Color, activity period, and eye structure in four lineages of ants: Pale, nocturnal species have evolved larger eyes and larger facets than their dark, diurnal congeners.

The eyes of insects display an incredible diversity of adaptations to enhance vision across the gamut of light levels that they experience. One commonly studied contrast is the difference in eye structure between nocturnal and diurnal species, with nocturnal species typically having features that enhance eye sensitivity such as larger eyes, larger eye facets, and larger ocelli. In this study, we compared eye structure between workers of closely related nocturnal and diurnal above ground foraging ant species (Hymenoptera: Formicidae) in four genera (Myrmecocystus, Aphaenogaster, Temnothorax, Veromessor). In all four genera, nocturnal species tend to have little cuticular pigment (pale), while diurnal species are heavily pigmented (dark), hence we could use cuticle coloration as a surrogate for activity pattern. Across three genera (Myrmecocystus, Aphaenogaster, Temnothorax), pale species, as expected for nocturnally active animals, had larger eyes, larger facet diameters, and larger visual spans compared to their dark, more day active congeners. This same pattern occurred for one pale species of Veromessor, but not the other. There were no consistent differences between nocturnal and diurnal species in interommatidial angles and eye parameters both within and among genera. Hence, the evolution of eye features that enhance sensitivity in low light levels do not appear to have consistent correlated effects on features related to visual acuity. A survey across several additional ant genera found numerous other pale species with enlarged eyes, suggesting these traits evolved multiple times within and across genera. We also compared the size of the anterior ocellus in workers of pale versus dark species of Myrmecocystus. In species with larger workers, the anterior ocellus was smaller in pale than in dark species, but this difference mostly disappeared for species with smaller workers. Presence of the anterior ocellus also was size-dependent in the two largest pale species.

Variation in predator response to short-wavelength warning coloration.

Warning coloration deters predators from attacking unpalatable prey, and is often characterized by long-wavelength colors, such as orange and red. However, warning colors in nature are more diverse and include short-wavelength colors, like blue. Blue has evolved as a primary defense in some animals but is not common. One hypothesis for the maintenance of this diversity is interspecific variation in predator responses to signals. We tested this hypothesis with galliform birds: Gambel's quail (Callipepla gambelii) and two domestic chicken breeds (Gallus gallus domesticus; Plymouth Rocks, Cochin Bantams). We measured innate avoidance and learning responses to only blue prey, only orange prey, and orange-and-blue prey, where the blue was iridescent to represent the natural coloration of the pipevine swallowtail butterfly (Battus philenor). We predicted birds would have similar responses to orange, but vary in response to blue. Upon first encounter, Cochin Bantams did not attack blue and Gambel's quail readily attacked, indicating innate avoidance by Cochin Bantams. Plymouth Rocks had no innate aversion to any color, lower attack latencies and attacked most prey items. Cochin Bantams and Gambel's quail both learned orange and orange-and-blue quicker than blue. Our results support the hypothesis that interspecific variation in predator response could maintain warning color diversity.

Environment-dependent attack rates of cryptic and aposematic butterflies.

Many organisms have evolved adaptive coloration that reduces their risk of predation. Cryptic coloration reduces the likelihood of detection/recognition by potential predators, while warning or aposematic coloration advertises unprofitability and thereby reduces the likelihood of attack. Although some studies show that aposematic coloration functions better at decreasing attack rate than crypsis, recent work has suggested and demonstrated that crypsis and aposematism are both successful strategies for avoiding predation. Furthermore, the visual environment (e.g., ambient lighting, background) affects the ability for predators to detect prey. We investigated these 2 related hypotheses using 2 well-known visually aposematic species of Heliconius butterflies, which occupy different habitats (open-canopy vs. closed-canopy), and one palatable, cryptic, generalist species Junonia coenia. We tested if the differently colored butterflies differ in attack rates by placing plasticine models of each of the 3 species in 2 different tropical habitats where the butterflies naturally occur: disturbed, open-canopy habitat and forested, closed-canopy habitat. The cryptic model had fewer attacks than one of the aposematic models. Predation rates differed between the 2 habitats, with the open habitat having much higher predation. However, we did not find an interaction between species and habitat type, which is perplexing due to the different aposematic phenotypes naturally occurring in different habitats. Our findings suggest that during the Panamanian dry season avian predation on perched butterflies is not a leading cause in habitat segregation between the 2 aposematic species and demonstrate that cryptically colored animals at rest may be better than aposematic prey at avoiding avian attacks in certain environments.

