Prey capture by the crab spider Misumena calycina (Araneae: Thomisidae)

Summary Crab spiders Misumena calycina (L.) in pasture rose Rosa carolina flowers regularly attacked bumble bees, smaller bees, and syrphid flies that visited these flowers. Attacks reached a maximum rate of over 20/h during mid morning, but only 1.6% of the most important prey item, bumble bees, were captured. The next most important food source, the most frequently taken item, syrphid flies Toxomerus marginatus (Say), were captured in 39% of the attempts. Since these flies have a biomass only 1/60th that of bumble bees, they comprised a much less important food source than did bumble bees. Spiders would obtain over 7% more food by specializing on bumble bees than by attacking all insect visitors, and as much as 20% more food at certain times of the day. However, they did not show a tendency to specialize at any time.     

Possible significance of the red patches of the female crab-spider, Misumena vatia

The female of the crab-spider, Misumena vatia, is one of the few spiders known to be able to change its colour reversibly. Adult females usually have red patches on each side of the abdomen. The most likely insect enemies of Misumena are red-blind, and because the red patches absorb ultraviolet exactly as does the rest of the abdomen the patches are not distinguishable to insects in the ultraviolet. It is suggested that the red patches function as a warning colour for birds and other vertebrates, which are not red-blind.     

Contrasting patch choice behaviour by immature ambush predators, a spider (Misumena vatia) and an insect (Phymata americana)

1. Spiderlings of the crab spider Misumena vatia and nymphs of the ambush bug Phymata americana normally ambush prey in flowers, as do the adults. The immatures of M. vatia are more mobile, by ballooning, than the ambulatory but largely sedentary adult female spiders, but the apterous immatures of P. americana are less mobile than the alate adults. The aim of the work reported here was to investigate how immatures, as compared with adults, select sites from which to ambush their prey. 2. Individual immatures of both species were released in experimental arenas with variously manipulated natural stems: normal, with and without leaves, and with and without flowers of three species of plants. 3. The spiderlings and nymphs showed discriminatory behaviour but not in accordance with optimal foraging in either case. The spiderlings chose more leafy stems than any other kind of stem. For the nymphs, the importance of leaves was less marked, food apparently being more important than shelter. 4. Neither of the predators discriminated among flowering stems of Solidago canadensis, Daucus carota, and Cirsium arvense. 5. In other experiments, in which flowered and deflowered stems were located in two separated groups in the arena and the individual was released in a bare area between the two groups, the spiderlings chose either group as predicted by chance but the nymphs showed a slight preference for the flowered sector. Nevertheless, once in either of the groups, the predators showed similar behaviour in choosing leafy stems more often than other stems. 6. Nymphs of P. americana are much more selective than adults but spiderlings of M. vatia are less selective than adult females. These results appear to accord with the time and energy costs of changing hunting sites, walking being slower than flying or ballooning. 7. The results indicate the need to include other parameters, such as enemy‐free space and time spent in not being able to forage, in the analysis of foraging behaviour.     

Previous foraging success influences web building in the spider Stegodyphus lineatus (Eresidae)

Stegodyphus lineatus (Eresidae) is a desert spider that buildsan aerial capture web on bushes in the Negev desert in southernIsrael. Web building for spiders is costly in energy, time,and risk of predation. Spiders should trade-off these costswith the benefits in terms of prey capture. We tested the hypothesisthat the previous foraging success of the spider influencesthe effort invested in foraging. Specifically, we asked whetheran increase in food intake causes spiders to reduce web renewalactivity and web size. Alternatively, time constraints on foragingand development, resulting from a short growing season, couldinduce spiders to continue foraging even when supplemented withprey. The cost of web building was measured as time and massloss. To build an average size web (about 150 cm²), we calculatedthat a spider requires 6 h and that spiders lose 3%-7% of their weight.In field experiments, spiders responded differently to food supplementationin 2 different years. In 1994, they improved their condition comparedto individuals whose webs were removed to reduce foraging opportunitiesand compared to control spiders. In 1995, spiders tested earlier inthe season than the previous year did not improve their conditionin response to prey supplementation. Nonetheless, in both years, food-supplementedspiders built significantly smaller webs than food-deprived andcontrol spiders. This result was confirmed in a laboratory experiment whereprey intake was controlled. We conclude that for S. lineatus immediateforaging risks outweigh the potential time constraints on foraging.     

