Copulatory mechanism in a sexually cannibalistic spider with genital mutilation (Araneae: Araneidae: Argiope bruennichi)

Genitalia are among the fastest evolving morphological traits as evidenced by their common function as diagnostic traits in species identification. Even though the main function of genitalia is the successful transfer of spermatozoa, the presence of diverse structures that are obviously not necessary for this suggests that genitalia are a target of sexual selection. The male genitalia of many spider species are extremely complex and equipped with numerous sclerites, plates and spines whose functions are largely unknown. Selection on male genitalia may be particularly strong in sexually cannibalistic spiders, where mating success of males is restricted to a single female. We investigated the copulatory mechanism of the sexually cannibalistic orb weaving spider Argiope bruennichi by shock freezing mating pairs and revealed a complicated interaction between the appendices and sclerites that make up the male gonopods (paired pedipalps). The plate that covers the female genital opening (scape) is secured between two appendices of the male genital bulb, while three sclerites that bear the sperm duct are unfolded and extended into the female copulatory opening. During copulation, females attack and cannibalise the male and males mutilate their genitalia in about 80% of cases. Our study demonstrates that (i) genital coupling is largely accomplished on the external part of the female genitalia, (ii) that the mechanism requires an interaction between several non-sperm-transferring structures and (iii) that there are two predetermined breaking points in the male genitalia. Further comparative work on the genus Argiope will test if the copulatory mechanism with genital mutilation indeed is an adaptation to sexual cannibalism or if cannibalism is a female counter adaptation to male monopolisation through genital plugging.     

Copulation patterns in the golden orb-web spider Nephila madagascariensis

A consequence of multiple mating by females can be that the sperm of two or more males directly compete for the fertilisation of ova inside the female reproductive tract. Selection through sperm-competition favours males that protect their sperm against that of rivals and strategically allocate their sperm, e.g., according to the mating status of the female and the morphology of the spermatheca. In the majority of spiders, we encounter the otherwise unusual situation that females possess two independent insemination ducts, both ending in their own sperm storage organ, the spermatheca. Males have paired mating organs, but generally can only fill one spermatheca at a time. We investigated whether males of the African golden orb-web spider Nephila madagascariensis can prevent rival males from mating into the same spermatheca and whether the mating status of the female and/or the spermatheca causes differences in male mating behaviour. There was no significant difference in the duration of copulations into unused spermathecae of virgin and mated females. We found that copulations into previously inseminated spermathecae were generally possible, but shorter than copulations into the unused side of mated females or with virgins. Thus, male N. madagascariensis may have an advantage when they mate with virgins, but cannot prevent future males from mating. However, in rare instances, parts of the male genitals can completely obstruct a female genital opening.     

Genital structures in the entelegyne widow spider Latrodectus revivensis (Arachnida; Araneae; Theridiidae) indicate a low ability for cryptic female choice by sperm manipulation

The female genital structures of the entelegyne spider Latrodectus revivensis are described using semithin sections and scanning electron microscopy. Apart from the tactile hairs overhanging the opening of the atrium, the contact zones of the female epigynum are devoid of any sensilla, indicating that the female does not discriminate in favor or against males due to their genital size or stimulation through copulatory courtship. The dumb-bell shape and the spatial separation of the entrance and the exit of the paired spermathecae suggest that they are functionally of the conduit type. Not described for other entelegyne spiders so far, the small fertilization ducts originating from the spermathecae of each side lead to a common fertilization duct that connects the spermathecae to the uterus externus. During oviposition, it is most likely that spermatozoa are indiscriminately sucked out of the spermathecal lumina by the low pressure produced by the contraction of the muscle extending from the epigynal plate to the common fertilization duct. As no greater amounts of secretion are produced by the female during oviposition, and no activated sperm are present within the female genital tract, the secretion produced by the spermathecal epithelium does not serve in displacement or (selective) activation of spermatozoa. These findings suggest that female L. revivensis are not able to exert cryptic female choice by selectively choosing spermatozoa of certain males.     

