Ultraviolet Umwelten: Exploring Semantic Organs Beyond the Visible Spectrum

doi:10.21203/rs.3.rs-8414405/v1

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Ultraviolet Umwelten: Exploring Semantic Organs Beyond the Visible Spectrum

https://doi.org/10.21203/rs.3.rs-8414405/v1

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Although not visible to human eyes, ultraviolet (UV) reflectance forms an essential part of the perceptual world of animals belonging to a wide range of taxa. It plays a role in their communication, orientation, and ecological interactions. Although ultraviolet patterns constitute a significant portion of biological signals, their role in species-specific Umwelten remains insufficiently understood, in part because human sensory limitations obscure this domain of meaning. In this essay, we highlight the significance of studying the ultraviolet Umwelten and propose that UV-reflecting surfaces can be interpreted as semantic organs whose biological relevance is based on a reciprocal dynamics of appearance and perception. To overcome the methodological constraints that have historically hindered access to UV-mediated communication, we provide a detailed, reproducible pipeline for UV reflectance photography. Our framework specifies the necessary optical equipment, illumination conditions, filtration strategies, calibration procedures, and postprocessing steps needed to obtain standardised and quantitatively interpretable UV images. This protocol should enable researchers to reliably visualise and compare UV patterns in ways that approximate their appearance to UV-sensitive organisms, thus granting empirical access to the aspects of their Umwelten which are normally hidden to human observers. By combining rigorous imaging methodology with a biosemiotic approach to organismal surfaces, we show how UV patterns contribute to the formation of ecological meaning and call for broader integration of UV modalities into the study of biological communication.

Ultraviolet reflectance

Ultraviolet Umwelt

Semantic organs

Biosemiotics of visual communication

UV photography methodology

Biological signals

The solar spectrum that reaches the surface of the Earth ranges from ultraviolet (UV) to infrared (IR) wavelengths. Human vision, however, is sensitive only to a small part of this spectrum, namely to wavelength of 400–750 nm. Below the violet end of this visible window lies a domain long excluded from human perceptual experience: the ultraviolet (UV) world, which – although invisible to us – forms a significant part of the Umwelt of many animals (Cuthill et al. 2000; Silberglied 1979; Stella and Kleisner 2022). Although sensitivity to UV radiation was first demonstrated in the nineteenth century by Sir John Lubbock’s pioneering observations on hymenopterans (Lubbock 1882a, 1882b), definitive psychophysical evidence of UV vision was established later by Karl von Frisch in his work on honeybees (Dyer et al. 2015; Frisch 1914). Systematic investigation proceeded with the work of Frank E. Lutz (1933; 1924) and Robert E. Silberglied, whose Communication in the Ultraviolet (1979) offered the first comprehensive synthesis of UV perception and signalling across taxa. Initially, the attention of scholars focused on UV reflectance in flowers and its role in plant–pollinator interactions (Daumer 1958; Lutz 1924). Later, Jocelyne Crane systematically described and published detailed spectrophotometric measurements of UV patterns on the wings of butterflies (Crane 1954). Although we cannot offer here a detailed history of UV research in the twentieth century, it is fair to say that it has become increasingly evident that UV radiation has a far-reaching impact on the behaviour, orientation, and social communication of an enormous variety of taxa, from insects to birds, reptiles, and fish (Fleishman et al. 2011; Osorio and Vorobyev 2008; Silberglied 1979; Stella and Kleisner 2022). This historical trajectory mirrors a broader conceptual shift in biology from the initial assumption that the visual world is coextensive with human perception towards a recognition of species-specific perceptual worlds (sensu Umwelten), each contingent upon particular sensory capacities. Moreover, the realisation that many animals perceive wavelengths inaccessible to humans has led biologists to a realisation of the epistemological limitations of their own sensory apparatus. Based on several examples, let us briefly describe how UV patterns and their perception function in various taxa.