Reproductive success in a socially polymorphic spider: social individuals experience depressed reproductive success in isolation

1. Correlated individual differences in behaviour across ecological contexts, or behavioural syndromes, can theoretically constrain individuals' ability to optimally adjust their behaviour for specific contexts. 2. Female Anelosimus studiosus exhibit a unique behavioural polymorphism: ‘social’ females are tolerant of conspecifics and aggregate in multi‐female colonies, while ‘solitary’ females aggressively defend their singleton webs from intrusion by adult female conspecifics. Previous work found that social females are also less aggressive toward prey and are more fearful of predators. 3. In this study we quantify potential fitness consequences of these correlated behaviours by examining the potential and realised fecundities of the two phenotypes in naturally occurring colonies, and by quantifying their ability to rear offspring as singleton individuals. 4. There were no differences in the fecundities of laboratory‐reared females between the phenotypes, nor were there differences in field‐collected brooding females from naturally occurring solitary and social nests. 5. Brooding females from solitary and social colonies that were isolated in new nests for the growing season were both capable of rearing their broods; however, females from solitary nests had significantly greater success. 6. These results suggest a fitness consequence to the reduced‐aggression syndrome of social females that may represent a general impediment to the evolution of sociality in spiders.     

Factors affecting the reproductive success of the crab spider Misumenoides formosipes: the covariance between juvenile and adult traits

Summary To examine the importance of covariance between stages in traits related to foraging, we quantified the relationships between reproductive success and sizerelated variability in weight gain in juvenile and adult instars of the crab spider Misumenoides formosipes (Araneae: Thomisidae). Prereproductive weight and fecundity are both highly correlated with carapace width, a linear measure of size which does not change within an instar. In field populations, adult females with larger carapaces gain more weight and are more likely to reproduce than females with smaller carapaces. The growth rate of spiders fed ad libitum in the laboratory is unrelated to size, suggesting that size-related differences in the field are due to variation in prey-capture success. Adult females with a carapace width less than 3.4 mm comprised 22% of the population, but were never found to reproduce. Of the individuals that did reproduce, a 17% increase in carapace width resulted in a 100% increase in fecundity. Juvenile stages must be examined to understand adult foraging and reproductive success, because the net weight gained by juvenile instars determines adult size. The final weight gained by spiders in the antepenultimate and penultimate instars explained nearly all the variation in carapace width in the penultimate and adult instars, respectively. We found that constraints on foraging in late juvenile stages are different from the adult stage. Penultimate foraging behavior differs from that of adults, because of constraints on foraging in the period preceding ecdysis. Additionally, in both late juvenile instars, carapace width had little or no effect on the final weight gained within the instar suggesting that factors that affect foraging are different between the juvenile and adult stages. These analyses stress the fact that to fully understand the effects of foraging on reproductive success, we must examine stage-specific constraints throughout an organism's life history.     

Reproductive success ofAmblyseius idaeus andA. anonymus on a diet of two-spotted spider mites

Amblyseius idaeus is a drought-resistant predatory mite of tetranychid spider mites. In this paper, life-history parameters ofA. idaeus and a morphologically similar species,A. anonymus, are measured and compared on a diet ofTetranychus urticae. Intrinsic rates of increase of both phytoseiids are similar, and are comparable to rates of phytoseiids that successfully controlTetranychus species. This makes both species promising candidates as biological control agents,A. idaeus especially under dry conditions.     