Northward geographic diversification of a kleptoparasitic spider Argyrodes lanyuensis (Araneae,Theridiidae) from the Philippine Archipelago to Orchid Island

Oceanic islands are unique geographic systems that promote local adaptations and allopatric speciation in many of their highly endemic taxa. This is a common case in the Philippine Archipelago, where numerous unrelated taxa on islands have been inferred to have diversified in isolation. However, few cases have been reported in invertebrates especially among parasitic organisms. Here, we tested for biogeographical structure in novel populations of the “generalist" kleptoparasitic spider, Argyrodes lanyuensis Yoshida, Tso & Severinghaus, 1998 in the Philippines. Results showed that, in addition to Orchid/Lanyu Island, this species has a wide geographic distribution in the Philippine Archipelago. The estimated divergence time of this lineage using the mitochondrial cytochrome oxidase 1 (mt‐CO1) suggests that this species diverged ca 3.12 MYA, during the Pliocene. Two reciprocal monophyletic clades were elucidated in A. lanyuensis, but with limited differentiation across Pleistocene Aggregate Island Complex (PAIC) boundaries and modern‐day islands. However, in our analyses of morphological variation, we identified two phenotypically differentiated units in males (Orchid Island, Taiwan + Luzon, Philippine PAIC populations vs. Palawan + West Visayan + Mindanao PAIC populations). We infer that this species diverged in the southern portion of the Philippine Archipelago and only recently colonized Orchid Island. Our study provides new information on the extensive distribution of A. lanyuensis outside Orchid Island, Taiwan, but we documented a very limited geographically associated genetic variation. Our study points to behavioral phenomena such as foraging behavior as essential contributor to the evolutionary process of species diversification, in contrast to the traditionally invoked geographic drivers of divergence.     

Stabilizing selection maintains exuberant colour polymorphism in the spider Theridion californicum (Araneae, Theridiidae)

Genetically controlled colour polymorphisms provide a physical manifestation of the operation of selection and how this can vary according to the spatial or temporal arrangement of phenotypes, or their frequency in a population. Here, we examine the role of selection in shaping the exuberant colour polymorphism exhibited by the spider Theridion californicum. This species is part of a system in which several distantly related spiders in the same lineage, but living in very different geographical areas, exhibit remarkably convergent polymorphisms. These polymorphisms are characterized by allelic inheritance and the presence of a single common cryptic morph and, in the case of T. californicum and its congener the Hawaiian happy-face spider Theridion grallator, numerous rare patterned morphs. We compare population differentiation estimated from colour phenotypic data to differentiation at neutral amplified fragment length polymorphisms (AFLP) loci and demonstrate that the colour polymorphism appears to be maintained by balancing selection. We also examine the patterns of selection in the genome-wide sample of AFLP loci and compare approaches to detecting signatures of selection in this context. Our results have important implications regarding balancing selection, suggesting that selective agents can act in a similar manner across disparate taxa in globally disjunct locales resulting in parallel evolution of exuberant polymorphism.     

Complex genital structures indicate cryptic female choice in a haplogyne spider (Arachnida,Araneae, Oonopidae,Gamasomorphinae)

Female genital structures with their allied muscles of the haplogyne spider Opopaea fosuma are described. A functional explanation of this system is given, which indicates that cryptic female choice may occur in these spiders: the anterior wall of their spermatheca is strongly sclerotized and possesses a cone-shaped hole in its upper part. A transverse sclerite that serves as muscle attachment bears a nail-like structure and lies in a chitinized area of the anterior wall of the uterus externus. Muscle contraction presses this nail into the hole of the spermatheca. In this way, the uterus externus gets both locked and fixed. Furthermore, as this occurs the copulatory orifice is enlarged and the resulting suction probably leads to previously deposited sperm being drawn from the spermatheca and dumped. This is a common mechanism used by females to influence a male's chances of fathering their offspring in a process known as cryptic female choice.     

