The principal eyes of a jumping spider have a telephoto component

Fine structural correlates of sensitivity in the eyes of the ctenid spider, Cupiennius salei Keys

We studied fine structural correlates of sensitivity in the principal and secondary eyes of the nocturnal hunting spider Cupiennius salei. In night-adapted eyes the four rhabdomeres of the principal eye photoreceptors are 58 microm long and occupy together 234 microm(2) in cross-section (average), whereas the two rhabdomeres of the secondary eye photoreceptors are about 49 microm long and measure 135-183 microm(2) in cross-section (average). The rhabdoms (photosensitive structures) consist of tightly packed microvilli (diameter 0.1 microm, maximum length 3.5 microm) and occupy up to 63% of the cross-sectional area of the retina. When calculating the amount of light the eyes of Cupiennius are able to capture according to their morphological characteristics, the values for sensitivity S(see Land, 1981, 1985) are between 78 and 109 microm(2). Cupiennius is more sensitive than any other hunting spider examined except Dinopis whose posterior median eyes are the most sensitive ones of all terrestrial arthropod eyes studied. In day-adapted eyes the rhabdomeral microvilli are almost completely degraded. The remaining microvillar surface amounts to only about one-tenth compared with the night-adapted state. Efferent synaptoid terminals have been found to contact the photoreceptors in all eyes of C. salei. The present fine structural data are compared to previous electrophysiological research and underline the significance of vision in Cupiennius.

Eye evolution: lens and cornea as an upgrade of animal visual system

Lens-containing eyes are a feature of surprisingly broad spectrum of organisms across the animal kingdom that represent a significant improvement of simple eye composed of just photoreceptor cells and pigment cells. It is apparent that such an upgrade of animal visual system has originated numerous times during evolution since many distinct strategies to enhance light refraction through the use of lens and cornea have been utilized. In addition to having an ancient role in prototypical eye formation Pax transcription factors were convergently recruited for regulation of structurally diverse crystallins and genes affecting morphogenesis of various lens-containing eyes.

Convergent evolution of eye ultrastructure and divergent evolution of vision-mediated predatory behaviour in jumping spiders

All jumping spiders have unique, complex eyes with exceptional spatial acuity and some of the most elaborate vision-guided predatory strategies ever documented for any animal of their size. However, it is only recently that phylogenetic techniques have been used to reconstruct the relationships and key evolutionary events within the Salticidae. Here, we used data for 35 species and six genes (4.8 kb) for reconstructing the phylogenetic relationships between Spartaeinae, Lyssomaninae and Salticoida. We document a remarkable case of morphological convergence of eye ultrastructure in two clades with divergent predatory behaviour. We, furthermore, find evidence for a stepwise, gradual evolution of a complex predatory strategy. Divergent predatory behaviour ranges from cursorial hunting to building prey-catching webs and araneophagy with web invasion and aggressive mimicry. Web invasion and aggressive mimicry evolved once from an ancestral spartaeine that was already araneophagic and had no difficulty entering webs due to glue immunity. Web invasion and aggressive mimicry was lost once, in Paracyrba, which has replaced one highly specialized predation strategy with another (hunting mosquitoes). In contrast to the evolution of divergent behaviour, eyes with similarly high spatial acuity and ultrastructural design evolved convergently in the Salticoida and in Portia.

Cross-modality effects during male-male interactions of jumping spiders

An important prediction from game theory is that the value of a resource influences the level to which male-male conflict escalates. Earlier experimental studies have shown that the seven salticid species we study here (Bavia aericeps, Euryattus sp., Hypoblemum albovittatum, Jacksonoides queenslandicus, Marpissa marina, Portia africana and Simaetha paetula) determine by sight whether a female is a conspecific or a heterospecific and then escalate the intensity with which they interact (i.e., they adopt behaviour that is likely to put them at greater risk of injury after detecting the presence of a conspecific female). Here the earlier studies are extended by using the odour of conspecific females (experimental tests) and heterospecific females (control tests), and by presenting each male with his mirror image as well as having two males interact with each other. Findings from this study suggest that, for J. queenslandicus and P. africana, the odour of conspecific females, more than the odour of heterospecific females, primes the male to escalate conflict with a potential rival. However, this was not found for the other five species tested.