Aging and foraging efficiency in an orb-web spider

Aging is often associated with reduced behavioral performance such as decreased locomotion or food consumption, related to a deterioration in physiological functions. In orb-web spiders, webs are used to capture prey and aging can affect web-building behavior and web structure. Here, we investigated the effect of aging on prey capture in the orb-web spider Zygiella x-notata. The ability of adult females to capture flies was examined at different ages. The rate of prey capture did not change with age, but older spiders took more time to subdue and capture the prey. Alterations which appeared in web structure with age (increase in the number of anomalies affecting radii and capture spiral) affected prey capture behavior. Furthermore, the analysis of individual performance (carried out on 17 spiders at two different ages) showed that older females spent more time handling the prey and finding it in the web. Our results suggest that, in the laboratory, age does not affect prey capture rates but it influences prey capture behavior by affecting web structure or/and spider motor functions.     

Reproductive biology of captive and free-ranging spider monkeys

Records from 42 zoos and from long-term studies of wild populations were analysed to describe the reproductive biology of spider monkeys (Ateles spp.). Both data sets suggested that spider monkey females typically have their first infant between 7 and 8 years of age with an interbirth interval of approximately 32–36 months. Infant sex ratio for zoo populations was approximately 1 male to 1 female; infant sex ratios from wild populations were variable. Zoo records provided adequate sample size to suggest that interbirth interval was not influenced by the sex of the infant produced, and that the sex ratio and the probability of infant survival did not change with the number of infants the mother had produced. The findings of this study have implications with respect to the conservation of New World primate species. Since spider monkeys take a long time to reach sexual maturity and their interbirth interval is longer than that expected based on their body size, their populations may be slow to recover following disturbances. Thus, particular care should be taken for the protection of these species.     

The vas deferens of the spider crab,Libinia emarginata

Sperm enter the anterior vas deferens individually in the spider crab male. There they become surrounded by secretion products from the cells of the vas deferens, and are compartmentalized into spermatophores of varying size. The anterior vas deferens can be divided into three regions. The epithelium of the anterior vas deferens varies regionally from low to high columnar. The cytoplasm contains vast arrays of rough endoplasmic reticulum and Golgi complexes but few mitochondria. Intercellular spaces contain septate junctions, gap junctions and vesicles. Once the spermatophores have been formed in the anterior vas deferens, they are moved posteriorly to the middle vas deferens where they are stored and surrounded by seminal fluids. The epithelial cells of the middle vas deferens contain large amounts of rough endoplasmic reticulum and Golgi complexes. Numerous micropinocytotic vesicles appear, forming at the cell surface and within the apical cytoplasm. Their suggested function is the resorption of secretion products of the anterior vas deferens which initiated compartmentalization of the spermatozoa into spermatophores. The posterior vas deferens functions primarily as a storage center for spermatophores until they are released at the time of copulation. Seminal fluid surrounding the spermatophores is produced in this region as well as in the middle vas deferens. The cells of this region contain vast arrays of vesicular rough endoplasmic reticulum and Golgi complexes. The cells are multinucleate. Microtubules are numerous throughout the length of the cells and appear to insert on the plasma membrane.     

Shoal composition determines foraging success in the guppy

The composition of an animal group can impact greatly on thesurvival and success of its individual members. Much recentwork has concentrated on behavioral variation within animalpopulations along the bold/shy continuum. Here, we screenedindividual guppies, Poecilia reticulata, for boldness usingan overhead fright stimulus. We created groups consisting of4 bold individuals (bold shoals), 4 shy individuals (shy shoals),or 2 bold and 2 shy individuals (mixed shoals). The performanceof these different shoal types was then tested in a novel foragingscenario. We found that both bold and mixed shoals approacheda novel feeder in less time than shy shoals. Interestingly,we found that more fish from mixed shoals fed than in eitherbold or shy shoals. We suggest that this can be explained bythe fact that nearly all the cases where one fish was followedinto the feeder by another occurred within mixed shoals andthat it was almost always a shy fish following a bold one. Theseresults suggest clear foraging benefits to shy individuals throughassociating with bold ones. Surprisingly, our results also suggestpotential foraging benefits to bold individuals through associatingwith shy individuals. This study highlights a possible mechanismby which interindividual variation in behavioral types is maintainedin a population.     