Microhabitat selection and audible sexual signalling in the wolf spider Hygrolycosa rubrofasciata (Araneae, Lycosidae)

We studied the microhabitat selection and male sexual signalling behaviour in the wolf spider Hygrolycosa rubrofasciata (Ohlert). Males strike dry leaves with their abdomen, producing an audible sexual drumming signal, and females use this signal to choose mating partners. In the field we followed male drumming rate and microhabitat selection using both the mark–recapture method and direct observations. In the laboratory we conducted an experiment on male microhabitat and drumming substrate selection. We found that in the field males were not distributed randomly among the habitat; fewer males were found in areas that had high sedge cover, low elevation, and low dry leaf cover. In the laboratory experiment, males spent more time on dry leaf substrate. Drumming rate in the field was positively correlated with dry leaf cover and in the laboratory males clearly preferred dry leaves as drumming substrate. Temperature was positively correlated with male drumming rate, and with male and female mobility. We conclude that in H. rubrofasciata male distribution and sexual signalling rate, and thus mating success, are greatly affected by environmental factors. Therefore, males may be sexually selected to make effective use of their signalling habitat by active microhabitat choice. Received: 27 December 1999 / Received in revised form: 14 February 2000 / Accepted: 14 February 2000     

Size matters: genital allometry in an African mole-rat (Family: Bathyergidae)

Typically, sexually selected traits show positive allometry and high coefficients of variation (CV). To date, many studies on the allometry of genitalia have focused on insects. In addition, studies have largely ignored the potential for sexual selection on female genitalia, despite male and female structures presumably co-evolving. Insects tend to show negative allometry in both male and female genitalia, while in contrast, the few studies carried out in mammals (males only) show positive allometry. Reasons for these differences between the taxa still remain unclear. However, in mammals, three main mechanisms have been proposed for genital evolution, namely, sperm competition, female cryptic choice and sexual conflict. In the first such study that we are aware of, we examined intra-specific genital allometry in both males and females of a mammal, the subterranean solitary Cape dune mole-rat, Bathyergus suillus. We found positive allometry occurring in male genitalia, which is consistent with previous vertebrate studies. Similarly, we found that female genitalia also exhibited positive allometry further supporting the notion of co-evolution of male and female genitalia. Although it is difficult to distinguish between the forces or mechanisms determining this directional selection, we suggest that several reproductive advantages are incurred as a result of positive allometric relationship of the genitalia in B. suillus and such advantages are also likely in other subterranean mammals. Our study further highlights the differences in genital allometry across taxa.     

中国银斑蛛属二新种(蜘蛛目:球蛛科)

记述了球蛛科银斑蛛属2新种:黄桑银斑蛛Argyrodes huangsangensis sp.nov. 和菱腹银斑蛛Argyrodes rhomboides sp.nov..并对环银斑蛛Argyrodes orbitus Zhu, 1988的雌蛛进行了补充描述.     

Evolution of deceit by worthless donations in a nuptial gift-giving spider

Males of the nursery web spider Pisaura mirabil usually offer an insect prey wrapped in white silk as a nuptial gift to facilitate copulation. Males exploit female foraging preferences in a sexual context as females feed on the gift during copula- tion. It is possible for males to copulate without a gift, however strong female preference for the gift leads to dramatically higher mating success for gift-giving males. Females are polyandrous, and gift-giving males achieve higher mating success, longer copulations, and increased sperm transfer that confer advantages in sperm competition. Intriguingly, field studies show that ap- proximately one third of males carry a worthless gift consisting of dry and empty insect exoskeletons or plant fragments wrapped in white silk. Silk wrapping disguises gift content and females are able to disclose gift content only after accepting and feeding on the gift, meanwhile males succeed in transferring sperm. The evolution of deceit by worthless gift donation may be favoured by strong intra-sexual competition and costs of gift-construction including prey capture, lost foraging opportunities and investment in silk wrapping. Females that receive empty worthless gifts terminate copulation sooner, which reduces sperm transfer and likely disadvantages males in sperm competition. The gift-giving trait may thus become a target of sexually antagonistic co-evolution, where deceit by worthless gifts leads to female resistance to the trait. We discuss factors such as female mating rate and intensity of sperm competition that may shape the evolution of male deception, and how ecological factors may influence the evolution and maintenance of worthless gifts as an evolutionarily stable alternative mating strategy by frequency dependent selection     

中国肥腹蛛属一新种记述(蜘蛛目:球蛛科)

2008～2010年在宁夏六盘山国家级自然保护区进行无脊椎动物科学考察,采集到蜘蛛目球蛛科Theridi-idae蜘蛛标本3370余头,经鉴定共有13属。发现肥腹蛛属Steatoda一新种,命名为彭阳肥腹蛛Steatoda pengyangensissp.nov.。模式标本保存于河北大学博物馆。     

