Copulatory behavior in a pholcid spider: males use specialized genitalic movements for sperm removal and copulatory courtship

Reproductive biology of harvestmen (Arachnida: Opiliones): a review of a rapidly evolving research field

Harvestmen are a major arachnid order that has experienced a dramatic increase in biological knowledge in the 21st century. The publication of the book Harvestmen: The Biology of Opiliones in 2007 stimulated the development of many behavioral studies. Although the book is relatively recent, our understanding of the reproductive biology of harvestmen is already outdated due to the fast accumulation of new data. Our goal is to provide an updated review of the subject to serve as a benchmark for the following years. In the pre-copulatory phase, we explore the evolution of facultative parthenogenesis, the factors that may affect the types of mating system, and the role of nuptial gifts in courtship. Regarding the copulatory phase, harvestmen are unique arachnids because they have aflagellate spermatozoa and a penis with complex morphology. We discuss the implications of these two features for sperm competition and cryptic female choice. In the post-copulatory phase, we connect oviposition site selection and climate conditions to the widespread occurrence of resource defense polygyny, alternative reproductive tactics, and sexual dimorphism in several clades of tropical harvestmen. Finally, we present the different forms of parental care in the order, and discuss the benefits and costs of this behavior, which can be performed either by females or males. Throughout the review, we indicate gaps in our knowledge and subjects that deserve further studies. Hopefully, the information synthesized here will stimulate researchers worldwide to embrace harvestmen as a study system and to improve our effort to unravel the mysteries of their reproductive biology.

Meta-analytical evidence that males prefer virgin females

Males are often predicted to prefer virgin over non-virgin females because of the reduced risk of sperm competition. Does this prediction hold across studies? Our systematic meta-analysis of 138 studies, mainly conducted in invertebrates, confirms that males generally prefer virgin females. However, males preferred virgin females even in species with last male sperm precedence, suggesting that sperm competition alone does not drive male preferences. Furthermore, our results suggest that males may reject mated females even when no alternative exists. Preference for virgins is unlikely to influence female reproductive success since virginity cannot be selected for, but strong preference for virgin females could swamp or reinforce selection on other traits. Our results add to growing evidence that males are not indiscriminate in mating. However, given the unexplained heterogeneity in effect sizes, we urge caution in assuming that males will prefer virgins and recommend considering the natural context of mating decisions.

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.

Successive matings affect copulatory courtship but not sperm transfer in a spider model

Previous studies have reported that males augment their reproductive success by increasing the number of females with which they copulate, and that such copulations are not energetically demanding in terms of trivial sperm production costs. However, we now know that males do pay reproductive costs. As males mate successively, a reduction in the performance of copulatory behaviours would be expected, as well as in the number of sperm transferred. Here we compared the duration of courtship, mating and post-insemination phase, the number of genital and non-genital copulatory courtship occurrences, and the number of sperm transferred in successive matings in Holocnemus pluchei spider males. As matings increased in males, there was no effect on the duration of courtship, mating or post-insemination phase. Interestingly, genital copulatory courtship varied in successive copulations depending on male size, but there was no change in the number of sperm transferred. In addition, the occurrence of non-genital copulatory courtship decreased along successive copulations. The negative effects of successive matings on copulatory courtship indicate that these behaviours are costly for males, except for the number of sperm transferred. Our research lays the foundation for future studies on male costs as a function of mating history in spiders.

The biology and evolution of spider venoms

Spiders are diverse, predatory arthropods that have inhabited Earth for around 400 million years. They are well known for their complex venom systems that are used to overpower their prey. Spider venoms contain many proteins and peptides with highly specific and potent activities suitable for biomedical or agrochemical applications, but the key role of venoms as an evolutionary innovation is often overlooked, even though this has enabled spiders to emerge as one of the most successful animal lineages. In this review, we discuss these neglected biological aspects of spider venoms. We focus on the morphology of spider venom systems, their major components, biochemical and chemical plasticity, as well as ecological and evolutionary trends. We argue that the effectiveness of spider venoms is due to their unprecedented complexity, with diverse components working synergistically to increase the overall potency. The analysis of spider venoms is difficult to standardize because they are dynamic systems, fine-tuned and modified by factors such as sex, life-history stage and biological role. Finally, we summarize the mechanisms that drive spider venom evolution and highlight the need for genome-based studies to reconstruct the evolutionary history and physiological networks of spider venom compounds with more certainty.