Multiple spectral channels in branchiopods. I. Vision in dim light and neural correlates.

Animals that have true color vision possess several spectral classes of photoreceptors. Pancrustaceans (Hexapoda+Crustacea) that integrate spectral information about their reconstructed visual world do so from photoreceptor terminals supplying their second optic neuropils, with subsequent participation of the third (lobula) and deeper centers (optic foci). Here, we describe experiments and correlative neural arrangements underlying convergent visual pathways in two species of branchiopod crustaceans that have to cope with a broad range of spectral ambience and illuminance in ephemeral pools, yet possess just two optic neuropils, the lamina and the optic tectum. Electroretinographic recordings and multimodel inference based on modeled spectral absorptance were used to identify the most likely number of spectral photoreceptor classes in their compound eyes. Recordings from the retina provide support for four color channels. Neuroanatomical observations resolve arrangements in their laminas that suggest signal summation at low light intensities, incorporating chromatic channels. Neuroanatomical observations demonstrate that spatial summation in the lamina of the two species are mediated by quite different mechanisms, both of which allow signals from several ommatidia to be pooled at single lamina monopolar cells. We propose that such summation provides sufficient signal for vision at intensities equivalent to those experienced by insects in terrestrial habitats under dim starlight. Our findings suggest that despite the absence of optic lobe neuropils necessary for spectral discrimination utilized by true color vision, four spectral photoreceptor classes have been maintained in Branchiopoda for vision at very low light intensities at variable ambient wavelengths that typify conditions in ephemeral freshwater habitats.

Multiple spectral channels in branchiopods. II. Role in light-dependent behavior and natural light environments.

Light is a primary environmental factor used by aquatic invertebrates for depth selection behavior. Many branchiopod crustaceans live in ephemeral aquatic habitats. All branchiopod crustaceans studied to date express four or more visual opsins in their compound eyes. We asked whether two branchiopods, Triops longicaudatus and Streptocephalus mackini, use multiple spectral channels to regulate their position in the water column. At the lowest intensities that elicited photonegative behavior, both species had broad spectral bandwidths, suggesting they use multiple spectral photoreceptor classes. Male S. mackini were more likely to maintain a vertical position 8.0-12.0 cm below the surface than females, independently of whether females were present. Male photopositive behavior at low intensity was restricted to a narrow bandwidth centered at 532 nm, suggesting a single photoreceptor class is used to maintain position above females. We compared ephemeral pools from two regions in Arizona and found that diffuse light attenuation coefficients were two orders of magnitude greater than the most heavily attenuating coastal waters. At less than 1 m of depth, pools were often dimmer than terrestrial habitats under starlight. Soil particle size distribution in each region affected spectral light environments, and behavioral responses of field-caught shrimp were adapted to the spectral properties of their region. The results suggest that branchiopods predominantly use luminance vision summed from multiple spectral photoreceptor classes for depth selection in dim, spectrally variable environments. The neuroanatomical basis for summation is described in a companion paper.

Indicators of recent mating success in the pipevine swallowtail butterfly (Battus philenor) and their relationship to male phenotype.

A key determinant of the intensity of sexual selection acting on a trait is how variation in that trait is related to variance in reproductive success of individuals. This connection compels efforts to assess lifetime mating number and how it varies among individuals in a population. In the Lepidoptera, female mating success can be assessed relatively easily by counting by the number of spermatophores in the female's copulatory sac but male mating success in the field can often only be documented by observing copulations. Here we report a method for identifying whether or not males have recently mated that relies on the effect of mating on the appearance of the male's reproductive tract in the pipevine swallowtail, Battus philenor. In this species laboratory experiments reveal that during mating, components of a male's reproductive tract become shorter, decrease in mass, and change in appearance, irrespective of male age. These changes persist for at least two days after mating. After documenting these indicators of recent mating, we examined the reproductive tract of 68 field-caught males and found that twelve (17.6%) showed strong evidence of having mated recently. We found that older males were more likely to have recently mated. In addition, the color of the dorsal hindwing, a feature that females use in mate choice, was significantly greener in males, that according to our criteria, had recently-mated than in males that had not.

