Sexual dimorphism in leg length among orb-weaving spiders: a possible role for sexual cannibalism

The degree and direction of sexual dimorphism across different species is commonly attributed to differences in the selection pressures acting on males and females. The extent of these differences is especially apparent in species that practise sexual cannibalism, where the female attempts to capture and eat a courting male. Here, we investigate the relationship between sexual dimorphism in size and leg length, sexual cannibalism and courtship behaviour in three taxonomic groups of orb-weaving spiders, using morphological data from 249 species in 36 genera. Females are larger than males in all three taxonomic groups, and males have relatively longer legs than females in both the Araneinae and Tetragnathidae. Across genera within each taxonomic group, male body size is positively correlated with both female body size and male leg length, and female body size is positively correlated with female leg length. Sexual size dimorphism is negatively correlated with relative male leg length within the Araneinae, but not within either the Tetragnathidae or the Gasteracanthinae. There was no negative correlation between sexual size dimorphism and relative female leg length in any taxonomic group. We argue that the relationship between sexual size dimorphism and relative male leg length within the Araneinae may be the result of selection imposed by sexual cannibalism by females.     

A revision of Danielssenia perezi Monard, D. paraperezi Soyer, D. eastwardae Coull (Harpacticoida; Paranannopidae) and their transfer to a new genus

The harpacticoid copepod Danielssenia perezi Monard, 1935 is redescribed on the basis of the only available material from the Scilly Isles off south-west Britain. Sentirenia gen. nov is erected because this species exhibits significant differences from the type species Danielssenia typica Boeck, 1872 in the structure of the antennule; the presence of sensory aesthetascs on, and structure of, the mandible, maxillula and maxilla; the form of the female P5; the setation of the swimming legs and details of the sexual dimorphism on the endopod of the male P2. Further, it is shown that the characters by which D. paraperezi Soyer, 1970 was originally distinguished from D. perezi are no longer valid and D. paraperezi is a junior synonym of S. perezi which now assumes a boreo-mediterranean distribution. A re-examination of the type material of D , eastwardae Coull, 1971 shows that this species is identical to S. perezi in the structure of the mouthparts (including the presence of aesthetascs on the mandible, maxillula and maxilla) and setation of P1–P5. However, within the genus Sentirenia, its specific status is maintained by virtue of a 5-segmented female antennule; a difference in the shape of the endopodal lobe of the female P5; the sexually dimorphic characters of the male P2 endopod; and variation in the ornamentation of some appendages.     

Spermatophore transfer in the hermit crab Clibanarius vittatus (Crustacea,Anomura, Diogenidae)

Although mating has been described in several hermit crab species, the mechanics of spermatophore transfer have not previously been demonstrated. Evidence from pleopod and gonopore morphology, video observations, and inseminated females indicates that in Clibanarius vittatus the male applies a spermatophoric mass directly onto the female via the gonopores rather than with modified pleopods 1-2 (gonopods) and/or genital papillae as in many other decapods. The single second pleopod of males of C. vittatus has a simple endopod with no apparent modifications for sperm transfer. There are no genital papillae extending from the male gonopores. The globular spermatophores are aligned in rows surrounded by a seminal secretion in the male ducts (vasa deferentia that terminate in ejaculatory ducts opening to the exterior via the gonopores). During copulation, described from time-lapse video recordings, the ventral surface of the last thoracic segment of the male, bearing the gonopores, was apposed to the ventral cephalothorax of the female. A massive amount of seminal secretion containing spermatophore ribbons, termed here the spermatophoric mass and described for the first time in a hermit crab species, was observed covering the sternites and coxae of pereopods 1-5 of a recently copulated female. It is suggested that during copulation the male emits the contents of the ejaculatory ducts directly onto the female without the aid of gonopods or genital papillae. Although spermatophore transfer is simple in C. vittatus, the presence of modified anterior pleopods or elongate genital papillae (sexual tubes) in other paguroidean species suggests the possibility of a more complex insemination process in these other hermit crabs.     