In plants, UV-reflective floral patterns serve as important visual cues for pollinators, especially bees and flies, in whom they contribute to flower recognition and guide foraging behaviour. These patterns are created not only by UV-absorbing pigments but also by specialised surface microstructures which modulate light reflection (Schulte et al. 2019). Many flowers also exhibit UV-reflective and UV-absorptive patterns that guide pollinators directly to the source of nectar; such patterns often take the form of a ‘bullseye’ or radial lines visible only in UV light (Klomberg et al. 2019). UV floral patterns evolve in relation to local types of pollinators and the levels of UV light, which points to both genetic control and their adaptive significance (Koski and Ashman 2014, 2016). UV signals also participate in complex biological phenomena such as mimicry; this is exemplified by orchids, which deceive pollinators by mimicking the UV reflectance of plants that would reward them with nectar (Scaccabarozzi et al. 2023)). In fruits, UV reflectance contributes to seed dispersal by attracting frugivorous birds and rodents who can perceive UV (Altshuler 2001).

UV reflectance plays a role also in lichens, which are lifeforms consisting of algae and fungal symbionts. They use secondary metabolites and pigments to reflect or absorb UV radiation, thus protecting themselves from damage. These UV-screening compounds often fluoresce and play a role in the survival of lichens under high UV exposure (Özyi̇ği̇toğlu et al. 2020). The ubiquity and evolutionary significance of UV reflection and UV perception suggest it is a fundamental component of biological communication systems.

In animals, UV reflectance has a wide range of functions, from mate recognition and sexual selection (Kemp et al. 2008; Pecháček et al. 2019; Silberglied 1979), to intrasexual competition (Kemp and Rutowski 2007; Rutowski 1985), species discrimination (Brunton and Majerus 1995; Stella et al. 2018), predator avoidance, and partial mimicry (Lyytinen et al. 2003; Olofsson et al. 2010), and all the way to camouflage through contrast matching or prey attraction through contrast manipulation (Heiling et al. 2005; Llandres and Rodríguez-Gironés 2011). It thus plays a role in both interspecific and intraspecific communication. For instance, butterflies and birds use UV patterns for mate selection, because stronger reflectance can signal a better physiological condition or genetic fitness (Stella and Kleisner 2022). Something similar has also been observed in fish, for instance, in the three-spined stickleback (Rick et al. 2006; Rick and Bakker 2008). In birds, UV patterns are often associated with courtship displays. It has been shown for instance in a comparative study of parrots (Hausmann et al. 2003), in whom UV light plays a unique role in that patterns are produced by both direct UV reflectance and UV-induced fluorescence. Moreover, there is no strong association between courtship displays and other visible colours (such as green, yellow, or red) that lack the UV component (Hausmann et al. 2003). This strengthens the argument that birds use UV light in a special way for sexual communication.

This brings us to the question of why is UV special in some communication contexts, why is it not just another colour? The UV channel seems to constitute a specialised signalling modality shaped by a unique combination of physical properties, ecological advantages, and evolutionary drivers. Physically, the short, high-energy wavelengths of UV light are prone to significant atmospheric scattering (Rayleigh scattering), which weakens the signal over long distances. This makes UV patterns effective for conspicuous short-range communication, for instance during courtship, while reducing the risk of eavesdropping by more distant predators or rivals (Burkhardt 1989; T. W. Cronin and Bok 2016). Ecologically, this short-range signalling is enhanced by the high contrast which UV creates against most natural backgrounds. Common substrates, such as the soil or the surrounding vegetation, tend to be poor reflectors of UV light, which is why UV-reflective animal markings can stand out vividly to a UV-sensitive receiver (Arnold et al. 2002; Endler 1990). This inherent contrast often is the reason why UV signals are so effective. Furthermore, many invertebrates and vertebrates, including birds and fish, have evolved photoreceptors dedicated to UV light perception, while most of their mammalian predators do not have this capacity. This sensory difference creates a ‘private’ communication channel that allows some animals (e.g., Xiphophorus nigrensis) to convey information to conspecifics while reducing the threat of detection and predation (Cummings et al. 2003; Stella and Kleisner 2022). Not surprisingly, exceptions can be found: for instance, the males of Cosmophasis umbratica spiders display UV-reflective markings that attract females but increase the risk of predation (Bulbert et al. 2015).

