Neurons and a sensory organ in the pedipalps of male spiders reveal that it is not a numb structure

A biosemiotic interpretation of certain genital morphological structures in the spiders Dysdera erythrina and Dysdera crocata (Araneae: Dysderidae)

A biosemiotic approach to the interpretation of morphological data is apt to highlight morphological traits that have hitherto gone unnoticed for their crucial roles in intraspecific sign interpretation and communication processes. Examples of such traits include specific genital structures found in the haplogyne spiders Dysdera erythrina (Walckenaer 1802) and Dysdera crocata (Koch 1838). In both D. erythrina and D. crocata, the distal sclerite of the male bulb and the anterior diverticulum of the female endogyne exhibit a striking, previously unreported correspondence in size and shape, allowing for a precise match between these structures during copulation. In D. erythrina, the sclerite at the tip of the bulb and the anterior diverticulum are semi-circular in shape, whereas in D. crocata they are rectangular. From the perspective of biosemiotics, which studies the production and interpretation of signs and codes in living systems, these structures are considered the morphological zones of an intraspecific sign interpretation process. This process constitutes one of the necessary prerequisites for sperm transfer and the achievement of fertilization. Therefore, these morphological elements deserve particular attention as they hold higher taxonomic value compared to morphological traits of the bulb for which a relevant role in mating and fertilization has not been proven. Thus, an approach to species delimitation based on biosemiotics, with its specific evaluation of morphological structures, provides new insights for the multidisciplinary endeavour of modern integrative taxonomy.

Copulatory behaviour and genital mechanics suggest sperm allocation by a non-intromittent sclerite in a pholcid spider

The male genitalia of pholcid spiders, which is one of the most species-rich spider families, are characterized by a procursus, which is a morphologically diverse projection of the copulatory organ. It has been shown that the procursus interacts with the female genitalia during copulation. Here, we investigate the function of the procursus in Gertschiola neuquena, a species belonging to the early branched and understudied subfamily Ninetinae, using behavioural and morphological data. Although many aspects of the copulatory behaviour of G. neuquena follow the general pattern described for the family, males use only one pedipalp during each copulation. Based on our micro-CT analysis of cryofixed mating pairs using virgin females, we can show that the long and filiform procursus is inserted deeply into the unpaired convoluted female spermatheca, and the intromittent sclerite, the embolus, is rather short and stout only reaching the most distal part of the female sperm storage organ. Histological data revealed that sperm are present in the most proximal part of the spermatheca, suggesting that the procursus is used to allocate sperm deeply into the female sperm storage organ. This represents the first case of a replacement of the sperm allocation function of the intromittent sclerite in spiders.

Male mating strategies to counter sexual conflict in spiders

When sexual conflict selects for reproductive strategies that only benefit one of the sexes, evolutionary arms races may ensue. Female sexual cannibalism is an extreme manifestation of sexual conflict. Here we test two male mating strategies aiming at countering sexual cannibalism in spiders. The “better charged palp” hypothesis predicts male selected use of the paired sexual organ (palp) containing more sperm for their first copulation. The “fast sperm transfer” hypothesis predicts accelerated insemination when cannibalism is high. Our comparative tests on five orbweb spider species with varying levels of female sexual cannibalism and sexual size dimorphism (SSD) reveal that males choose the palp with more sperm for the first copulation with cannibalistic females and that males transfer significantly more sperm if females are cannibalistic or when SSD is biased. By supporting the two hypotheses, these results provide credibility for male mating syndrome. They, however, open new questions, namely, how does a male differentiate sperm quantities between his palps? How does he perform palp choice after assessing his cannibalistic partner? By conducting follow-up experiments on Nephilengys malabarensis, we reveal that it is sperm volume detection, rather than left-right palp dominance, that plays prominently in male palp choice.

Male spiders make choices about how to maximise their mating success when at risk of cannibalisation by females.

Limitations of sperm transfer in the complex reproductive system of spiders

In spiders, sperm transfer from the male to the female is indirect via secondary copulatory structures, the pedipalps. At the time of transfer the sperm are not mobile and the ejaculate needs to move through narrow male and female ducts to the female sperm storage organ. In addition, copulation duration can be very short, often limited to just a few seconds. Finally, sexual cannibalism and genital damage limits male life-time mating opportunities. These features of the reproductive biology in spiders are likely to result in sperm transfer constraints. Here we review the intrinsic and extrinsic sperm transfer limitations and conduct a meta-analysis on sperm transfer data from published data. Most of the information available relates to orb-web spiders, but our meta-analysis also includes non-orb-web spiders. Our review identifies some of the behavioural factors that have been shown to influence sperm transfer, and lists several morphological and physiological traits where we do not yet know how they might affect sperm transfer.