A predator from East Africa that chooses malaria vectors as preferred prey

Background

All vectors of human malaria, a disease responsible for more than one million deaths per year, are female mosquitoes from the genus Anopheles. Evarcha culicivora is an East African jumping spider (Salticidae) that feeds indirectly on vertebrate blood by selecting blood-carrying female mosquitoes as preferred prey.

Methodology/Principal Findings

By testing with motionless lures made from mounting dead insects in lifelike posture on cork discs, we show that E. culicivora selects Anopheles mosquitoes in preference to other mosquitoes and that this predator can identify Anopheles by static appearance alone. Tests using active (grooming) virtual mosquitoes rendered in 3-D animation show that Anopheles' characteristic resting posture is an important prey-choice cue for E. culicivora. Expression of the spider's preference for Anopheles varies with the spider's size, varies with its prior feeding condition and is independent of the spider gaining a blood meal.

Conclusions/Significance

This is the first experimental study to show that a predator of any type actively chooses Anopheles as preferred prey, suggesting that specialized predators having a role in the biological control of disease vectors is a realistic possibility.

A spider that feeds indirectly on vertebrate blood by choosing female mosquitoes as prey

Spiders do not feed directly on vertebrate blood, but a small East African jumping spider (Salticidae), Evarcha culicivora, feeds indirectly on vertebrate blood by choosing as preferred prey female mosquitoes that have had recent blood meals. Experiments show that this spider can identify its preferred prey by sight alone and by odor alone. When presented with two types of size-matched motionless lures, E. culicivora consistently chose blood-fed female mosquitoes in preference to nonmosquito prey, male mosquitoes, and sugar-fed female mosquitoes (i.e., females that had not been feeding on blood). When the choice was between mosquitoes of different sizes (both blood- or both sugar-fed), small juveniles chose the smaller prey, whereas adults and larger juveniles chose the larger prey. However, preference for blood took precedence over preference for size (i.e., to get a blood meal, small individuals took prey that were larger than the preferred size, and larger individuals took prey that were smaller than the preferred size). When presented with odor from two prey types, E. culicivora approached the odor from blood-fed female mosquitoes significantly more often the odor of the prey that were not carrying blood.

Sexual selection research on spiders: progress and biases

The renaissance of interest in sexual selection during the last decades has fuelled an extraordinary increase of scientific papers on the subject in spiders. Research has focused both on the process of sexual selection itself, for example on the signals and various modalities involved, and on the patterns, that is the outcome of mate choice and competition depending on certain parameters. Sexual selection has most clearly been demonstrated in cases involving visual and acoustical signals but most spiders are myopic and mute, relying rather on vibrations, chemical and tactile stimuli. This review argues that research has been biased towards modalities that are relatively easily accessible to the human observer. Circumstantial and comparative evidence indicates that sexual selection working via substrate-borne vibrations and tactile as well as chemical stimuli may be common and widespread in spiders. Pattern-oriented research has focused on several phenomena for which spiders offer excellent model objects, like sexual size dimorphism, nuptial feeding, sexual cannibalism, and sperm competition. The accumulating evidence argues for a highly complex set of explanations for seemingly uniform patterns like size dimorphism and sexual cannibalism. Sexual selection appears involved as well as natural selection and mechanisms that are adaptive in other contexts only. Sperm competition has resulted in a plethora of morphological and behavioural adaptations, and simplistic models like those linking reproductive morphology with behaviour and sperm priority patterns in a straightforward way are being replaced by complex models involving an array of parameters. Male mating costs are increasingly being documented in spiders, and sexual selection by male mate choice is discussed as a potential result. Research on sexual selection in spiders has come a long way since Darwin, whose spider examples are reanalysed in the context of contemporary knowledge, but the same biases and methodological constraints have persisted almost unchanged through the current boom of research.

One-encounter search-image formation by araneophagic spiders

An experimental study of search-image use by araneophagic jumping spiders (i.e., salticid spiders that prey routinely on other spiders) supports five conclusions. First, araneophagic salticids have an innate predisposition to form search images for specific prey from their preferred prey category (spiders) rather than for prey from a non-preferred category (insects). Second, single encounters are sufficient for forming search images. Third, search images are based on selective attention specifically to optical cues. Fourth, there are trade-offs in attention during search-image use (i.e., forming a search image for one type of spider diminishes the araneophagic salticid's attention to other spiders). Fifth, the araneophagic salticid's adoption of search images is costly to the prey (i.e., when the araneophagic salticid adopts a search, the prey's prospects for surviving encounters with the araneophagic salticid are diminished). Cognitive and ecological implications of search-image use are discussed.

