Spiders fluoresce variably across many taxa

The evolution of fluorescence is largely unexplored, despite the newfound occurrence of this phenomenon in a variety of organisms. We document that spiders fluoresce under ultraviolet illumination, and find that the expression of this trait varies greatly among taxa in this species-rich group. All spiders we examined possess fluorophores in their haemolymph, but bright fluorescence appears to result when a spider sequesters fluorophores in its setae or cuticle. By sampling widely across spider taxa, we determine that fluorescent expression is labile and has evolved multiple times. Moreover, examination of the excitation and emission properties of extracted fluorophores reveals that spiders possess multiple fluorophores and that these differ among some families, indicating that novel fluorophores have evolved during spider diversification. Because many spiders fluoresce in wavelengths visible to their predators and prey (birds and insects), we propose that natural selection imposed by predator-prey interactions may drive the evolution of fluorescence in spiders.     

Can the circadian system of a diurnal and a nocturnal rodent entrain to ultraviolet light?

Spectral measurements of sunlight throughout the day show close correspondence between the timing of above ground activity of the European ground squirrel and the presence of ultraviolet light in the solar spectrum. However, in a standard entrainment experiment ground squirrels show no entrainment to ultraviolet light, while Syrian hamsters do entrain under the same protocol. Presented transmittance spectra for lenses, corneas, and vitreous bodies may explain the different results of the entrainment experiment. We found ultraviolet light transmittance in the colourless hamster lens (50% cut-off at 341 nm), but not in the yellow ground squirrel lens (50% cut-off around 493 nm). Ultraviolet sensitivity in the ground squirrels based upon possible fluorescence mechanisms was not evident. Possible functions of ultraviolet lens filters in diurnal mammals are discussed, and compared with nocturnal mammals and diurnal birds. Species of the latter two groups lack ultraviolet filtering properties of their lenses and their circadian system is known to respond to ultraviolet light, a feature that does not necessarily has to depend on ultraviolet photoreceptors. Although the circadian system of several species responds to ultraviolet light, we argue that the role of ultraviolet light as a natural Zeitgeber is probably limited.     

Structure of eyes in trombidioid mites (Acariformes: Trombidioidea)

The eyes or ocelli of trombidioid mite larvae of Euschoengastia rotundata, Hirszutiella zachvatkini and Camerotrombidium pexatum, and larvae and adults of Platytrombidium fasciatum were studied by means of transmission electron microscopy. These species together with larvae of Odontacarus efferus, Ericotrombidium hasgelum, Walchia chinensis and adult E. rotundata and H. zachvatkini were also studied under scanning electron microscope. The eyes of larvae are not inverted and characterized by an epicuticular lamellar lens. The group of phoreceptor cells with rhabdomeres arranged typically of Chelicerata is underlaid by a pigment cup. The eyes of adult mites are inverted, perikarions of photoreceptor cells are situated between the lens and rhabdomeres; tapetum occupies the space between the pigment cup and rhabdomeres. Sensitivity of eyes to light is similar to that of primary eyes of spiders dwelling on soil surface.     

Immunohistochemical evidence for multiple photosystems in box jellyfish

Cubomedusae (box jellyfish) possess a remarkable visual system with 24 eyes distributed in four sensory structures termed rhopalia. Each rhopalium is equipped with six eyes: two pairs of pigment cup eyes and two unpaired lens eyes. Each eye type probably captures specific features of the visual environment. To investigate whether multiple types of photoreceptor cells are present in the rhopalium, and whether the different eye types possess different types of photoreceptors, we have used immunohistochemistry with a range of vertebrate opsin antibodies to label the photoreceptors, and electroretinograms (ERG) to determine their spectral sensitivity. All photoreceptor cells of the two lens eyes of the box jellyfish Tripedalia cystophora and Carybdea marsupialis displayed immunoreactivity for an antibody directed against the zebrafish ultraviolet (UV) opsin, but not against any of eight other rhodopsin or cone opsin antibodies tested. In neither of the two species were the pigment cup eyes immunoreactive for any of the opsin antibodies. ERG analysis of the Carybdea lower lens eyes demonstrated a single spectral sensitivity maximum at 485 nm suggesting the presence of a single opsin type. Our data demonstrate that the lens eyes of box jellyfish utilize a single opsin and are thus color-blind, and that there is probably a different photopigment in the pigment cup eyes. The results support our hypothesis that the lens eyes and the pigment cup eyes of box jellyfish are involved in different and specific visual tasks.     