Spermatophore Transfer and Subsequent Sperm Development in a Homalorhagid Kinorhynch

The cuticular morphology and precise location of male and female gonopores and penile spines of the homalorhagid kinorhynch Kinorhynchus phyllotropis Brown & Higgins, 1983 are described and illustrated. In this species spermatozoa are transferred from male to female by a spermatophore. This is the first record of the mechanism of sperm transfer in a kinorhynch. The spermatophore is presumably extruded through the male gonopore and directed towards the female by the ductless penile spines. Spermatozoa in the spermatophore are rod-shaped and catenulate. The spermatophore is pressed directly against the cuticular plates of the female, and usually covers the female gonopores. The spermatophore contains a mass of intertwined spermatids and spermatozoa surrounded by clear material covered with a layer of debris. Spermatozoa are found in the female lodged in the seminal receptacle tissue applied to the dorsal aspect of posterior oocytes. There the spermatozoa complete their development. Nuclei change from filiform to geniculate, and oval corpuscles surrounding the nuclei disappear, so that the spermatozoa are seen as densely-packed, polyhedral cells. These observations conform with literature reports of aberrant spermatozoa of unknown origin seen in female Pycnophyes . The fertilization process remains unknown.     

Correlated evolution of female neoteny and flightlessness with male spermatophore production in fireflies (Coleoptera: Lampyridae)

The beetle family Lampyridae (fireflies) encompasses ～100 genera worldwide with considerable diversity in life histories and signaling modes. Some lampyrid males use reproductive accessory glands to produce spermatophores, which have been shown to increase female lifetime fecundity. Sexual dimorphism in the form of neotenic and flightless females is also common in this family. A major goal of this study was to test a hypothesized link between female flight ability and male spermatophore production. We examined macroevolutionary patterns to test for correlated evolution among different levels of female neoteny (and associated loss of flight ability), male accessory gland number (and associated spermatophore production), and sexual signaling mode. Trait reconstruction on a molecular phylogeny indicated that flying females and spermatophores were ancestral traits and that female neoteny increased monotonically and led to flightlessness within multiple lineages. In addition, male spermatophore production was lost multiple times. Our evolutionary trait analysis revealed significant correlations between increased female neoteny and male accessory gland number, as well as between flightlessness and spermatophore loss. In addition, female flightlessness was positively correlated with the use of glows as female sexual signal. Transition probability analysis supported an evolutionary sequence of female flightlessness evolving first, followed by loss of male spermatophores. These results contribute to understanding how spermatophores have evolved and how this important class of seminal nuptial gifts is linked to other traits, providing new insights into sexual selection and life-history evolution.     

Male Aggression,Female Mimicry and Female Choice in the Rove Beetle,Aleochara curtula (Coleoptera,Staphylinidae)

Causes, consequences and pheromonal regulation of male contest and female choice in the staphylinid beetle, Aleochara curtula (Goeze), have been investigated in the field and in the laboratory. At the feeding and mating site (carcass) the sex ratio is male biased. Polyandry is affected by prolonged copulations, spermatophore plugs and anti-aphrodisiac pheromones transferred from the male, and by female repulse behaviour as well. Aggression of competing males leads to expulsion of inferior males from the carcass. Young, starved and multiply mated males, which need access to the food resource, produce the female sex pheromone. They release homosexual responses, but also avoid intrasexual aggression. On the other hand, females behave aggressively towards individuals bearing the female sex pheromone or repulse their copulatory attempts. Those males of insufficient physiological condition produce a lighter spermatophore and fertilize less eggs. The adaptive significance of female mimicry, male mating tactics, and female choice is discussed.     

Increase in song frequency decreases spermatophore size: correlative evidence of a macroevolutionary trade-off in katydids (Orthoptera: Tettigoniidae)

In many katydids, the male feeds his mate with a large gelatinous spermatophore. Males of most species also produce elaborate calling songs. We predicted a negative relationship between spermatophore size and call frequency because of trade-offs between these two costly traits. Our comparative analysis controlling phylogeny and body size supported this prediction. Although call frequency is expected to decrease with increasing body size, after controlling for phylogeny, both variables were not related. Finally, given that song frequency and spermatophore size are likely targets of sexual selection, we examined the relationship between these variables and sexual size dimorphism (SSD) which can be influenced by sexual selection on body size. We found that only female body size was positively related to SSD, suggesting that natural and/or sexual selection on female body size may be stronger than sexual selection on male and spermatophore size.     