Benefits of size dimorphism and copulatory silk wrapping in the sexually cannibalistic nursery web spider,Pisaurina mira

In sexually cannibalistic animals, male fitness is influenced not only by successful mate acquisition and egg fertilization, but also by avoiding being eaten. In the cannibalistic nursery web spider, Pisaurina mira, the legs of mature males are longer in relation to their body size than those of females, and males use these legs to aid in wrapping a female''s legs with silk prior to and during copulation. We hypothesized that elongated male legs and silk wrapping provide benefits to males, in part through a reduced likelihood of sexual cannibalism. To test this, we paired females of random size with males from one of two treatment groups—those capable of silk wrapping versus those incapable of silk wrapping. We found that males with relatively longer legs and larger body size were more likely to mate and were less likely to be cannibalized prior to copulation. Regardless of relative size, males capable of silk wrapping were less likely to be cannibalized during or following copulation and had more opportunities for sperm transfer (i.e. pedipalpal insertions). Our results suggest that male size and copulatory silk wrapping are sexually selected traits benefiting male reproductive success.     

Host-dependent differences in prey acquisition between populations of a kleptoparasitic spider Argyrodes kumadai (Araneae: Theridiidae)

Abstract.  1. A kleptoparasitic spider, Argyrodes kumadai , is known to use phylogenetically unrelated host species in different regions – Cyrtophora moluccensis (Araneidae) in south-west Japan and Agelena silvatica (Agelenidae) in north-east Japan. The work reported here examined whether differences in host characters affect prey acquisition of A. kumadai .
2. Field surveys showed that prey-biomass capture rate of Argyrodes was significantly higher in populations parasitising Cyrtophora than in populations parasitising Agelena . Although Argyrodes appeared to catch fewer prey within Cyrtophora webs, they were able to feed upon substantially larger prey.
3. Differences in prey-biomass capture rate were found to reflect differences in host traits rather than regional differences in potential prey availability. Individuals in populations parasitising Cyrtophora were observed to acquire prey via a number of foraging tactics that included stealing wrapped food bundles, feeding upon prey remains and, in the case of large prey items, feeding together with the host. In contrast, individuals in populations parasitising Agelena were only ever observed to feed upon small prey items ignored by its host.
4. This variability in prey acquisition between kleptoparasite populations reflected different opportunities for feeding within their respective host webs – opportunities that were primarily determined by the foraging behaviour of the host. One key trait associated with host foraging behaviour was host-web structure, namely the presence/absence of a retreat.     

Habitat preferences of the urban wall spider Oecobius navus (Araneae, Oecobiidae)

Abstract  To assess the relative impact of a range of habitat variables on spider abundance, field and laboratory experiments were conducted on populations of the urban wall spider Oecobius navus in suburban Perth, Western Australia. Habitat characteristics investigated were: substrate type, wind speed, rainfall, sunlight exposure, relative humidity, air temperature, substrate temperature, artificial lighting and prey type/abundance. In the field, O. navus was found to be associated with high humidity, low air temperature and shelter from sunlight and rainfall. Oecobius navus was more abundant at sites with greater prey abundance. The most common prey item was the red meat ant Iridomyrmex chasei . Juvenile spiders were more abundant than adult spiders; however, patterns between spider abundance and habitat variables were similar for both adults and juveniles. Laboratory experiments showed that O. navus preferred to build webs on wooden substrates, and pitted limestone walls. These findings indicate that O. navus may be vulnerable to desiccation and/or thermal stress, and thus survives better on sheltered walls.     