Active sensing in a freely walking spider: look where to go

The Central American hunting spider Cupiennius salei, like most other spiders, has eight eyes, one pair of principal eyes and three pairs of secondary eyes. The principal eyes and one pair of the secondary eyes have almost completely overlapping visual fields, and presumably differ in function. The retinae of the principal eyes can be moved independently by two pairs of eye muscles each, whereas the secondary eyes do not have such eye muscles. The behavioural relevance of retinal movements of freely moving spiders was investigated by a novel dual-channel telemetric registration of the eye muscle activities. Walking spiders shifted the ipsilateral retina with respect to the walking direction before, during and after a turning movement. The change in the direction of vision in the ipsilateral anterior median eye was highly correlated with the walking direction, regardless of the actual light conditions. The contralateral retina remained in its resting position. This indicates that Cupiennius salei shifts it visual field in the walking direction not only during but sometimes previous to an intended turn, and therefore “peers” actively into the direction it wants to turn.     

Characterization of fluorescent eye markers for mammalian transgenic studies

Genotyping mice by DNA based methods is both laborious and costly. As an alternative, we systematically examined fluorescent proteins expressed in the lens as transgenic markers for mice. A set of eye markers has been selected such that double and triple transgenic animals can be visually identified and that fluorescence intensity in the eyes can be used to distinguish heterozygous from homozygous mice. Taken together, these eye markers dramatically reduce the time and cost of genotyping transgenics and empower analysis of genetic interaction.     

Regulation of the number of spiders participating in collective prey transport in the social spider Anelosimus eximius (Araneae, Theridiidae)

In an experimental study, mechanisms by which cooperative prey transport is achieved in social spiders were clarified. Factors that could influence the number of individuals that participate in prey transport (prey mass, length and vibration) were investigated. Results show that two factors are fundamental: the vibrations and the prey length. Prey mass did not seem to influence spiders' participation. Thus, the single fact that individuals respond locally to environmental stimuli (intensity of vibration, available site on the prey) explains how spiders cooperate and efficiently capture a wide range of prey types without complex communication systems.     

The accoutrements of spiders' eggs (Araneae) with an exploration of their functional importance

Analysis is presented of the characteristics of spheres coating the chorion surface of the eggs from 41 species belonging to 15 families of spiders. Examples of eggs from the arachnid orders Opiliones, Amblypygi and Uropygi are illustrated for the first time. The spheres are unique, among arachnids, to the Araneae. Their form and size class distributions overlap considerably between families and the spheres are present in all 22 families of Araneae that have been examined; they represent a very conservative morphological feature in the evolution of the Araneae. Evidence of specialized regions of the egg surface (micropyllar areas and plastrons) was found only in an opilionid. As spiders' eggs are laid in silk egg sacs, mostly in clutches, the possible effects of these characteristics are discussed from evidence in the literature and from a functional theoretic stance.     

Ocular changes accompanying eye loss in the spider family uloboridae

In uloborid spiders, eye loss is accompanied by increased visual angles, optical material investment, and potential visual acuity of the retained eyes. Relative to carapace volume, the six-eyed Hyptiotes cavatus and two four-eyed Miagrammopes species have greater retinal hemisphere areas and lens volumes than do the eight-eyed uloborids Waitkera waitkerensis, Uloborus glomosus, and Octonoba sinensis. In Waitkera, in which the eyes have little visual overlap, and in Miagrammopes, in which eye loss simplifies the spiders' patterns of visual overlap, increased retinal cell density enhances potential visual acuity. However, this occurs at the expense of potential retinal cell sensitivity.     

Behavioural evidence of UV sensitivity in jumping spiders (Araneae: Salticidae)

Jumping spiders are known to possess ultraviolet (UV) receptors in the retinas of their large-principal eyes. The existence of UV visual cells, however, does not prove that jumping spiders can see into the UV part of spectrum (300–400 nm) or whether such an ability plays any role in salticid intra-specific interactions. In the study reported herein, we performed behavioural experiments to test whether a UV−reflecting jumping spider, Cosmophasis umbratica, is sensitive to UV wavelengths and whether UV cues are important in intra-specific communication. The absence of UV cues not only affected intra-specific behaviour by significantly reducing the frequency of agonistic displays, but also elicited unprecedented courtship displays in males towards their own mirror images and conspecific opponents. Furthermore, C. umbratica males were able to respond rapidly to changes in UV cues of conspecific mirror images by switching between agonistic and courtship displays. These findings clearly demonstrate that C. umbratica males are capable of seeing UV wavelengths and that UV cues are necessary and sufficient for this species to enable the agonistic displays. Hence, UV light may have an important role to play in intra-specific communication in jumping spiders.     