Microspectrophotometry of the visual pigment of the spider crab Libinia emarginata

Summary Isolated dark-adapted rhabdoms from the spider crab Libinia emarginata were examined by microspectrophotometry to determine the visual pigments present and their light-sensitive characteristics. The rhabdoms contain a single pigment with _max=493 nm. Upon one minute irradiation with bright orange light this pigment forms a light-stable photoproduct with nearly the same _max as the parent pigment but with slightly greater absorption to the long wavelength side of the absorption peak. On exposure to orange or yellow light in the presence of 5% glutaraldehyde, however, the pigment of Libinia rhabdoms bleaches slowly.The photosensitive pigment of properly oriented, transversely illuminated rhabdoms shows isotropic and dichroic regions, corresponding to layers of the rhabdom in which the microvilli are respectively parallel and perpendicular to the direction of propagation of the measuring beam. The maximum dichroic ratio is about 2, with most absorption when the plane of polarization is parallel to the microvillar axes.

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Zusammenfassung Die isolierten, dunkeladaptierten Rhabdome von Libinia emarginata wurden mikrospektrophotometrisch untersucht, um die Sehfarbstoffe und ihre lichtempfindlichen Eigenschaften zu entdecken. Die Rhabdome enthalten ein einziges Pigment (_max 493 nm). Nach einer Bestrahlungszeit von 1 min mit hellem orangem Licht entsteht ein lichtfestes Photoprodukt mit beinahe dem gleichen _max wie das ursprüngliche Pigment, aber mit etwas größerer Absorption gegen die langwellige Seite des Absorptionsmaximums hin. Das Pigment von Libinia-Rhabdomen bleicht langsam aus, wenn man es orangem oder gelbem Licht in Gegenwart von 5% Glutaraldehyd aussetzt.Werden richtig orientierte Rhabdome von der Seite belichtet, so zeigt das lichtempfindliche Pigment teils Isotropismus teils Dichroismus. Im ersten Fall sind die Mikrovilli parallel zur Richtung des Meßstrahles orientiert, im zweiten Fall stehen sie senkrecht dazu. Das größte Dichroismus-Verhältnis liegt bei 2. Man erhält die größte Absorption, wenn die Polarisationsrichtung parallel zur Achse der Mikrovilli steht.

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This work was supported by U.S.P.H.S. grant NB-03333. Some of the experiments were done at The Marine Biological Laboratory, Woods Hole, Mass.     

Subject: Reproductive effort and success

Research Notes on Avian Biology 1994: Selected Contributions from the 21st International Ornithological CongressGeneral Biology: Reproductive Success

Subject: Reproductive effort and success     

Prey size-distributions and size-specific foraging success of Ambystoma larvae

Summary We examined how prey size-distributions influence size-specific foraing rate and food gain, i.e., food intake scaled to metabolic demands, in Jefferson's and small-mouth salamander larvae. Ambystoma jeffersonianum larvae sampled on 17 dates from a farm pond whose fauna was dominated by macrozooplankton and chironomid larvae were rarely gape-limited, and total volume of food in the stomach (VS) showed only a slight tendency to increase with larval size. Although 15 of 17 correlation coefficients of VS with larval size were positive, only 1 of 17 correlations were statistically significant, and body size explained only 8% of the overall variation in VS. Correlation coefficients of food gain and body size were positive in 9 cases and negative in 8, but only 3 were statistically significant.In contrast, Ambystoma texanum larvae in 42 samples taken from five sites dominated by macrozooplankton as well as relatively large isopods and amphipods were almost always gape-limited, and VS tended to increase markedly with larval size. 40 of 42 correlation coefficients of VS and larval size were positive, and 19 correlations were statistically significant. Body size in turn explained about 35% of the overall variation in VS. Correlation coefficients of food gain and larval size were positive in 32 of 42 samples, and 9 of 10 significant correlations were positive.When food is limiting and prey selection is not limited by gape, smaller larvae may grow as fast or in some cases faster than larger larvae because they are nearly as effective foragers, but have lower metabolic demands. Larger larvae may in turn grow faster than smaller larvae in environments which support a broad size spectrum of prey, particularly when gape limitations are highly disproportionate among size classes. The growth rate of larvae in one size class relative to another depends primarily on the extent to which increased foraging rate compensates for higher energy demands as body size increases. Size-specific foraging rate may in turn be strongly influenced by the prey size-distribution within a habitat. These relationships suggest that relative size is not always a good a priori predictor of exploitative competitive ability.     

