since several females were unreceptive after a single copulation, directly diminishing the male potential reproductive rate. This is the first experimental approach to estimate tarantula mating systems, their determinants and the consequences of the strategies shown by each sex.