The evolution of semantic UV-sensitive organs is also intricately linked to the structure of their surface. In many butterflies and birds, coloration is produced by nanostructures that create iridescence, so that the colour and intensity shift dramatically with the angle of view. This is particularly notable in butterflies (Thayer and Patel 2023), where the flapping of wings produces dynamic, stroboscopic UV flashes which are thought to be critical for mate recognition. For example, wing scale nanostructures in the form of beads enhance the whiteness of pierid butterfly wings, making them brighter than in species that lack these bead-like structures (Stavenga et al. 2004). In Colias eurytheme, UV signals are created by thin-film interference from nano-ridge structures on the wing scales, which generate bright and direction-dependent iridescent reflections (Rutowski et al. 2005). The density and orientation of these nano-ridges directly correlate with the brightness and angle dependence of UV reflectance (see also Morhpo sp. Figure 1). This structural control explains the variation in signalling and its potential use in courtship (Kemp et al. 2006).

The evolution of such vibrant signals related to mating may also be explained by the principle of sensory exploitation, where the males evolve traits that tap into a pre-existing sensory bias in females (Ryan 1990). If females already possess a heightened sensitivity to UV for other tasks, such as foraging for UV-reflective food sources, males can evolve UV signals to exploit this bias and increase their mating success (Briscoe et al. 2010; Finkbeiner and Briscoe 2021). The special nature of UV communication thus seems to be linked to a powerful synergy of UV light’s physical properties related to short-range signalling, its high contrast against natural backdrops, its relative invisibility to many predators, and exploitation of the sensory systems of receivers.

Aside from the various communicative and functional roles of organs sensitive to UV light, research has also found macroecological patterns in the distribution of these particular traits and their specific appearance on a continental scale. For example, empirical analyses across Lepidopteran species show consistent sexual dimorphism, where males typically exhibit a stronger UV reflectance (Pecháček et al. 2014; Stella et al. 2018), but geographic and climatic variables modulate both the extent and configuration of UV patches (Pecháček et al. 2019; Stella et al. 2016). In green-veined white (Pieris napi, see Fig. 2), for instance, UV reflectance inversely correlates with environmental productivity: individuals from colder, less hospitable environments exhibit higher UV reflectance (Stella et al. 2016). This holds also in the other direction: in common brimstone (Gonepteryx rhamni), structurally generated UV patches become larger and more intensive with increasing habitat temperature and humidity, which is consistent with the notion of condition-dependent sexual ornamentation (Pecháček et al. 2014). These results indicate that UV traits respond to both developmental constraints and local ecological regimes, and their bearers thus integrate into their visible morphology physiological, genetic, and environmental information.

This implies that UV-sensitive organs are highly morphologically and functionally diverse. Nevertheless, for us, humans, the notion of perceiving UV patterns and ornaments on flowers, birds’ feathers, or butterflies’ wings evokes a sense of a mysterious world hidden from our senses. We find it hard to fully appreciate that UV reflection and absorption patterns are widespread across multiple kingdoms of life and can play a critical role in ecological signalling and perception (see Fig. 4). The exposed surfaces of organisms form not just the outer layer of the body but an active and dynamic interface where interactions take place. These surfaces function as semantic organs (Kleisner 2015), carriers of potential meaning which emerge from structural, developmental, and genetic foundations and interact with other animals’ sensory systems and, more broadly, with the perceptual and cognitive capacities of other living entities. The character of semantic organs thus depends not only on the sources of phenotypic variability and their underlying biological foundations, but also on differences between species, groups, and individuals in the perception of these structures. One component of this perceptual variability is the observer’s sensitivity to reflected light across different wavelengths – in our case, to UV light.