The life-cycle of Ergasilus briani Markewitsch, 1993 (Copepoda: Poecilostomatoida)

The life cycle of Ergasilus briani is shown to consist of six nauplius stages, five copepodid stages and the adults. Only the adult female is parasitic. The free-living stages are described in detail. The presence of three aesthetascs on the antennules of all copepodid stages and the adult is noted for the first time. Unusual aspects of E. briani development include: the antennule is sexually dimorphic from copepodid IV onwards, the dimorphism being expressed in the numbers of setae present on the more proximal segments, and the first appearance of the male maxilliped is at the copepodid II stage. The endopodal setation of the male maxilliped is the maximum known within the Poecilostomatoida. The striking pigmentation of the developmental stages of ergasilids is noted and discussed.     

Psammic Procyclopina (Copepoda, Cyclopoida) from the coast of Brazil

Procyclopina polyarthra Herbst is redescribed and three new species, P. maricopeba, P. feiticeira and P. uguaipuku , are described from intertidal interstitial waters. Procyclopina is distinguished within the family by the enditic setation of the maxilliped 0,1,3,4, the 19-segmented antennule, and three elements in the leg 5 exopod of the female. Procyclopina has its distribution restricted to the coast of Säo Paulo, Brazil. Procyclopina maricopeba sp. n. is distinguished from P. polyarthra by the three-segmented leg 5 in the male and by the length: width ratio of the genital double somite in the female. Procyclopina feiticeira sp. n. is unique in the genus by lacking the inner seta on legs 14 exp-I, and by the incomplete fusion of genital (seventh thoracic) and first abdominal somites in the female. Procyclopina uguaipuku sp. n. is characterized by the antennulary armature, by the caudal ramus (much longer than wide), and by the position of the proximal seta of the leg 4 enp-2 which is set on a protuberance on the posterior face. Morphological characters of Procyclopina are compared with those of other cyclopinids.     

The Role of Body Size in Mating Interactions of the Sexually Cannibalistic Fishing Spider Dolomedes triton

Some arachnids display extreme sexual size dimorphism (SSD) with adult females being several times larger than adult males. One explanation for SSD in species that exhibit pre‐copulatory sexual cannibalism (female attack, kill and consumption of the male prior to mating) is that smaller males may be less likely victims of predatory attacks by females. However, in some sexually cannibalistic species SSD is relatively moderate (i.e. males are similar in size to females) suggesting benefits of large male body size. Here, I report the results of an experiment designed to explore the ramifications of body size in mating interactions of the sexually cannibalistic, North American fishing spider (Dolomedes triton). Results suggest that male size does not influence courtship behavior, the likelihood of being attacked, or the male's ability to secure a mounting. However, large males were superior at gaining copulations once mounted. Sexual cannibalism may also be predicated on female size. Female condition (mass/cephalothorax area) did not explain any of these behaviors from the copulatory sequence, however, females with a smaller cephalothorax area were more likely to attack courting males. Finally, analysis of the ratio of female size to male size showed that when SSD is weak males are more likely to escape attacks and mate successfully. Results are discussed in light of several hypotheses for sexual cannibalism, and the benefits of large male body size illustrated here are put forth as potential explanations for the relatively moderate extent of SSD found in this sexually cannibalistic species.     

Seasonal Variation in Mate Choice of Photinus ignitus Fireflies

Mate choice by either sex may vary with changes in the associated costs and benefits, determined by factors such as the availability of potential mates and variation in mate quality. We examined seasonal variation in operational sex ratio, courtship behavior, spermatophore mass, egg count, and the relationship between morphological traits and mating success in Photinus ignitus fireflies to determine if mate choice in either sex varied with the availability and relative reproductive investment of fertilizable females and sexually active males. Successfully mating males had larger lanterns than unsuccessful males when the operational sex ratio was male‐biased. In addition, female responsiveness to male signals increased as the number of courting males decreased, and male spermatophore mass decreased with body size across the mating season. Successfully mating females had larger body mass than unsuccessful females. Female body mass predicted egg count and female rejection by males increased as the season progressed and female size decreased. These results suggest that both male and female P. ignitus exhibit mate choice, and that such choice is influenced by seasonal variation in the abundance and quality of potential mates.     

