Copulatory courtship by bushcricket genital titillators revealed by functional morphology, μCT scanning for 3D reconstruction and female sense structures

Copulatory mechanics of ghost spiders reveals a new self-bracing mechanism in entelegyne spiders

Spiders evolved a distinctive sperm transfer system, with the male copulatory organs located on the tarsus of the pedipalps. In entelegyne spiders, these organs are usually very complex and consist of various sclerites that not only allow the transfer of the sperm themselves but also provide a mechanical interlock between the male and female genitalia. This interlocking can also involve elements that are not part of the copulatory organ such as the retrolateral tibial apophysis (RTA)-a characteristic of the most diverse group of spiders (RTA clade). The RTA is frequently used for primary locking i.e., the first mechanical engagement between male and female genitalia. Despite its functional importance, some diverse spider lineages have lost the RTA, but evolved an apophysis on the femur instead. It can be hypothesized that this femoral apophysis is a functional surrogate of the RTA during primary locking or possibly serves another function, such as self-bracing, which involves mechanical interaction between male genital structures themselves to stabilize the inserted pedipalp. We tested these hypotheses using ghost spiders of the genus Josa (Anyphaenidae). Our micro-computed tomography data of cryofixed mating pairs show that the primary locking occurs through elements of the copulatory organ itself and that the femoral apophysis does not contact the female genitalia, but hooks to a projection of the copulatory bulb, representing a newly documented self-bracing mechanism for entelegyne spiders. Additionally, we show that the femoral self-bracing apophysis is rather uniform within the genus Josa. This is in contrast to the male genital structures that interact with the female, indicating that the male genital structures of Josa are subject to different selective regimes.

Functional morphology of the praying mantis male genitalia (Insecta: Mantodea)

Male genitalia in praying mantids are highly complex, but we know little of how they function. I combined the micro-computed tomography of a copulating pair of the European mantis (Mantis religiosa) with public videos of copulation in various species of Mantodea and an analysis of literature. The function of each major element is reviewed. Copulation is divided into three phases: opening, anchoring and deposition. The opening is achieved by pulling the female subgenital plate with the male apical process. Multiple cases of female cooperation or resistance were observed and one case of coercion by the male. In species with the reduced apical process, female cooperation is mandatory. The male subgenital plate may participate in the opening as an integral part of the genitalia. After the opening, the conformation of the genitalia drastically changes, revealing activity of the genital papilla. Tight grasp on female genitalia is maintained solely by the clamp on the right phallomere, despite the overall complexity and predictions of sexual conflict theory. Other prominent elements show rhythmic motions, but their functions are not entirely clear and evidently involve spermatophore deposition, female stimulation or rival sperm removal. The opening and anchoring are similar in Mantodea and Blattodea, but achieved with non-homologous elements.

The effect of baculum shape and mating behavior on mating-induced prolactin release in female house mice

Male genitalia are subject to rapid divergent evolution, and sexual selection is believed to be responsible for this pattern of evolutionary divergence. Genital stimulation during copulation is an essential feature of sexual reproduction. In mammals, the male intromittent genitalia induces a cascade of physiological and neurological changes in females that promote pregnancy. Previous studies of the house mouse have shown that the shape of the baculum (penis bone) influences male reproductive success and responds to experimentally imposed variation in sexual selection. Here, we test the hypothesis that the baculum is subject to sexual selection due to a stimulatory function during copulation. We selected male and female house mice (Mus musculus domesticus) from families with breeding values at the extremes of baculum shape and performed two series of experimental matings following which we examined the concentration of prolactin in the blood of females either 15 (“early”) or 75 (“late”) min after ejaculation. Our results provide evidence of a mating-induced release of prolactin in the female house mouse early after ejaculation, the level of which is dependent on an interaction between the shape of the baculum and male sexual behavior. Our data thereby provide novel insight into the mechanism(s) of sexual selection acting on the mammalian baculum.