In the following, we provide a description of a methodological framework and instrumental equipment required for the acquisition of UV photography. This technique allows us to peek into the UV Umwelten of animals and better appreciate the forms of UV-sensitive semantic organs and intensity of UV perception.

2.1 General Properties of UV Reflectance and Its Photographic Image

UV photography enables the visualisation and quantification of patterns of reflectance and absorption in the spectral range of 300–400nm, which is invisible to human eyes but ecologically relevant to a high proportion of the animal kingdom. Correct acquisition requires controlled illumination, calibrated optical equipment, and standardised postprocessing procedures. The following protocol outlines each step for capturing the UV reflectance of biological surfaces such as butterfly wings, bird plumage, or plant flowers and leaves (Fig. 3).

At the outset, it is crucial to differentiate between UV-reflected photography and UV-induced fluorescence. UV-induced fluorescence is the imaging of visible or IR light that is emitted when UV light hits a surface, while in UV-reflected photography, the subject is illuminated directly by UV light and the camera captures only the reflected part of the UV spectrum.

The invention of digital photography offered a significantly faster and more cost-effective way of producing images, and it enabled a subsequent image analysis. Conventional cameras regularly detect light in both visible and IR spectrum (for butterfly wings in IR light, see, e.g., Krishna et al. 2020), but digital UV photography is somewhat more specific in its requirements. First of all, one must have a UV-sensitive camera. For instance, FujiFilm IS Pro is a digital camera suitable for UV photography due to its broad sensitivity spanning from 330 to 900nm. Unfortunately, generally speaking, information regarding the range of light sensitivity of cameras is a technological secret and therefore rarely available. Furthermore, the camera needs to be equipped with an uncoated UV-transmitting non-distorting lens, for instance a quartz lens, whereby generally speaking the older and simpler, the better. What one needs is a prime lens with few optical elements. Then one mounts a set of filters (e.g., B + W) on the lens to filter out the visible and IR parts of the spectrum. Finally, one should use an artificial source of stable UV light, such as a mercury lamp, flashlight, or a LED source specially adapted for emitting in known UV spectra (for an analysis of properties of several commonly used UV light sources, see Rutowski and Macedonia 2008).

Reflection standards for UV photography, such as Spectralon or Kodak grey card, are another required element for the acquisition of UV photographs (Knüttell and Fiedler 2000). They allow standardisation (linearisation) of all photographs in a dataset (Troscianko and Stevens 2015). Cameras are usually designed to produce images that look good – in other words, they are not developed to record reality. This so-called non-linearity in capturing RGB signals is the standard in most conventional modern cameras; linear outputs are usually found only in technical and high-specification cameras (sensors).

To emphasise the nontriviality of taking UV photography, we should also mention the shift of focus. Due to a different wavelength, the plane of focus in the UV range is closer to the focal plane (Primack 1982). This can be partly remedied by stopping the aperture. Some authors use the term ‘harmonisation’ to describe setups that which address all these concerns (Crowther, 2019). In the past decade or so, several publications have described UV photography in relative detail (Arribas 2012; Crowther 2019; Dalrymple et al. 2018; Stella et al. 2018; Stevens and Cuthill 2007; Tedore and Nilsson 2019; Troscianko and Stevens 2015) .