Male wing color properties predict the size of nuptial gifts given during mating in the Pipevine Swallowtail butterfly (Battus philenor).

In many animals, males bear bright ornamental color patches that may signal both the direct and indirect benefits that a female might accrue from mating with him. Here we test whether male coloration in the Pipevine Swallowtail butterfly, Battus philenor, predicts two potential direct benefits for females: brief copulation duration and the quantity of materials the male passes to the female during mating. In this species, males have a bright iridescent blue field on the dorsal hindwing surface, while females have little or no dorsal iridescence. Females preferentially mate with males who display a bright and highly chromatic blue field on their dorsal hindwing. In this study, we show that the chroma of the blue field on the male dorsal hindwing and male body size (forewing length) significantly predict the mass of material or spermatophore that a male forms within the female's copulatory sac during mating. We also found that spermatophore mass correlated negatively with copulation duration, but that color variables did not significantly predict this potential direct benefit. These results suggest that females may enhance the material benefits they receive during mating by mating with males based on the coloration of their dorsal hindwing.

Warning color changes in response to food deprivation in the pipevine swallowtail butterfly, Battus philenor.

Predation on distasteful animals should favor warning coloration that is relatively conspicuous and phenotypically invariable. However, even among similarly colored individuals there can be variation in their warning signals. In butterflies, individual differences in larval feeding history could cause this variation. The warning signal of the pipevine swallowtail butterfly, Battus philenor L. (Lepidoptera: Papilionidae) consists of both a blue iridescent patch and pigmentbased orange spots on the ventral hindwing. B. philenor males also display a dorsal surface iridescent patch that functions as a sexual indicator signal. A previous study of iridescence in B. philenor found that the iridescent blue on both the dorsal and ventral hind wings is variable and significantly different between lab-reared and field-caught individuals. These differences could be the result of larval food deprivation in the field. Through experimental manipulation of larval diet, larval food deprivation was evaluated as a potential cause of the differences observed between lab and field individuals, and if food deprivation is a source of inter-individual variation in warning signals. B. philenor larvae were food restricted starting at two points in the last larval instar, and one group was fed through pupation. Adult coloration was then compared. Food deprivation led to poorer adult condition, as indicated by lower adult body mass, forewing length, and fat content of stressed individuals. As the level of food deprivation increased, the hue of the iridescent patches on both the dorsal and ventral hind wing shifted to shorter wavelengths, and the chroma of the orange spots decreased. The shifts in iridescent color did not match the differences previously found between lab and field individuals. However, the treatment differences indicate that food deprivation may be a significant source of warning color variation. The differences between the treatment groups are likely detectable by predators, but the effect of the variation on signal effectiveness and function remains to be empirically explored.

Variation in compound eye structure: effects of diet and family.

Studies of compound eyes have revealed that variation in eye structure can substantially affect visual performance. Here, we investigate the degree to which a stressful rearing environment, which decreases body size, affects the eye phenotype. Full siblings of the Orange Sulphur butterfly, Colias eurytheme, were collected from known parents and split within families among two diet treatments that varied in quality. In both sexes, individuals reared on the high-quality diet had larger eye height and anterior facet diameter, and therefore, by inference, superior vision. However, relative to their reduced body size, individuals reared on low-quality diet had proportionally larger eyes and facets than individuals reared on high-quality diet. We interpret this finding as evidence that butterflies encountering nutritional stress increased proportional investment in eye development to reduce loss of visual performance. We also found significant broad-sense genetic variation underlying eye structure in both males and females, and report novel heritability estimates for eye height and facet diameter. Surprisingly, there was greater genetic variation in eye height among males than among females, despite apparently stronger directional selection on male vision. We discuss the implications of these data for our understanding of eye development and evolution.

In the eyes of the beholders: Female choice and avian predation risk associated with an exaggerated male butterfly color.