Correlated sexual selection on male genitalia, copulatory performance and nuptial gifts in a bushcricket (Orthoptera: Tettigoniidae) indicated by allometric scaling

We adopt an allometric framework of scaling relationships for comparison between mating-related traits in the middle European bushcricket Roeseliana roeselii (Hagenbach, 1822). Eight characters, covering ontogenetic fitness (size traits; fixed at final moult), male condition (mass traits) and mating motivation (reproductive behaviours), were analysed in unrestricted matings and in matings involving genital manipulation. Shortening the male titillators had no effect on mating-related traits in males. However, titillators, known to be under sexual selection, scale hyperallometrically, with larger males possessing proportionally longer titillators, performing more titillator movements and exhibiting a reduced duration of copulation. Scaling was also hyperallometric for spermatophore mass, with larger males being heavier and transferring heavier nuptial gifts. Both titillator length and spermatophore mass might be condition-dependent indicators, because their variances were nearly twice as large those of body size or body mass. Mass traits were also dynamic, increasing by 11% for male body mass and 17% for spermatophore mass between the first and second matings. Sexual selection by female choice seems to favour larger trait size in the bushcricket R. roeselii, acting in concert on titillator length, intensity of titillator movements and spermatophore mass.

Mechanical properties of a female reproductive tract of a beetle and implications for penile penetration

Coevolution of male and female genitalia is widespread in animals. Nevertheless, few studies have examined the mechanics of genital interactions during mating. We characterized the mechanical properties of the elongated female genitalia, the spermathecal duct, of the small cassidine beetle, Cassida rubiginosa. The data were compared with the mechanical properties of the elongated male genitalia, the flagellum. We analysed the material distributions of the spermathecal duct using a microscopy technique, established a tensile test setup under a light microscope and conducted tensile tests. Diameter and tensile stiffness gradients were present along the spermathecal duct, but its Young's modulus and material distribution were more or less homogeneous. The results confirmed the hypothesis based on numerical simulations that the spermathecal duct is more rigid than the flagellum. In the study species, the penile penetration force is simply applied to the base of the hyper-elongated flagellum and conveyed along the flagellum to its tip. Considering this simple penetration mechanism, the relatively low flexibility of the spermathecal duct, compared to the flagellum, is likely to be essential for effective penetration of the flagellum.

High-throughput microCT scanning of small specimens: preparation, packing, parameters and post-processing

High-resolution X-ray microcomputed tomography, or microCT (μCT), enables the digital imaging of whole objects in three dimensions. The power of μCT to visualize internal features without disarticulation makes it particularly valuable for the study of museum collections, which house millions of physical specimens documenting the spatio-temporal patterns of life. Despite the potential for comparative analyses, most μCT studies include limited numbers of museum specimens, due to the challenges of digitizing numerous individuals within a project scope. Here we describe a method for high-throughput μCT scanning of hundreds of small (< 2 cm) specimens in a single container, followed by individual labelling and archival storage. We also explore the effects of various packing materials and multiple specimens per capsule to minimize sample movement that can degrade image quality, and hence μCT investment. We demonstrate this protocol on vertebrate fossils from Queensland Museum, Australia, as part of an effort to track community responses to climate change over evolutionary time. This system can be easily modified for other types of wet and dry material amenable to X-ray attenuation, including geological, botanical and zoological samples, providing greater access to large-scale phenotypic data and adding value to global collections.

A comprehensive and user-friendly framework for 3D-data visualisation in invertebrates and other organisms

Methods for 3D‐imaging of biological samples are experiencing unprecedented development, with tools such as X‐ray micro‐computed tomography (μCT) becoming more accessible to biologists. These techniques are inherently suited to small subjects and can simultaneously image both external and internal morphology, thus offering considerable benefits for invertebrate research. However, methods for visualising 3D‐data are trailing behind the development of tools for generating such data. Our aim in this article is to make the processing, visualisation and presentation of 3D‐data easier, thereby encouraging more researchers to utilise 3D‐imaging. Here, we present a comprehensive workflow for manipulating and visualising 3D‐data, including basic and advanced options for producing images, videos and interactive 3D‐PDFs, from both volume and surface‐mesh renderings. We discuss the importance of visualisation for quantitative analysis of invertebrate morphology from 3D‐data, and provide example figures illustrating the different options for generating 3D‐figures for publication. As more biology journals adopt 3D‐PDFs as a standard option, research on microscopic invertebrates and other organisms can be presented in high‐resolution 3D‐figures, enhancing the way we communicate science.