2.2 Standardised Workflow for UV Reflectance Imaging

2.2.1 Equipment

2.2.1.1 Camera Body

For UV reflectance imaging, one can use a digital camera body modified to capture the full-spectrum (e.g., Nikon D70/D70s, Canon EOS 6D, Sony A7 II), from which the manufacturer’s internal UV/IR-cut (‘hot-mirror’) filter was removed and replaced by a quartz or fused-silica window to preserve sensor focus and enable near-UV transmission. Silicon CCD/CMOS sensors are usually inherently sensitive to light ranging from the near-UV to the near-IR, but the stock hot-mirror blocks parts of this range. This conversion thus ‘unlocks’ the sensor’s native response in the ≈ 330–380nm UV band while retaining IR sensitivity up to ~ 1100nm (Cosentino 2015). It should be noted that UV fluorescence (UVF) photography does not require a modified camera, because the detector records visible/IR emission under UV excitation, but UVF is methodologically distinct from UV reflectance and not a substitute for reflectance work such as described below. It requires purpose-built or factory-modified bodies (e.g., Fuji IS Pro UVIR – recently not sold), eventually field conversions (e.g., Nikon D70s) (Garcia et al., 2014). Because manufacturers seldom publish spectral quantum-efficiency curves for consumer sensors, empirical verification of UV throughput (body + cover glass + microlenses) is recommended after conversion (Stella et al., 2017, 2018; Pecháček et al., 2019).

2.2.1.2 Lens

For UV reflectance imaging, the choice of lens choice determines both the spectral throughput and image formation. Because many modern optics use multicoating, cemented groups, and polymer elements that attenuate light in the 320–400nm wavelengths, one ought to opt for a simple, fixed-focal-length prime with few elements and minimal coatings; older enlarger/repro lenses and early single-coated primes (such as Helios 44 − 2 58 mm f/2 lens) typically pass more near-UV light and are less prone to internal flare than complex zooms (Cosentino 2015). Where available, dedicated UV-transmitting optics (quartz or CaF₂ constructions such as CoastalOpt 60 mm f/4 UV-VIS-IR APO Macro) offer a better transmission and reduce focus shift, but they do so at a substantially higher cost, which is why their use is largely confined to forensic/scientific applications.

Regardless of the lens type, UV focus lies at a different conjugate than the focus of visible light (VIS); critical focus therefore requires either live-view focusing under UV illumination or a pre-measured focus offset. Stopping down (increase of aperture value) improves tolerance but should be balanced against diffraction and signal loss (Cosentino et al., 2015). Close-range work should be preferentially conducted using glass-free extensions (tubes/bellows) rather than add-on dioptres, which introduce UV-absorbing glass. Historical practice shows that inexpensive UV bandpass filtration can be adapted to ordinary primes (e.g., Wratten 18A on a standard lens) but such setups require a stricter control of leakage and focus calibration (Hill et al., 1977).

2.2.1.3 Filters

UV reflectance imaging with a full-spectrum body requires spectral isolation: the passband should transmit near-UV light (≈ 320–400nm) while suppressing both visible and IR light. We therefore recommend a two-filter stack that combines a UV-pass element with an IR-blocking (hot-mirror) / blue-green glass. B + W 403 (UV-pass, non-fluorescent; suited for close-ups) stacked with B + W BG 38 (hot-mirror) effectively passes near-UV while attenuating the strong IR leakage typical of UV-pass glasses; B + W BG 38 cannot be used alone for UV reflectance because it still passes visible light and is designed primarily to stop IR. Functionally similar IR-blocking substitutes (such as Hoya U360, Schott UG-11x or Heliopan BG 38) are acceptable if the same stack logic is applied, but all options require verification of the combined transmission to ensure deep rejection across the 420–700 nm (visible) and 700–1100 nm (near-IR) range. Historical single-filter solutions (e.g., Wratten 18A on standard primes) can work but require a stricter control of leakage and focus calibration and are generally inferior to modern two-filter stacks.

In practice, we have mounted slim-ring filters to minimise vignetting, blackened any exposed edges to reduce stray-light, and oriented the stack to minimise internal reflections (we have tested both orders: UV-pass forward and IR-block forward), because coatings and glass fluorescence vary by the brand and sample. To guard against IR contamination from lamps or warm backgrounds, we performed leak tests (i.e., identical exposure with UV source off and identical exposure replacing the IR-block with clear glass) and quantified differences in RAW counts. Because spectral data for consumer filters can be incomplete or vary by sample, we recommend empirical throughput screening of each assembled stack on your actual body + lens combination before deployment.