Rearing environment affects behaviour of jumping spiders

We tested the effect of rearing conditions on the behaviour of jumping spiders, Phidippus audax. Spiders were assigned randomly to either small or large cages that either were empty or contained a painted dowel. Laboratory-reared spiders were raised from second instar to adult in these environments. Field-caught adults also were randomly assigned to these containers and were held for approximately 4 months prior to testing. We presented spiders with three tests designed to examine a range of behaviours. Field-caught spiders were more likely than laboratory-reared spiders to (1) react to videotaped prey, (2) progress further on a detour test, and (3) be less stereotactic and more active in an open field. Larger cage size and the presence of the dowel also improved performance in several tests. Our results suggest that the rearing conditions we used, which are commonly employed by behavioural researchers, may profoundly influence the behaviour of adult spiders. Copyright 2000 The Association for the Study of Animal Behaviour.

Scanning and route selection in the jumping spider Portia labiata

Jumping spiders Portia labiata were tested in the laboratory on three different kinds of detours. In one, both routes led to the lure. In the other variants, one of the routes had a gap, making that route impassable. When tested with only one complete route, Portia chose this route after visually inspecting both routes. An analysis of scanning showed that, at the beginning of the scanning routine, the spiders scanned both the complete and the incomplete route but that, by the end of the scanning routine, they predominantly scanned only the complete route. Two rules seemed to govern their scanning: (1) they would continue turning in one direction when scanning away from the lure along horizontal features of the detour route; and (2) when the end of the horizontal feature being scanned was reached, they would change direction and turn back towards the lure. These rules 'channelled' the spiders' scanning on to the complete route, and they then overwhelmingly chose to head towards the route they had fixated most while scanning. Copyright 1999 The Association for the Study of Animal Behaviour.

Post-embryonic development of the principal retina of a jumping spider. II. The acquisition and reorganization of rhabdomeres and growth of the glial matrix

An accompanying paper by Blest ( Phil. Trans. R. Soc. Lond. B 320, 489 (1988 b )) describes the conformational changes undergone by principal retinae of a salticid spider during post-embryonic development, and identifies the origins of the tiered receptor mosaics. We now discuss the ultrastructural morphogenesis of the individual receptive segments, with particular reference to the growth of microvilli, to the achievement of light-guide properties by rhabdomeres of the foveal mosaic of Layer I, and to putative mechanisms that ensure that inter-receptor spacings are optically apt. 1. Establishment of the tiered retina by conformational changes precedes the first appearance of plasmalemmal microvilli in all four Layers of receptive segments. Microvilli are first differentiated in the ventral retina. 2. Short, irregular microvilli, initially interdigitated, appear on the plasmalemmae of receptive segments around day 10 of post-embryonic development. They progressively lengthen, cease to be inter-digitated, and become more regular throughout subsequent development, during the moult to the second instar at day 12, and until day 15 when spiderlings become fully independent and principal retinae attain their final conformation. 3. Until late in development, after the moult to the second instar, foveal Layer I receptive segments are contiguous, and densely occupied by organelles. The six glial strands that flank each receptive segment are prominent. A transformation between days 14 and 15 variously reduces or eliminates the glial columns, and, by endocytosis, the plasmalemmal areas of the receptive segments. The Layer I cytoplasm is stripped of organelles other than microtubules. Consequently, receptive segments become separated from each other, and their rhabdomeres can perform as light-guides. 4. Over the same period, receptive segments of Layers II-IV slowly acquire the dense population of mitochondria that have been shown to equilibrate the refractive indices of rhabdomeres and their surrounds. 5. Final conformational changes to the retinae and the establishment of definitive spacings between receptive segments seem to be assured by the differential growth and local involution of the finely divided glial processes intercalated between them. Despite some caveats, it is argued that this protracted sequence of events is consistent with a limited degree of ontogenetic recapitulation. In particular, it is remarkable that the early states of the nascent microvilli so closely resemble those found in less advanced families of spiders.