A New Species of Praying Mantis from Peru Reveals Impaling as a Novel Hunting Strategy in Mantodea (Thespidae: Thespini)

A new species of lichen-mimicking praying mantis, Carrikerella simpira n. sp., is described from Tingo María region in Peru. The new species differs from its congeners in having reduced tergal lobes, a relatively sinuous pronotum, and it is found in the highland tropical rainforest of the Central Andes. Behavioral observations conducted on captive individuals revealed that juveniles and adults hunt by impaling prey using modified foretibial structures. Anatomical examinations of the incumbent trophic structures revealed functional adaptations for prey impaling in the foretibiae, primarily consisting of prominent, forwardly oriented, barbed spines. We provide an overall description of this novel hunting behavior in Mantodea and hypothesize on its evolutionary origin and adaptive significance for the Thespidae.

The mosaic distribution pattern of two sister bush-cricket species and the possible role of reproductive interference

Reproductive interference can shape regional distribution patterns in closely related species, if prezygotic isolation barriers are weak. The study of such interaction could be more challenging in nuptial gift-giving species due to the direct nutritional effects on both sexes of both species during copulation. We mapped the distribution of two sister bush-cricket species, Pholidoptera aptera and Pholidoptera transsylvanica, at the northern margin of their overlapping ranges in Europe, and with a behavioral experiment, we tested the possibility of heterospecific mating. We found a very rare coexistence of species locally (0.5%, n = 391 sites) with mostly mutually exclusive distribution patterns, resulting in a mosaic pattern of sympatry, whereas they occupied the same climate niche in forest-dominated mountain landscape. Over 14 days of a mating experiment with seven mixed groups of conspecifics and heterospecifics (n = 56 individuals in total), the number of received spermatophores per female was 3-6 in P. aptera and 1-7 in P. transsylvanica. In total, we found 8.1% of heterospecific copulations (n = 99 transferred spermatophores with genetic identification of the donor species), while we also confirmed successful transfer of heterospecific sperms into a female's reproductive system. Because bush-cricket females also obtain required nutrition from a heterospecific spermatophylax what should increase their fitness and fecundity, we suggest that their flexibility to mate with heterospecifics is beneficial and drives reproductive interference. This may substantially limit the reproductive success of the less frequent species (P. transsylvanica), coupled with eventual detrimental effects from hybridization, and result in the competitive exclusion of that species from their areas of coexistence.

Sexual selection on bushcricket genitalia operates in a mosaic pattern

In most species with internal fertilization, male genitalia evolve faster than other morphological structures. This holds true for genital titillators, which are used exclusively during mating in several bushcricket subfamilies. Several theories have been proposed for the sexual selection forces driving the evolution of internal genitalia, especially sperm competition, sexually antagonistic coevolution (SAC), and cryptic female choice (CFC). However, it is unclear whether the evolution of genitalia can be described with a single hypothesis or a combination of them. The study of species-specific genitalia action could contribute to the controversial debate about the underlying selective evolutionary forces. We studied female mating behaviors in response to experimentally modified titillators in a phylogenetically nested set of four bushcricket species: Roeseliana roeselii, Pholidoptera littoralis littoralis, Tettigonia viridissima (of the subfamily Tettigoniinae), and Letana inflata (Phaneropterinae). Bushcricket titillators have several potential functions; they stimulate females and suppress female resistance, ensure proper ampulla or spermatophore attachment, and facilitate male fixation. In R. roeselii, titillators stimulate females to accept copulations, supporting sexual selection by CFC. Conversely, titillator modification had no observable effect on the female's behavior in T. viridissima. The titillators of Ph. l. littoralis mechanically support the mating position and the spermatophore transfer, pointing to sexual selection by SAC. Mixed support was found in L. inflata, where manipulation resulted in increased female resistance (evidence for CFC) and mating failures by reduced spermatophore transfer success (evidence for SAC). Sexual selection is highly species-specific with a mosaic support for either cryptic female choice or sexually antagonistic coevolution or a combination of both in the four species.