2.2.1.4 Light Sources and Reference Standards

For UV reflectance imaging, we have used a UVP MRL-58 Multiple-Ray Lamp fitted with an F8T5 long-wave 365nm mercury fluorescent tube (8 W, 230 V, 50 Hz, 0.16 A). Long-wave UVA at ~ 365nm is in effect the standard for UV reflectance work in biology, because it offers a good compromise between specimen safety, available flux, and compatibility with band-pass filter stacks (Stella et al. 2018). Fluorescent tubes deliver a lower irradiance than high-power LEDs or mercury spot lamps, which is why we used long exposures on a tripod and stable geometry to maintain Signal-to-Noise ratio (SNR) and repeatability. Because fluorescent sources exhibit mains-linked flicker, we have exposed below the power-line period (≤ 1/50s equivalent via multi-second exposures, in our case usually 15s; based on the power of the light source) to average such fluctuations. To minimise the visible-violet spill and stray reflections, which are common with tube lamps, we have combined the lamp with our lens-side filter stack (UV-pass + IR-block; see Filters) and worked in a darkened environment with matte black baffles. We have positioned the lamp at 45° incidence at a distance of app. 40 cm, and where needed used two oblique positions of the light source to reduce specular highlights. We avoided direct backlighting and verified illumination uniformity by checking RAW counts on a PTFE/Spectralon patch (targeting < 5–10% CV).

Prior to acquisition, the lamp should be warmed for 5 to 10 minutes to stabilise output. We ran leak tests (UV off; IR-block removed) to confirm that any recorded signal is indeed produced by near-UV reflectance rather than by VIS/IR contamination. While higher-flux options exist (e.g., 365nm LED arrays; mercury spotlights for ‘light-painting’), our portable tube-based setup – in conjunction with rigorous filtration, geometry control, and radiometric normalisation – yielded stable, reproducible UV reflectance images appropriate for downstream quantitative analysis.

We have included an X-Rite ColorChecker Passport in every specimen image and used its neutral grey patch as our working standard. All photographs should be ideally captured as RAW (12–14-bit) with in-camera processing disabled. During acquisition, we kept the grey patch well below clipping (mid-range RAW values) and maintained consistent lamp distance and angle. In post-processing, each image was normalised by a single exposure gain so that the grey patch reached the same RAW level across images. This resulted in a comparable brightness across sessions and helped detect any drift in illumination or focus (Stella et al. 2018). Because the ColorChecker has been designed for managing visible-light colours, its pigments are not UVA-flat. We have therefore used it only for consistency and exposure control, not to derive absolute UV reflectance values or UV-specific camera profiles.

2.2.2 Specimen Preparation

Specimens should be prepared under standard museum curation conditions, using dried and properly stored material. Butterflies (or flowers, feathers, etc.) can be mounted on matte black or neutral grey backgrounds to suppress secondary reflections; museum specimens should be positioned with wings (or petals, feathers) spread flat and secured using non-fluorescent entomological pins (Cosentino 2015). All surfaces can be cleaned with compressed, oil-free air to remove dust and loose scales, which could otherwise generate artefactual UV scatter. A metric scale bar can be placed next to each specimen to preserve spatial reference for subsequent morphometric and patch-size analyses. Although properly curated material is generally stable in the UV range, specimen condition remains a critical covariate: multiple studies have reported only negligible differences in UV reflectance between freshly killed and well-preserved museum specimens protected from light, abrasion, and chemical degradation (Ramos and Hulshof 2019; Stella et al. 2018), and fumigation with chloroform or paradichlorobenzene does not seem to alter UV spectral properties (Crane 1954). For example, it has been shown that the structural colours of Polyommatus icarus males can persist over both short developmental timescales and for over a century in collections (Kertész et al. 2019), and other lepidopterans exhibit only a minor UV brightening that can be attributed to pigment decay (Peet-Pare 2017). Even so, there are notable exceptions, such as age-related UV increases in Anartia fatima (Taylor 1973) and comparable fading in some feathers, dragonflies, and salticids (Lim et al. 2007), which underscores the need to record the specimen age, storage regimen, and known handling history in all imaging metadata.