Color ornaments are often viewed as products of countervailing sexual and natural selection, because more colorful, more attractive individuals may also be more conspicuous to predators. However, while evidence for such countervailing selection exists for vertebrate color ornaments (e.g., Trinidadian guppies), similar studies have yet to be reported in invertebrates. Indeed, evidence for female mate choice based on extant variation in male coloration is limited in invertebrates, and researchers have not explicitly asked whether more attractive males are also more conspicuous to predators. Here we provide evidence that more chromatic male cabbage white butterflies (Pieris rapae) are more attractive to females but should also be more conspicuous to predators. Female P. rapae preferentially mate with more chromatic males when choosing from populations of males with naturally occurring or commensurate, experimentally induced color variation. Mathematical models of female color vision confirm that females should be able to discriminate color differences between prospective mates. Further, chromatic and luminance contrast scores from female visual system models better predicted male mating success than did measures of male color derived more directly from color spectra. Last, models of avian color vision suggest that preferred males should be more conspicuous to known avian predators.

Comment on "Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators".

Whitney et al. (Reports, 2 January 2009, p. 130) investigated the mechanism of iridescence in hibiscus and tulip flowers and suggested that bumblebees are able to use this iridescence as a pollination cue. However, their study failed to isolate iridescence from other coincident visual cues, leaving open questions regarding the importance of iridescent stimuli in foraging-based associative learning in bumblebees.

Iridescence: views from many angles.

Iridescent colours have been fascinating to humans throughout history; they are flashy, shimmering, dynamic, and examples surround us, from the commonly seen iridescent sheen of oily street puddles to the exotic, gaudy displays of birds-of-paradise featured in nature documentaries. Iridescent colours and the structures that produce them have unique properties in comparison with other types of colourants found in nature. Scientists from a variety of disciplines study the optics, development, heritability, chemical make-up, origin, evolution, functions and biomimetic technological applications of naturally occurring iridescent colours. For the first time, graduate students at Arizona State University brought together these scientists, along with educators and artists, at 'Iridescence: more than meets the eye', a conference to promote interdisciplinary communication and collaboration in the study of iridescent coloration from all of these perspectives. Here, we summarize the outcomes of this conference, introduce the papers that follow in this special journal issue and briefly review the current status of our understanding of iridescence.

Diversity in structural ultraviolet coloration among female sulphur butterflies (Coliadinae, Pieridae).

In some species of sulphur butterflies (Pieridae: Coliadinae) females as well as males display bright structural reflectance on their dorsal wing surfaces, although comparatively little attention has been paid to this coloration in females. We examined the spectral properties of female dorsal coloration and scale structure in three species of sulphurs for which published images show bright UV reflectance in females: the Neotropical Anteos clorinde and two species of Indo-Australian Eurema, E. hecabe and E. candida. In A. clorinde and E. hecabe, female UV reflectance is iridescent and produced by thin film interference in a system of ridges and lamellae, as it is in conspecific males. Female A. clorinde exhibit the same spatial distribution and chromaticity of UV reflectance as seen in males, but the UV reflectance in female E. hecabe is much smaller in area compared to that of conspecific males and is both less bright and less chromatic than observed in males. In contrast, UV reflectance in E. candida females is diffuse, and arises from a lack of pterin pigments in the wings, which permits a broad-band scattered reflection to be seen. This is the mechanism that is known to produce bright UV reflectance in females of the confamilial whites. Our results highlight the diversity of UV reflectances and underlying mechanisms in sulphurs and suggest multiple evolutionary pathways leading to this diversity in female sulphur butterflies.

Condition dependence, quantitative genetics, and the potential signal content of iridescent ultraviolet butterfly coloration.

Structural colors result from an interaction between light and the fine-scale physical structure of a surface, and are often extremely bright, chromatic, and iridescent. Given that these visual features depend upon the aggregate abundance and architectural precision of photonic structures, structurally colored sexual ornaments seem well placed to indicate a range of mate quality characteristics. We tested this hypothesis by investigating the signaling potential of structural coloration in the sexually dimorphic butterfly Colias eurytheme. Males of this species display iridescent ultraviolet (UV) markings (arising from multilayer thin films) that overlay a broad area of yellowish-orange pigmentation on their dorsal wing surface. Only the structural UV has demonstrated function as a sexual signal; hence we predicted that it should contain more reliable phenotypic and/or genetic quality information, which would be indicated by phenotypic and/or genetically mediated condition dependence. In two split-family breeding experiments we manipulated condition by exposing full siblings to different stressors at two different juvenile life-history stages: (1) reduced larval host-plant quality and (2) transient heat/cold shocks during metamorphosis. Both stressors had profound effects on key developmental and life-history traits. Each stressor also significantly affected male dorsal coloration; thus, the expression of both structural and pigmentary coloration is phenotypically condition dependent. As predicted, the strongest condition dependence was evident in the brightness and angular visibility (i.e., iridescence) of the UV. Characteristics of both the iridescent UV and pigmentary orange also exhibited moderate-high and significant heritability (H(2) approximately h(2) approximately 0.4-0.9). However, genetic and residual variances did not increase under stress; thus, the observed condition dependence was not genetically mediated as predicted if wing color trait signals "good" genes for the ability to either withstand or circumvent developmental stress. The heightened stress sensitivity of the iridescent UV suggests that it offers an informative lifetime indicator of juvenile environments and, henceforth, adult male phenotypic condition, which may be salient to females seeking a highly fertile and/or nutritious male ejaculate.