Copulatory mechanics in the wolf spider Agalenocosa pirity reveals a hidden diversity of locking systems in Lycosidae (Araneae)

Genital traits are among the fastest to evolve, and the processes that drive their evolution are intensively studied. Spiders are characterized by complex genitalia, but the functional role of the different structures during genital coupling is largely unknown. Members of one of the largest spider groups, the retrolateral tibial apophysis (RTA) clade, are characterized by a RTA on the male palp, which is thought to play a crucial role during genital coupling. However, the RTA was lost in several families including the species-rich wolf spiders (Lycosidae) leading to the hypothesis that the genital coupling is achieved by alternative mechanisms. Here, we investigate the genital interactions during copulation in the wolf spider Agalenocosa pirity (Zoicinae) on cryofixed mating pairs using electron, optical and X-ray microscopy and compare our findings with other lycosids and entelegyne spiders. We found an unprecedented coupling mechanism for lycosid spiders involving the palea and a membranous cuticle folding adjacent to the epigynal plate. Additionally, we show an uncommon coupling between the median apophysis and the contralateral genital opening, and confirmed that the terminal apophysis acts as functional conductor, as previously hypothesized for males of Zoicinae. Phylogenetic mapping of RTA indicated that the basal tibial process found in Agalenocosa is a secondary acquisition rather than a modified RTA.

Demonstrating sexual selection by cryptic female choice on male genitalia: What is enough?

Rapid divergence in external genital structures occurs in nearly all animal groups that practice internal insemination; explaining this pattern is a major challenge in evolutionary biology. The hypothesis that species-specific differences in male genitalia evolved under sexual selection as courtship devices to influence cryptic female choice (CFC) has been slow to be accepted. Doubts may stem from its radical departure from previous ideas, observational difficulties because crucial events occur hidden within the female's body, and alternative hypotheses involving biologically important phenomena such as speciation, sperm competition, and male-female conflicts of interest. We assess the current status of the CFC hypothesis by reviewing data from two groups in which crucial predictions have been especially well-tested, Glossina tsetse flies and Roeseliana (formerly Metrioptera) roeselii bushcrickets. Eighteen CFC predictions have been confirmed in Glossina and 19 in Roeseliana. We found data justifying rejection of alternative hypotheses, but none that contradicted CFC predictions. The number and extent of tests confirming predictions of the CFC hypothesis in these species is greater than that for other generally accepted hypotheses regarding the functions of nongenital structures. By this criterion, it is reasonable to conclude that some genital structures in both groups likely involved sexual selection by CFC.

The evolution of female genitalia

Female genitalia have been largely neglected in studies of genital evolution, perhaps due to the long-standing belief that they are relatively invariable and therefore taxonomically and evolutionarily uninformative in comparison with male genitalia. Contemporary studies of genital evolution have begun to dispute this view, and to demonstrate that female genitalia can be highly diverse and covary with the genitalia of males. Here, we examine evidence for three mechanisms of genital evolution in females: species isolating 'lock-and-key' evolution, cryptic female choice and sexual conflict. Lock-and-key genital evolution has been thought to be relatively unimportant; however, we present cases that show how species isolation may well play a role in the evolution of female genitalia. Much support for female genital evolution via sexual conflict comes from studies of both invertebrate and vertebrate species; however, the effects of sexual conflict can be difficult to distinguish from models of cryptic female choice that focus on putative benefits of choice for females. We offer potential solutions to alleviate this issue. Finally, we offer directions for future studies in order to expand and refine our knowledge surrounding female genital evolution.

The sperm pump and genital coupling of Panorpodes kuandianensis (Mecoptera: Panorpodidae)

Males of Panorpodidae possess a special sperm pump, through which they directly transfer seminal fluid to the female spermatheca. However, the sperm pump has not been studied in Panorpodes to date. Here, the structure of the sperm pump and the internal coupling of genitalia were investigated in the short-faced scorpionfly Panorpodes kuandianensis Zhong, Zhang, and Hua, 2011 using light and scanning electron microscopy. The sperm pump mainly consists of a piston, a pumping chamber, the anterior region of the aedeagal complex, the posterior region of the ejaculatory sac, and associated muscles. The piston as a propulsion apparatus is controlled by levator and depressor muscles. Its posterior region connects dorsally to the aedeagus via a joint. The pumping chamber is located between the piston and the aedeagus. The dorsal and ventral parameres were attached by retractor muscles. During copulation, the male phallotreme connects to the female copulatory pore to transfer sperm. Male gonostyli and parameres grasp the female to restrict the genitalia movement and impede her medigynium from retreating. The sperm ejaculatory mechanism of Panorpodes and the evolution of sperm transfer mode in insects are briefly discussed based on the structure of the sperm pump and the internal coupling of genitalia.