Paternity analyses in wild‐caught and laboratory‐reared Caribbean cricket females reveal the influence of mating environment on post‐copulatory sexual selection

Polyandry is ubiquitous in insects and provides the conditions necessary for male‐ and female‐driven forms of post‐copulatory sexual selection to arise. Populations of Amphiacusta sanctaecrucis exhibit significant divergence in portions of the male genitalia that are inserted directly into the female reproductive tract, suggesting that males may exercise some post‐copulatory control over fertilization success. We examine the potential for male–male and male–female post‐copulatory interactions to influence paternity in wild‐caught females of A. sanctaecrucis and contrast our findings with those obtained from females reared in a high‐density laboratory environment. We find that female A. sanctaecrucis exercise control by mating multiple times (females mount males), but that male–male post‐copulatory interactions may influence paternity success. Moreover, post‐copulatory interactions that affect reproductive success of males are not independent of mating environment: clutches of wild‐caught females exhibit higher sire diversity and lower paternity skew than clutches of laboratory‐reared females. There was no strong evidence for last male precedence in either case. Most attempts at disentangling the contributions of male–male and male–female interactions towards post‐copulatory sexual selection have been undertaken in a laboratory setting and may not capture the full context in which they take place – such as the relationship between premating and post‐mating interactions. Our results reinforce the importance of designing studies that can capture the multifaceted nature of sexual selection for elucidating the role of post‐copulatory sexual selection in driving the evolution of male and female reproductive traits, especially when different components (e.g. precopulatory and post‐copulatory interactions) do not exert independent effects on reproductive outcomes.     

Mate plugging via genital mutilation in nephilid spiders: an evolutionary hypothesis

Nephilid spiders are known for gigantic females and tiny males. Such extreme sexual dimorphism and male-biased sex ratios result in fierce male–male competition for mates. Intense sperm competition may be responsible for behaviors such as mate guarding, mate binding, opportunistic mating, genital mutilation, mating plugs and male castration (eunuchs). We studied the mating biology of two phylogenetically, behaviorally and morphologically distinct south-east Asian nephilid spider species ( Herennia multipuncta, Nephila pilipes ) in nature and in the laboratory. Specifically, we established the frequencies and effectiveness of plugging (a plug is part of the male copulatory organ), and tested for male and female copulatory organ reuse. Both in nature and in the laboratory, plug frequencies were higher in H. multipuncta (75–80% females plugged) compared with N. pilipes (45–47.4%), but the differences were not significant. Plugs were single and effective (no remating) in H. multipuncta but multiple and ineffective (remating possible) in N. pilipes . In Herennia , the males plugged when the female was aggressive and in Nephila plugging was more likely when mating with previously mated and larger females. Further differences in sexual biology are complete palpal removal and higher sexual aggressiveness in Herennia (sexual cannibalism recorded for the first time), and mate binding in Nephila . Thus, we propose the following evolutionary hypothesis: nephilid plugging was ancestrally successful and enabled males to monopolize females, but plugging became ineffective in the phylogenetically derived Nephila . If the evolution of nephilid sexual mechanisms is driven by sexual conflict, then the male mechanism to monopolize females prevailed in a part of the phylogeny, but the female resistance to evade monopolization ultimately won the arms race.     

Male Condition, Female Choice, and Extreme Variation in Repeated Mating in a Scaly Cricket, Ornebius aperta (Orthoptera: Gryllidae: Mogoplistinae)

Mating in the scaly cricket Ornebius aperta often includes the transfer of many spermatophores to individual females during extended copulatory interactions. We manipulated male condition in staged matings to determine whether this could explain variation in the number of repeated copulations seen across pairs. Males on a high nutrient diet were in good condition, were more likely to mate repeatedly, and transferred more spermatophores on average than low-diet males (in poor condition). High-diet males were more likely to produce a vibratory signal that increased female receptivity to repeated mating attempts. Courtship and copulatory interactions were always terminated by females, and in every case males had already formed a spermatophore when deserted by females. We conclude that variation in male repeated mating success may be due to female choice rather than an inability or unwillingness of low-diet males to produce spermatophores.     