Photoreceptor spectral sensitivities of the Small White butterfly <Emphasis Type="Italic">Pieris rapae crucivora</Emphasis> interpreted with optical modeling

The compound eye of the Small White butterfly, Pieris rapae crucivora, has four classes of visual pigments, with peak absorption in the ultraviolet, violet, blue and green, but electrophysiological recordings yielded eight photoreceptors classes: an ultraviolet, violet, blue, double-peaked blue, green, blue-suppressed-green, pale-red and deep-red class. These photoreceptor classes were identified in three types of ommatidia, distinguishable by the different eye shine spectra and fluorescence; the latter only being present in the eyes of males. We present here two slightly different optical models that incorporate the various visual pigments, the light-filtering actions of the fluorescent, pale-red and deep-red screening pigment, located inside or adjacent to the rhabdom, and the reflectance spectrum of the tapetum that abuts the rhabdom proximally. The models serve to explain the photoreceptor spectral sensitivities as well as the eye shine.     

Function of being colorful in web spiders: attracting prey or camouflaging oneself?

Bright body colorations of orb-weaving spiders have been hypothesizedto be attractive to insects and thus function to increase foragingsuccess. However, the color signals of these spiders are alsoconsidered to be similar to those of the vegetation background,and thus the colorations function to camouflage the spiders.In this study, we evaluated these 2 hypotheses by field experimentsand by quantifying the spiders' visibility to insects. We firstcompared the insect interception rates of orbs constructed bythe orchid spider, Leucauge magnifica, with and without thespider. Orbs with spiders intercepted significantly more insectsthan orbs without. Such a result supported the prey attractionbut not the camouflaging hypothesis. We then tested whetherbright body colorations were responsible for L. magnifica'sattractiveness to insects by manipulating the spiders' colorsignals with paint. Alteration of color signals significantlyreduced L. magnifica's insect interception and consumption rates,indicating that these spiders' bright body parts were attractiveto insects. Congruent with the finding of field manipulationswere the color contrasts of various body parts of these spiders.When viewed against the vegetation background, the green bodyparts were lower, but the bright parts were significantly higherthan the discrimination threshold. Results of this study thusprovide direct evidence that bright body colorations of orbweavers function as visual lures to attract insects.     

The comparative study of the predatory behaviour of Myrmarachne, ant-like jumping spiders (Araneae: Salticidae)

The predatory behaviour of 31 species of Myrmarachne , ant-like salticids, was studied in the laboratory and the field. The ant-like morphology and locomotion of these spiders appears to function primarily in Batesian mimicry. No evidence was found of Myrmarachne feeding on ants. However, predatory sequences were found to differ considerably from those typical of salticids. Instead of stalking and leaping on prey, Myrmarachne lunged at prey from close range. Myrmarachne used its legs I to tap prey before lunging, another unusual behaviour for a salticid. Myrmarachne fed on a wide range of arthropod prey in nature and the laboratory, but appears to be especially efficient at catching moths. Also, Myrmarachne tends to open up, or enter into, other spiders' nests and eat other spiders' eggs. Myrmarachne males were less efficient than females, in laboratory tests, at catching various types of arthropod prey, but they appear to be as efficient as females at oophagy. Myrmarachne tend to use webs of other spiders as nest sites, but no evidence was found of Myrmarachne preying on spiders in webs. It appears that the unusual features of Myrmarachne's predatory and nesting behaviour are important in enabling these spiders to preserve their ant-like appearance.     

Pupil size in spider eyes is linked to post-ecdysal lens growth

In this study we describe a distinctive pigment ring that appears in spider eyes after ecdysis and successively decreases in size in the days thereafter. Although pigment stops in spider eyes are well known, size variability is, to our knowledge, reported here for the first time. Representative species from three families (Ctenidae, Sparassidae and Lycosidae) are investigated and, for one of these species (Cupiennius salei, Ctenidae), the progressive increase in pupil diameter is monitored. In this species the pupil occupies only a fourth of the total projected lens surface after ecdysis and reaches its final size after approximately ten days. MicroCT images suggest that the decrease of the pigment ring is linked to the growth of the corneal lens after ecdysis. The pigment rings might improve vision in the immature eye by shielding light rays that would otherwise enter the eye via peripheral regions of the cornea, beside the growing crystalline lens.     