Reproductive limits and heterogeneity of male twospotted spider mites

This paper determines reproductive limits and variation in performance of male twospotted spider mites Tetranychus urticae Koch when virgin females are provided ad libitum over the first eight days of adult life. Theree phases of male reproduction were studied: copulations, insemination and reproduction. An average one day old male copulated 15 times, inseminated 15 females, and contributed to 336 daughters. All parameters declined with age. The average male inseminated 70 females and contributed to 1145 daughters in the first days. Variability between males was small for all parameters considered. These data suggest that the number of sperm transferred at each insemination, rather than the number of inseminations, is the limiting step to higher male reproductive output.

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Résumé La reproduction est un processus qui nécessite généralement un mâle et une femelle. Cependant la biologie des populations ne s'est traditionnellement intéressée qu'à la contribution des femelles dans ce processus. Un effet de cet accent sur les femelles est que l'activité reproductrice des mâles a été très ignorée. Cet article examine, dans un contexte démographique, l'activité reproductrice des mâles de T. urticae, acarien arrhénotoque et phytophage. Trois séquences de l'activité reproductrice des mâles ont été examinées: 1) la copulation, 2) l'insémination, 3) la reproduction.En moyenne un mâle de 1 jour a copulé 15 fois, inséminé 15 femelles et a eu 336 filles. Pendant les 8 premiers jours, le mâle moyen a inséminé 70 femelles et a eu 1145 filles. Ces résultats suggèrent que le nombre de spermatozoïdes transférés à chaque insémination, plus que le nombre d'inséminations, est le facteur limitant d'une forte contribution du mâle à la reproduction.

     

Sex-specific mediation of foraging in the shore crab, Carcinus maenas

Experiments were conducted to investigate the sex-specific differences to feeding responses of the shore crab Carcinus maenas throughout the year. Results demonstrate that female shore crabs exhibit stronger feeding responses than males throughout the year with a significantly reduced feeding response in males during the summer months' reproductive season. We also studied the possible function(s) of the moulting hormone, 20-hydroxyecdysone (Crustecdysone) that has been described as a potential female-produced sex pheromone to initiate male reproductive behaviour in a number of crustaceans. We recently presented evidence that for shore crabs this is not the case and now show that the steroid is instead functioning as a sex-specific feeding deterrent protecting the moulting 'soft' female crabs. Whilst male shore crabs were deterred from prey (Mytilus edulis) and synthetic feeding stimulants glycine and taurine when these feeding stimulants were spiked with crustecdysone, intermoult female crabs were significantly less affected and rarely deterred from feeding. This sex specificity of the moulting hormone, in combination with the female sex pheromone, which has no anti-feeding properties, ensures that male crabs mate with soft-shelled, moulted females rather than engage in cannibalism, such as found frequently in cases when soft-shelled females are exposed to intermoult females.     

Reproductive success for social Hymenoptera

An organism's “reproductive success” is a quantity designed to measure the extent to which genes descended from that organism will be represented in distant future generations. A mathematical justification is given for the formulation of reproductive success used in a recent study of the evolution of sex ratio in eusocial Hymenopteran communities. Reproductive success is also defined and evaluated in the context of a more general model for Hymenopteran populations. The expressions for reproductive success of males and females involve the community sex ratio and the proportion of males in the community which are offspring of the queen (rather than of workers).