Extreme Costs of Mating for Male Two-Spot Ladybird Beetles

Male costs of mating are now thought to be widespread. The two-spot ladybird beetle (Adalia bipunctata) has been the focus of many studies of mating and sexual selection, yet the costs of mating for males are unknown. The mating system of A. bipunctata involves a spermatophore nuptial gift ingested by females after copulation. In this study, we investigate the cost to males of mating and of transferring spermatophores in terms of lifespan, ejaculate production and depletion of nutritional reserves. We found that males faced a strong trade-off between mating and survival, with males that were randomly assigned to mate a single time experiencing a 53% reduction in post-mating lifespan compared to non-mating males. This is among the most severe survival costs of a single mating yet reported. However, spermatophore transfer did not impact male survival. Instead, the costs associated with spermatophores appeared as a reduced ability to transfer spermatophores in successive matings. Furthermore, males ingested more food following spermatophore transfer than after matings without spermatophores, suggesting that spermatophore transfer depletes male nutritional reserves. This is to our knowledge the first report of an effect of variation in copulatory behaviour on male foraging behaviour. Overall, our study highlights the advantages of assessing mating costs using multiple currencies, and suggests that male A. bipunctata should exhibit mate choice.     

Mimetic butterflies support Wallace's model of sexual dimorphism

Theoretical and empirical observations generally support Darwin's view that sexual dimorphism evolves due to sexual selection on, and deviation in, exaggerated male traits. Wallace presented a radical alternative, which is largely untested, that sexual dimorphism results from naturally selected deviation in protective female coloration. This leads to the prediction that deviation in female rather than male phenotype causes sexual dimorphism. Here I test Wallace's model of sexual dimorphism by tracing the evolutionary history of Batesian mimicry-an example of naturally selected protective coloration-on a molecular phylogeny of Papilio butterflies. I show that sexual dimorphism in Papilio is significantly correlated with both female-limited Batesian mimicry, where females are mimetic and males are non-mimetic, and with the deviation of female wing colour patterns from the ancestral patterns conserved in males. Thus, Wallace's model largely explains sexual dimorphism in Papilio. This finding, along with indirect support from recent studies on birds and lizards, suggests that Wallace's model may be more widely useful in explaining sexual dimorphism. These results also highlight the contribution of naturally selected female traits in driving phenotypic divergence between species, instead of merely facilitating the divergence in male sexual traits as described by Darwin's model.     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

Determinants of Mating Frequency in the Spiny Orbweaving Spider,Micrathena gracilis (Araneae: Araneidae)

Male Micrathena gracilis require two copulations, separated by a dismount, in order to inseminate both reproductive tracts of the female. We examined several factors that might influence a male's copulatory success. Web structure influenced male courtship and dismount tactics, but not copulatory frequency. The presence of another male reduced the likelihood of a given male copulating with both tracts, a limitation mediated by sexual responsiveness of the female. Mating status of the female did influence copulatory frequency; males were less likely to copulate a second time with nonvirgin females. In summary, males modify mating activities to reduce predation by females, to reduce intermale competition, and to avoid expending gametes when there is little chance of fertilization. Females influence males by predatory activities, mediated through web structure, and enhancing sperm competition among males.     

AGGRESSION,DENSITY, AND SEXUAL DIMORPHISM IN CHERNETID PSEUDOSCORPIONS (ARACHNIDA: PSEUDOSCORPIONIDA)

The determinants of sexual dimorphism in a family of false scorpions (Pseudoscorpionida: Chernetidae) were investigated experimentally and with a literature analysis of comparative morphometric and habitat data. Species vary in the extent to which males and females differ in size of the pedipalps, and, in particular, in size of the pedipalpal chelae. A statistical analysis of dimorphism patterns within the Chernetidae suggests that dimorphism is a highly variable condition, relatively unconstrained by phylogenetic influences. The evolution of species with enlarged male pedipalps appears to be associated with a change from nonpairing to pairing sperm-transfer behavior and with aggressive mate acquisition by males. Experiments with Dinocheirus arizonensis demonstrate a strong correlation between male combat ability and chela size. Manipulations also suggest that superior combat ability of large males results in increased mating success only under high-density conditions. The greater success in spermatophore transfer among large males can be attributed to increased opportunity at high density for large males to interrupt matings and aggressively displace smaller males. Comparative analysis showed a positive correlation between density and enlarged male chela size across chernetid species.