Diversification through gustatory courtship: an X-ray micro-computed tomography study on dwarf spiders

BACKGROUND

Sexual selection has been considered to promote diversification and speciation. Sexually dimorphic species have been used to explore the supposed effect, however, with mixed results. In dwarf spiders (Erigoninae), many species are sexually dimorphic-males possess marked prosomal modifications. These male traits vary from moderate elevations to bizarre shapes in various prosomal regions. Previous studies established that male dwarf spiders produce substances in these prosomal modifications that are taken up by the females. These substances can act as nuptial gifts, which increase the mating probability of males and the oviposition rate in females. Therefore, these dimorphic traits have evolved in the context of sexual selection. Here, we explore the evolutionary lability of this gustatory trait complex with the aim of assessing the role of this trait complex in species divergence by investigating (1) if erigonine modified prosomata are inherently linked to nuptial-gift-producing glands, (2) if the evolution of the glands evolution preceded that of the modified prosomal shapes, and by assessing (3) the occurrence of convergent/divergent evolution and cryptic differentiation.

RESULTS

We reconstructed the position and extent of the glandular tissue along with the muscular anatomy in the anterior part of the prosoma of 76 erigonine spiders and three outgroup species using X-ray micro-computed tomography. In all but one case, modified prosomata are associated with gustatory glands. We incorporated the location of glands and muscles into an existing matrix of somatic and genitalic morphological traits of these taxa and reanalyzed their phylogenetic relationship. Our analysis supports that the possession of glandular equipment is the ancestral state and that the manifold modifications of the prosomal shape have evolved convergently multiple times. We found differences in gland position between species with both modified and unmodified prosomata, and reported on seven cases of gland loss.

CONCLUSIONS

Our findings suggest that the occurrence of gustatory glands in sexually monomorphic ancestors has set the stage for the evolution of diverse dimorphic external modifications in dwarf spiders. Differences among congeners suggest that the gland position is highly susceptible to evolutionary changes. The multiple incidences might reflect costs of glandular tissue maintenance and nuptial feeding. Our results indicate divergent evolutionary patterns of gustatory-courtship-related traits, and thus a likely facilitating effect of sexual selection on speciation.

First records and three new species of the family Symphytognathidae (Arachnida, Araneae) from Thailand, and the circumscription of the genus Crassignatha Wunderlich, 1995

The family Symphytognathidae is reported from Thailand for the first time. Three new species: Anapistula choojaiae sp. nov., Crassignatha seeliam sp. nov., and Crassignatha seedam sp. nov. are described and illustrated. Distribution is expanded and additional morphological data are reported for Patu shiluensis Lin & Li, 2009. Specimens were collected in Thailand between July and August 2018. The newly described species were found in the north mountainous region of Chiang Mai, and Patu shiluensis was collected in the coastal region of Phuket. DNA sequences are provided for all the species here studied. The relations of these symphytognathid species were tested using previously published phylogenetic analyses on micro orb-weavers. Also, we used micro CT analysis to build 3D models of the male genitalia and somatic characters of two species of Crassignatha Wunderlich, 1995. The molecular phylogeny and 3D models were used to discuss the taxonomy and circumscription of the currently valid symphytognathid genera, with focus on Crassignatha and Patu Marples, 1951. Based on this, three new combinations are suggested: Crassignatha bicorniventris (Lin & Li, 2009), comb. nov., Crassignatha quadriventris (Lin & Li, 2009), comb. nov., and Crassignatha spinathoraxi (Lin & Li, 2009), comb. nov. A new record of Crassignatha danaugirangensisMiller et al. 2014 is reported from Brunei.