Role of the Age Factor in Eye Regeneration in the Gastropod Achatina fulica

The dependence of the ability to regenerate the eye on the age of experimental animals was studied in the snail Achatina fulica. The degree of regeneration was estimated by light-microscopic and electrophysiological methods and by analyzing the motor response to visual stimuli. In older age groups, the number of regenerated eye-bearing tentacles decreased, whereas the period of regeneration increased. The regenerated eyes of the snails operated at the age of more than two months remained smaller than normal eyes even after six months. Regeneration of the distal part of the optic nerve was observed, and the regenerated eyes recovered the ability to respond to stimulation by light. In the electroretinogram, the responses of the regenerated eye, compared to the control, were characterized by lower amplitude and longer repolarization and refractory periods. Manifestations of the motor response to visual stimuli in the young snails with regenerating eyes could be regarded as evidence for the recovery of connection between the organ of sight and the central ganglia.     

Potential predation of non-webbuilding spider assemblage on cotton pests Helicoverpa armigera and Spodoptera littoralis (Lepidoptera: Noctuidae)

The relative feeding rates and preferences of a hunting-spider assemblage inhabiting southern Spanish cotton fields for two major cotton pests, Helicoverpa armigera (Hübner) and Spodoptera littoralis (Boisdubal) (Lepidoptera: Noctuidae), were analyzed under laboratory conditions. First, a no-choice feeding test was used to determine relative feeding rates for hunting-spider families and species, offering a fixed number of 10 neonate larvae of H. armigera or S. littoralis and observing predation after 2 h, 4 h, 8 h, and 24 h. In a second test, Drosophila melanogaster, a very palatable alternative prey, was used to determine the degree of preference for cotton pest larvae. The mean number of first-instar lepidoptera larvae consumed by hunting spiders after 24 h was 8.57±0.25. As expected, spiders showed no preference for either of the two cotton pest species H. armigera and S. littoralis over the other. Results also showed that cursorial spiders of the families Miturigidae (represented here by Cheiracanthium pelasgicum) and Philodromidae consumed significantly higher percentages of larvae than crab spiders belonging to the Thomisidae family after 2 h and 24 h, respectively. In the prey choice test, Cheiracantium pelasgicum displayed a strong preference for cotton pest larvae while Thomisidae and Oxyopidae showed no significant preference. In addition, as the attack sequence progressed, Ch. pelasgicum showed a clear tendency towards the alternation of prey while Thomisidae, and more irregularly Oxyopidae, maintained their preference for D. melanogaster. These findings confirmed both the considerable potential value of some cursorial spiders (e.g. Ch. pelasgicum) in the biological control of lepidopteran cotton pests and the relatively low impact of other hunting spiders, e.g. Thomisidae, on pests of this kind.     

The extracellular products of Anabaena cylindrica Lemm. II. Fluorescent substances containing serine and threonine,and their role in extracellular pigment formation

From aged-culture filtrates of Anabaena cylindrica Lemm., two similar compounds were isolated which fluoresced strongly in ultraviolet light. Hydrolysis of each gave a single different fluorescent moiety, and serine and threonine in a probable ratio of 2 : 1. No carboxy- or amino-terminal amino acid was present. On chromatograms exposed to light, the fluorescent peptides turned yellow-brown. Analysis of the results of this reaction indicated conversion to similar but yellow substances, followed by polymerisation to give a macromolecular brown pigment-peptide, indistinguishable from a similar complex which represents a major extracellular product of the alga.     

Polarizing optics in a spider eye

Many arthropods including insects and spiders exploit skylight polarization for navigation. One of the four eye pairs of the spider Drassodes cupreus is dedicated to detect skylight polarization. These eyes are equipped with a tapetum that strongly plane-polarizes reflected light. This effectively enhances the polarization-sensitivity of the photoreceptors, improving orientation performance. With a multidisciplinary approach, we demonstrate that D. cupreus exploits reflective elements also present in non-polarizing tapetal eyes of other species such as Agelena labyrinthica. By approximately orthogonal arrangement of two multilayer reflectors consisting of reflecting guanine platelets, the tapetum uses the mechanism of polarization by reflection for polarizing reflected light.     

Circadian photoreception: spotlight on the brain

In addition to its visual function, the mammalian eye detects light for a range of behavioral and physiological responses that are separate and apart from sight. Recent studies have begun to shed light on the areas of the brain that respond to such 'non-visual' photoreception in the human eye.