2.2.3 Image Acquisition

As described above, UV reflectance images can be acquired using a camera modified to capture the full spectrum of light and fitted with a UV-transmitting lens and a filter stack, whereby with all images should be captured in complete darkness except for controlled UV-A (Ultraviolet A refers to the longest wavelength of ultraviolet light from the sun-315-400 nm) illumination. In our collection of UV images of butterflies, we kept the exposure parameters constant across the datasets (ISO 400, 15s, f/22); they were chosen to balance the depth of field with the reduced photon flux characteristic of UV-A imaging. Autofocus should be disabled and critical focus set manually under visible light before filter exchange, after which the focus is locked to avoid wavelength-dependent focal shift. All frames should be recorded in the RAW format (or NEF, .CR2) to retain a linear sensor output and avoid in-camera gamma correction. White balance should be fixed to 5500K to maintain consistency across sessions; absolute UV colorimetry is determined later by reflectance normalisation rather than by in-camera settings (Stella et al. 2018).

The camera should be mounted on a rigid tripod and exposures triggered via remote release to eliminate vibration. Neither the specimen nor the camera should move at all between the captures in visible and UV light captures – this ensures geometric congruence for later comparative analyses. Even with suitable hardware, optical geometry is non‑trivial: as noted above, shorter wavelengths shorten the focal distance by shifting sharpness towards the sensor (Primack 1982). Stopped‑down apertures reduce but do not eliminate the chromatic focus error. Researchers therefore speak of a ‘harmonisation’, meaning a holistic alignment of optics, illumination, filtration, and capture parameters (Crowther 2019). Detailed protocols now exist for various taxa and contexts (Arribas 2012; Dalrymple et al. 2018; Stella et al. 2018; Stevens and Cuthill 2007; Tedore and Nilsson 2019).

2.2.4 Calibration and Image Processing

RAW files can be imported into Adobe Camera Raw or Adobe Lightroom with all automatic tone curves, sharpening, and noise reduction disabled to retain the linear sensor output required for quantitative UV analyses. Each frame contains a calibration chart, allowing exposure linearisation and subsequent conversion to 16-bit TIFF in either AdobeRGB or a linear RGB colorspace. Reflectance normalisation is performed on per-image basis by scaling pixel intensities relative to the white and black standards included in every frame; in this way, one corrects variation in illumination, compensates for sensor–filter spectral bias, and ensures comparability across sessions. All analyses should be conducted only on these normalised 16-bit files.

2.2.5 Notes on Field Applications

The acquisition and processing workflow described above can be in principle adapted for field conditions using portable full-spectrum camera bodies, UV-pass filter stacks, and battery-powered 365nm LED sources. In practice, however, the long exposure times necessitated by low UV irradiance impose substantial limitations on standardised outdoor imaging, especially when ambient light cannot be fully excluded or even minimal wind leads to some movement of the specimen or camera. Although all technical settings and calibration procedures could be in theory transferred to field setups, the constraining factor is the intensity and spectral purity of portable UV illumination, which rarely matches laboratory sources. For these reasons, and due to our so far limited experience with fully standardised UV acquisition in situ, field UV photography should currently be regarded as feasible mainly for robust and immobile targets imaged under controlled shading or in light-isolating enclosures, rather than as a routine analogue to laboratory-based protocols.

2.2.6 Data Archiving

All images should be archived in both RAW and 16-bit TIFF formats together with complete metadata describing camera body, lens and filter configuration, exposure parameters, illumination spectra, and calibration standards. Metadata should be encoded and validated using the EXIFTool and then stored with the image files in institutional repositories to ensure long-term preservation and reproducibility. In accordance with the FAIR data principles, derivative 16-bit TIFFs and their associated standardised metadata can be deposited in open-access biodiversity infrastructures such as GBIF, iDigBio, or LepNet, which enable high-throughput comparative analyses and provide interoperability with existing digital specimen pipelines. The original RAW files, which contain the full linear sensor output, are retained in institutional storage and can be made available upon reasonable request for verification or re-analysis. This archiving regime ensures data transparency and reusability while maintaining practical file size and repository constraints.