Imperfect and askew: A review of asymmetric genitalia in araneomorph spiders (Araneae: Araneomorphae)

Bilateral asymmetry in the genitalia is a rare but widely dispersed phenomenon in the animal tree of life. In arthropods, occurrences vary greatly from one group to another and there seems to be no common explanation for all the independent origins. In spiders, genital asymmetry appears to be especially rare. Most known species show almost perfectly symmetrical genitals with the right and left sides being mirror images of each other. However, some examples of asymmetric genitalia have been studied and many other reports are scattered in the taxonomic literature. Based on a broad literature survey, we found several species in thirteen families with evidence of genital asymmetry, mostly expressed only in females. Our review suggests that spider genital asymmetries, although rare, are more common than previously thought and taxonomic descriptions and illustrations are a useful but not entirely reliable tool for studying them. Here we also report on directional asymmetry in the liocranid spider Teutamus politus, the first known case of morphologically asymmetric male genitals in Entelegynae spiders. Generalities, evolution and categorization of asymmetry in spiders are further discussed.

Genital morphology and the mechanics of copulation in the millipede genus Pseudopolydesmus (Diplopoda: Polydesmida: Polydesmidae)

Mate choice, copulation, genital morphology, and sperm storage are not very well understood in millipedes. The use of three-dimensional x-ray computed tomography (μCT) provides new morphological data regarding millipede reproductive systems in both the female and male, including chitinous sclerites and membranes, muscles, glands, oviducts, and sperm conduits. Here we present a complete integrated account of the morphology and function of the female genital organs in the family Polydesmidae (Diplopoda: Polydesmida) using μCT, UV fluorescence imaging, and scanning electron microscopy. These data allow us to consider competing hypotheses regarding millipede vulva formation. We additionally present the morphology of copulatory interface in Pseudopolydesmus Attems, 1898 using images of a mating pair in copula and by simulating the interface of the organs using 3D models from μCT, allowing us to tentatively identify a lock-and-key-like mechanism. Finally, we use μCT to reveal the topology of the seminal canal in the gonopod of male Pseudopolydesmus, a topic that has remained unresolved for nearly 80 years.

The innervation of the male copulatory organ of spiders (Araneae) - a comparative analysis

Background

Nervous tissue is an inherent component of the many specialized genital structures for transferring sperm directly into the female’s body. However, the male copulatory organ of spiders was considered a puzzling exception. Based on the recent discovery of nervous tissue in the pedipalps of two distantly related spider species, we investigated representatives of all major groups across the spider tree of life for the presence of palpal nerves. We used a correlative approach that combined histology, micro-computed tomography and electron microscopy.

Results

We show that the copulatory organ is innervated in all species investigated. There is a sensory organ at the base of the sperm transferring sclerite in several taxa and nervous tissue occurs close to the glandular tissue of the spermophor, where sperm are stored before transfer.

Conclusions

The innervation of the copulatory organ by the bulb nerve and associated efferent fibers is part of the ground pattern of spiders. Our findings pave the way for unraveling the sensory interaction of genitalia during mating and for the still enigmatic mode of uptake and release of sperm from the male copulatory organ.

The sensory equipment of a sandokanid: An extreme case of tarsal reduction in harvestmen (Arachnida, Opiliones, Laniatores)

The study of sensory structures has the potential to provide insights into the natural history and evolution of animals. The sensory structures of arachnids are usually concentrated on the pedipalps (the tritocerebral appendages) or on the distal podomere (tarsus) of the anterior walking legs, the latter being the case for armored harvestmen (Opiliones, Laniatores). Therefore, modifications of the tarsus could have direct impacts on the sensory equipment of these animals. Using scanning electron microscopy, we investigated the sensory equipment in an extreme case of reduction in tarsal articles in the harvestman Sandokan truncatus (Sandokanidae), which bears a single tarsomere in all legs, and the potential consequences of this reduction. Additionally, we review the literature on the natural history of the family Sandokanidae. Tarsomeres of all legs are equipped with gustatory sensilla, mechanoreceptors, and a pore organ, but wall-pored olfactory sensilla are restricted to tarsi I and II. Tarsi II present a higher density of olfactory sensilla and also putative campaniform sensilla (strain detectors), which indicates a special sensory function of this pair of legs. Other podomeres are covered with shelled sensilla, a probable chemoreceptor previously unreported in Opiliones. Overall, S. truncatus has types of sensilla largely comparable to harvestmen with longer and subdivided tarsi. However, S. truncatus also exhibits extra-tarsal sensory fields of sensilla basiconica (putative thermo-/hygroreceptors) in previously undescribed sites, and the unique pore organs. Our results establish a basis for further research investigating the natural history, as well as the evolutionary correlations and mechanistic causes of the tarsal reduction in this enigmatic lineage.