Securing paternity: mating plugs in the dwarf spider Oedothorax retusus (Araneae: Erigoninae)

One of the various male strategies to prevent or impede female remating is the production of a mating plug that covers the female genital opening or remains inside of the female genital tract after mating. Such structures have been described for many species in many animal taxa; however, in most cases, we know little or nothing about their specific adaptive value. Our investigations demonstrate that females of the dwarf spider species Oedothorax retusus (Westring, 1851) (Linyphiidae, Erigoninae) exhibit a substance on one or both of her paired genital openings only after copulation. We performed double-mating trials and forced the second male to mate into the previously used or unused spermathecal duct of the female by amputating one of his paired male gonopods (pedipalps). Furthermore, to investigate whether the duration of the first mating has an effect on the size and efficiency of the mating plug, we interrupted first matings after either 1 or 3 min, categorized plug size and recorded mating behaviour of subsequent males. The amount of secretion transferred was larger in long compared to short copulations. A long first copulation successfully prevented subsequent males from mating into the used ducts, whereas mating success after short first matings was similar to matings into unused copulatory ducts of the females. The present study demonstrates that a male O. retusus can prevent a rival from transferring sperm into the same spermatheca by applying a mating plug, but only if he mates for long enough.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 574–583.     

Colonization and diversification of the spider genus Pholcus Walckenaer, 1805 (Araneae, Pholcidae) in the Macaronesian archipelagos: evidence for long-term occupancy yet rapid recent speciation

Macaronesian archipelagos stand apart from other oceanic islands reputed as laboratories for the study of evolution by their proximity to the mainland, lack of subsidence, and steep ecological gradients. The genus Pholcus Walckenaer, 1805, commonly known as daddy-long-leg spiders, is one of the most speciose arthropod groups in the region, with 25 endemic species. In the present study, we use information from four mitochondrial genes, along with morphological data, to examine the phylogenetic relationships and diversification patterns of the genus in the region. Phylogenetic analyses support monophyly of Macaronesian Pholcus including the Moroccan species Pholcus vachoni and hence a single colonization event in the archipelagos. Madeira colonizers most likely originated from the Canaries, and a back-colonization of the nearby mainland receives further support. Estimated lineage divergence times suggest a long-time presence of Pholcus in the region, but also reveal that most present-day species are the result of recent, and probably rapid, speciation events. Diagnostic characters among Macaronesian Pholcus are confined to structures involved in copulation. Coupled with the extremely high diversification rate, the highest recorded for spiders, these copulatory characters suggest that sexual selection has played a key role in the local diversification of Pholcus in Macaronesia.     

Natural history of the non-solitary sheetweaving spider Anelosimus jucundus (Araneae: Theridiidae)

Field observations were made on one Panamanian population of Anelosimus jucundus. Almost all webs were found in a relatively exposed area, on small shrubs and composites. This is in contrast to A. eximius , a colonial congeneric which in our study site built its large webs along moist, fern-covered banks. Web structure varied with different plant substrates; for example, only some of the webs incorporated a clearly defined lower sheet. Webs were built by penultimate instar females, and perhaps by adult females and males. In some cases, several adult females inhabited a web, but they remained under separate leaf canopies; during census observations co-operative efforts were not noted. Prey consisted mainly of winged insects. Juvenile sex ratio was about equal. The degree of sociality demonstrated by A. jucundus is discussed.     

State-dependent prey type preferences of a kleptoparasitic spider Argyrodes flavescens (Araneae: Theridiidae)

Spiders from the theridiid genus Argyrodes exhibit considerable variation in foraging tactics. However, little is known about the conditions under which Argyrodes spiders switch foraging tactics. Argyrodes flavescens (Pickard-Cambridge) is commonly found in the webs of another spider Nephila pilipes (Fabricius) in Singapore. In this study, a series of prey-choice tests were conducted for A. flavescens , both in the presence and absence of N. pilipes , to investigate the state-dependent prey type preference of A. flavescens . It was found that, in the absence of N. pilipes , well-fed A. flavescens took houseflies more than fruit flies, but starved A. flavescens took more fruit flies than houseflies. Whether N. pilipes spiders were present or absent, both well-fed and starved A. flavescens preferred living prey and rarely took wrapped prey of any kind. When well fed, A. flavescens rarely took mealworms. However, when starved, A. flavescens tended to take freshly captured prey, and also tended to feed together with N. pilipes on a housefly or mealworm captured by N. pilipes . Whether A. flavescens were absent or present, both well-fed and starved N. pilipes took mealworm larvae more often than they took houseflies, and they never attacked fruit flies. This is the first study to show that Argyrodes spiders alter their foraging tactics depending on hunger level, prey type, or the presence of the host. In doing so, Argyrodes spiders may maximize their energy gain and minimize predation risk in different circumstances.