Material heterogeneity of male genitalia reduces genital damage in a bushcricket during sperm removal behaviour

Sperm removal behaviour (SRB) is known in many animals, and male genital structures are often involved in the SRB, e.g. rubbing female genitalia vigorously. However, it remains unclear how those male genital structures function properly without severe genital damage during SRB. In the present study, we focused on the bushcricket Metaplastes ornatus and examined the biomechanics of male and female genital structures, involved in their SRB as a model case. During an initial phase of mating, males of this species thrust their subgenital plate with hook-like spurs and many microscopic spines into the female genital chamber. By moving the subgenital plate back-and-forth, males stimulate females, and this stimulation induces the ejection of sperm previously stored in females. We aimed to uncover the mechanics of the interaction between the subgenital plate and genital chamber during SRB. The genital morphology and its material composition were investigated using modern imaging and microscopy techniques. The obtained results showed a pronounced material heterogeneity in the subgenital plate and the genital chamber. The material heterogeneity was completely absent in that of a second bushcricket species, Poecilimon veluchianus, which does not exhibit SRB. Finite element simulations showed that the specific material heterogeneity can redistribute the stress in the subgenital plate of M. ornatus and, thereby, reduces stress concentration during SRB. This may explain why only a few examined males had a broken spur. We suggest that the observed structural features and material heterogeneity in M. ornatus are adaptations to their SRB.

Sperm competition when transfer is dangerous

Aggressive and cannibalistic female spiders can impose strong selection on male mating and fertilization strategies. Furthermore, the distinctive reproductive morphology of spiders is predicted to influence the outcome of sperm competition. Polyandry is common in spiders, leading to defensive male strategies that include guarding, plugging and self-sacrifice. Paternity patterns are highly variable and unlikely to be determined solely by mating order, but rather by relative copulation duration, deployment of plugs and cryptic female choice. The ability to strategically allocate sperm is limited, either by the need to refill pedipalps periodically or owing to permanent sperm depletion after mating. Further insights now rely on unravelling several proximate mechanisms such as the process of sperm activation and the role of seminal fluids. This article is part of the theme issue 'Fifty years of sperm competition'.

Copulatory behaviour increases sperm viability in female spiders

One remarkable reproductive feature in animals with internal fertilization is a reduction in sperm viability over time in females. Whether this reduction is driven by male–male competition and/or cryptic female choice is unclear. From the perspective of cryptic female choice, we postulated that sperm viability is affected by a particular male copulatory behaviour. In this study, we investigated the following aspects: (1) sperm viability in mated females vs. males; (2) whether sperm viability varies temporally after mating; and (3) whether male copulatory behaviour covaries positively with sperm viability within females. We used the spider Holocnemus pluchei, whose males use several copulatory behaviours to court females. We found that females that stored sperm for 4 or 15 days showed no difference in sperm viability but had lower sperm viability compared with males, and males that performed a longer post-insemination behaviour had higher sperm viability inside the female. It is unclear how sperm viability is reduced and how male post-insemination behaviour affects this. It is possible that extending copulation allows males to induce females to keep sperm alive for longer. This result is predicted by theory whereby males induce females to facilitate sperm to reach and fertilize eggs based on male postcopulatory behaviour.

Female secondary sexual traits in spiders: Adaptive interpretations of the sternum projection in the pholcid Holocnemus pluchei

Sexually dimorphic traits are the outcome of a network of selective pressures acting upon each sex. While male secondary sexual traits are frequently restricted to intrasexual competition, female ornaments or weapons are mostly associated to social interactions for the access to limited resources. Here we investigate a sexually dimorphic trait expressed within the female's sternum of the spider species Holocnemus pluchei (Pholcidae), and its adaptive maintenance under the three leading hypotheses: sexual receptivity signaling, mate choice and sexual conflict hypothesis. We provide fine-scaled behavioral descriptions of the copulatory behavior of Holocnemus pluchei, corroborating that mating interactions are mediated by the female's sternum projection. The single moment of contact between the female projection and male body occurs during sperm transfer, always followed by a reduction on the intensity of the male genital movement. Additionally, biometrical properties of the female sternum projection corroborate our functional interpretation for the trait as restricting the intensity of males' copulatory movements. We claim that female sternum projection is a sexually selected trait mediating sexual conflict, with several adaptive consequences upon the sexes.