The eyes of a patrolling butterfly: visual field and eye structure in the Orange Sulphur, Colias eurytheme (Lepidoptera, Pieridae).

Sensory information plays a critical role in determining an animal's behavior on both proximate and evolutionary timescales. Butterflies, like many other insects, use vision extensively over their lifetimes, and yet relatively little work has been published to date on their visual capabilities. We describe the visual system of a pierid butterfly, Colias eurytheme, with the ultimate goal of better understanding its role in shaping the behavior of this animal. We made several measurements: visual field dimensions, eye surface area, interommatidial angle (Deltaphi), facet diameter (D), and eye parameter (p). C. eurytheme had a large visual field and considerable regional variation in visual acuity, as inferred by Deltaphi and D. When compared to females, males had larger eye surface areas, smaller Deltaphi, and larger D in all regions except ventrally. Both sexes had proportionally large eye surface areas compared to other butterflies. Minimum p in males was small, indicating that some regions of their eyes may operate close to the diffraction limit. Finally, we found that both eye surface area and D scaled positively, but with negative allometry to body size. We discuss the relevance of these visual characteristics to the biology and behavior of C. eurytheme.

Visual field structure in the Empress Leilia, Asterocampa leilia (Lepidoptera, Nymphalidae): dimensions and regional variation in acuity.

Male Empress Leilia butterflies ( Asterocampa leilia) use a sit-and-wait tactic to locate mates. To see how vision might influence male behavior, we studied the morphology, optics, and receptor physiology of their eyes and found the following. (1) Each eye's visual field is approximately hemispherical with at most a 10 degrees overlap in the fields of the eyes. There are no large sexual differences in visual field dimensions. (2) In both sexes, rhabdoms in the frontal and dorsal ommatidia are longer than those in other eye regions. (3) Interommatidial angles are smallest frontally and around the equator of the eye. Minimum interommatidial angles are 0.9-1 degrees in males and 1.3-1.4 degrees in females. (4) Acceptance angles of ommatidia closely match interommatidial angles in the frontal region of the eye. We conclude that vision in these butterflies is mostly monocular and that males have more acute vision than females, especially in the frontal region (large facets, small interommatidial angles, small acceptance angles, long rhabdoms, and a close match between interommatidial angles and acceptance angles). This study also suggests that perched males direct their most acute vision where females are likely to appear but show no eye modifications that appear clearly related to a mate-locating tactic.

Adaptive and incidental consequences of the alba polymorphism in an agricultural population of Colias butterflies: female size, fecundity, and differential dispersion.

A sex-limited color dimorphism occurs in many populations of Colias eurytheme. Alba females exhibit different patterns of resource allocation and are less attractive than orange females. This study examines some of the consequences of these differences in terms of reproductive success and population structure in a high density agricultural population.Alba females produced more eggs than orange females on a daily basis, but the morphs did not differ in three measures of size. Spermatophore counts revealed that fresh and worn females did not differ in mating frequency between the two morphs, but very worn alba females gained more matings than very worn orange females.In two mark-release-recapture experiments, alba females exhibited longer residence times than orange females. Changes in population structure over time suggest that this was due to dispersal of orange females from the mature field. Evidence is presented that orange females emigrate in response to male harassment at high density while alba females, exposed to less harassment, remain behind. We suggest that the persistence of the polymorphism in this agricultural population is at least partially facilitated by the cyclic cutting of the alfalfa.