Retinal mosaics of the principal eyes of two primitive jumping spiders, Yaginumanis and Lyssomanes : clues to the evolution of Salticid vision

Retinae of the principal eyes of jumping spiders (Salticidae) have four tiers of photoreceptors. A previous paper described the receptor mosaics in all four layers at the retinal fovea for two forms, Lagnus and Portia , shown by behavioural experiments to possess high visual acuities. Only Layer I farthest from the dioptric apparatus has a mosaic quality that can sustain the demonstrated visual discriminations, leaving the roles of Layers II—III open to question. We now describe the retinal mosaics of two presumptively primitive species Yaginumanis is placed with Portia in a newly erected subfamily of Salticidae, the Spartaeinae, whose members possess functional posterior median eyes; those of Lyssomanes are vestigial as in advanced Salticids, but the genus is usually considered primitive and its ethology indicates restricted visual capabilities. Layer I receptors of Yaginumanis each bear two rhabdomeres on opposite faces of the cell which are contiguous with those of adjacent receptors, so that optical pooling substantially limits acuity. The same arrangement is present in receptors in the peripheral retina of Lyssomanes but at the fovea each cell bears only a single rhabdomere, as in Salticids with high visual acuities. In both species, Layer II receptors have two rhabdomeres throughout the retina and centre-to-centre spacings that approximately match those of the Layer I receptors that they overlie. Layers III and IV are essentially the same in all Salticids: Layer III amounts to a virtually continuous sheet of rhabdomeres, and Layer IV is divided into three regions with a complex organisation. Neither is suitable to sustain fine visual discriminations. The organization of the salticid principal retina emerges as anatomically conservative, and contrasts with that of the secondary eyes which varies greatly between primitive and advanced species. A working hypothesis is proposed to explain its evolution.

Post-embryonic development of the principal retina of a jumping spider. I. The establishment of receptor tiering by conformational changes

The tiered principal retinae of salticid spiders are unique. No other arthropod ocellar retina resembles them anatomically, or sustains equivalently fine spatial acuities. The phylogeny of spiders guarantees that they evolved from far simpler, untiered retinae. The obscure systematic affiliations of the Salticidae preclude a search for extant forms that might offer precursors of the tiered retina, and it is probable that no such forms now exist. We examine the post-embryonic morphogenesis of a salticid principal retina, making a heuristic assumption that the concept of the ontogenetic recapitulation of phylogeny (Haeckel’s 'Law of Recapitulation', now in a literal sense discredited) can, cautiously applied, either (1) indicate how the retina might have evolved, or (2) exclude some arbitrary models of its evolution. By day 3 of post-embryonic development, a presumptive principal retina is established as a simple hemisphere of primordial receptive segments, to which new receptive segments are inferred to be still in the process of being added at the equator. At around days 4-5, the hemisphere of receptive segments starts rapidly to narrow. By around day 10, horizontal narrowing has transformed the initial hemisphere to an approximation of the mature, dorsoventrally aligned, boomerang-shaped retina. At day 12, spiderlings moult to the second instar, but conformational changes to the retina continue until day 15, when spiders leave their brood sacs and become independent. The major conformational changes imply that distal receptive segments (e.g. those of Layers III and IV) are derived from those that initially lie at or close to the equator of the primordial hemisphere; and that foveal Layer I receptive segments remain at the retinal pole. Extreme horizontal narrowing ensures that equatorial receptive segments come to overlie those at the retinal pole, or at least near to it. These findings exclude a hypothesis that receptors of different spectral sensitivities were randomly distributed over a primitive retinal hemisphere, and that tiering was generated during phylogeny simply by their differential migration along lines parallel to the optical axis. Nevertheless, limited local migrations of nascent receptive segments along such lines are inferred to contribute to the final pattern of tiering.

Direct observation of receptors and images in simple and compound eyes

The relation between the quality of the optical image and the fineness of the retinal mosaic has been studied in eyes of three different optical types: the simple eyes of spiders, the superposition compound eyes of moths, and the apposition compound eyes of butterflies. In all three it is possible to observe both the receptor mosaic and the image in the living eye, using appropriate ophthalmoscopic techniques. Whereas in humans the retinal sampling frequency approaches the optical cut-off frequency quite closely, in diurnal insects of both types the image is undersampled by a factor of 2-3, and in crepuscular spiders this factor may be greater than 100. Reasons for these differences are discussed.