Copulatory mechanism and functional morphology of genitalia and anal horn of the scorpionfly Cerapanorpa dubia (Mecoptera: Panorpidae)

The scorpionfly genus Cerapanorpa is characterized by the male possessing a single finger-like anal horn on the posterior portion of tergite VI. However, the functional morphology of this anal horn and the genitalia have not been studied to date. Herein, we investigated the functional morphology of the genitalia and the nongenital structures of the scorpionfly Cerapanorpa dubia by observing the mating process and dissecting the freeze-fixated pairs in copula to reveal the copulatory mechanism. The male C. dubia provides a solid salivary mass to the female as a nuptial gift prior to copulation. When the female starts to feed on the gift, the male uses his notal organ and complex genital structures to control the closest wing and genitalia of the female to establish a V-shaped mating position. In the maintenance phase of copulation, the male uses his anal horn in cooperation with the basally-constricted abdominal segment VII to clamp female abdominal segment VIII. The male hypovalves grasp female cerci, and move up and down rhythmically. The paired parameres clasp both sides of female tergite IX. The basal processes on male gonostyli grip the pleural membranes of the female genital chamber. In the sperm transfer phase, the male aedeagus directly couples with the female medigynium to transmit sperm by connecting his phallotreme to the female's copulatory pore. The evolution of the male complex grasping structures in Panorpidae is also briefly discussed.

Female perception of copulatory courtship by male titillators in a bushcricket

Males of the bushcricket Metrioptera roeselii bear paired titillators that are spiny genital structures supposedly functioning as copulatory courtship devices. During copulation, the male inserts its titillators into the female's genital chamber, where they rhythmically tap on the sensilla-covered dorsal surface of the genital fold. Here, we investigated the stimulatory function of male titillators during mating in M. roeselii Tracer backfills of presumptive mechanosensory sensilla at the female genital fold revealed a thick bundle of sensory axons entering the last unfused abdominal ganglion (AG-7). Electrophysiological recordings of abdominal nerves demonstrated that females sense mechanical stimulation at their genital fold. The mechanosensory responses, however, were largely reduced by the insecticide pymetrozine that selectively blocks scolopidia of internal chordotonal organs but not campaniform and hair sensilla on the outer cuticle surface. In mating experiments, the females showed resistance behaviours towards males with asymmetrically shortened titillators, but the resistance was largely reduced when mechanoreceptors at the female's genital fold were either pharmacologically silenced by pymetrozine or mechanically blocked by capping with UV-hardened glue. Our findings support the hypothesis that the male titillators in these bushcrickets may serve as copulatory courtship devices to mechanically stimulate the female genitalia to reduce resistance behaviour.

Copulatory courtship by internal genitalia in bushcrickets

Male genital organs are among the fastest evolving morphological structures. However, large parts of the male's genitalia are often hidden inside the female during mating. In several bushcricket species, males bear a pair of sclerotized genital appendices called titillators. By employing synchrotron-based in vivo X-ray cineradiography on mating couples, we were able to visualize titillator movement and spermatophore attachment inside the female. Titillators are inserted and retracted rhythmically. During insertion the titillator processes tap the soft and sensillae-covered dorsal side of the female's flap-like genital fold, which covers the opening of the female's genitalia, without tissue penetration. Titillators thus appear to be initially used for stimulation; later they may apply pressure that forces the female's genital fold to stay open, thereby aiding mechanically in spermatophore transfer.

Investigating the functional morphology of genitalia during copulation in the grasshopper Melanoplus rotundipennis (Scudder, 1878) via correlative microscopy

We investigated probable functions of the interacting genitalic components of a male and a female of the flightless grasshopper species Melanoplus rotundipennis (Scudder, 1878) (frozen rapidly during copulation) via correlative microscopy; in this case, by synergizing micro-computed tomography (micro-CT) with digital single lens reflex camera photography with focal stacking, and scanning electron microscopy. To assign probable functions, we combined imaging results with observations of live and museum specimens, and function hypotheses from previous studies, the majority of which focused on museum specimens with few investigating hypotheses in a physical framework of copulation. For both sexes, detailed descriptions are given for each of the observed genitalic and other reproductive system components, the majority of which are involved in copulation, and we assigned probable functions to these latter components. The correlative microscopy approach is effective for examining functional morphology in grasshoppers, so we suggest its use for other animals as well, especially when investigating body regions or events that are difficult to access and understand otherwise, as shown here with genitalia and copulation. J. Morphol. 278:334-359, 2017. © 2017 Wiley Periodicals, Inc.