UV photography has been evolving and used in science for almost one hundred years, ever since Lutz captured the very first UV pictures using a pinhole camera (Lutz 1924). This American entomologist was the first to use UV photography to investigate UV-reflective patterns and their function in communication among animals or between plants and their pollinators (Lutz 1933; Lutz 1924). Since that time, however, UV photography has received in science much less attention than for instance IR or standard visible spectrum photography. Even so, numerous studies from various areas of science have described diverse approaches and dealt with different challenges with respect to UV photography, and various companies and scientists came up with new techniques of acquisition and standardisation of still UV photography (Brues 1941; Eastman Kodak Company 1972, 1972; Ferris 1975; Kevan et al. 1973) and UV cinematography (Aneshansley and Eisner 1975). There exist studies on UV light as such (Allman 1973), UV photography in chemical analysis (Luner 1968), UV use in skin analysis (Crowther 2019), UV photography in archaeology (Baker 2011) and forensic science (Arribas 2012), on aerial UV photography (Cronin et al. 1968; Lavigne 1976; Lavigne and Øritsland 1974), studies on the visual system of bees and pollination ecology (Daumer 1958; De Bruin 1961; Hill 1977), and at last but not least work on UV patterns in butterflies (Crane 1954; Ferris 1972; Mazokhin-Porshnyakov 1957). Nevertheless, compared to visible light or IR imaging, the UV modality remained marginal, hampered by low commercial demand, rapidly shifting photographic materials, and the absence of field‑wide standards (Nekrutenko and Didmanidze 1975; Silberglied 1979).

3.1 The Umwelt Theory, Semiotic Co-option, and the Evolution of UV Semantic Organs

Despite growing empirical knowledge, the study of UV patterns has remained largely anchored in the functionalist paradigm of the signalling theory (Johnstone 1997; Maynard Smith and Harper 2003). But to better understand how and why UV signals emerge and acquire a biological meaning, we need an interpretive framework that accounts for both expression and perception as complementary aspects of organismal communication (Brejcha et al. 2019, 2021; Kleisner and Maran 2014a; Kull 2000; Maran 2017; Witzany 2014). The concept of Umwelt, formulated by Jakob von Uexküll and later expanded in biosemiotics (Brentari 2015; Ferreira and Caldas 2013; Karel Kleisner and Maran 2014b; Kull 2010; Salthe 2014; Uexküll 1921, 1928), offers one such perspective. Most briefly, it states that each organism inhabits a world composed not of physical stimuli in general but of meaningful cues, i.e., cues that correspond to its sensory and effectorial capacities. In this sense, the UV Umwelt of a butterfly, bird, or fish is not a hidden extension of human reality but a distinct semiotic domain co-constituted by the animal’s physiology, behaviour, and environment.

UV patterns can thus be understood as semantic organs: percepto-morphological interfaces which both display and mediate information between the organism and its environment. They are produced by developmental processes that embody the organism’s ecological and evolutionary history, and they operate within perceptual loops which include the sender and the receiver. The structural complementarity of ‘seeing and appearing’ (Brejcha et al. 2019; Kleisner 2024) thus becomes a central principle in understanding UV communication. The surface that reflects UV light and the eye that perceives it are not independent entities – they are reciprocally shaped components of a coupled system, a system that evolved to channel specific wavelengths as carriers of meaning within a species’ Umwelt.

How does the UV Umwelt relate to evolutionary adaptation? Uexküll himself in his Bedeutungslehre (1956) provides an illustrative example of how a glass bowl can take on different meanings depending on how it is used. We can, for instance, embed it into the wall of a house so that it lets light into the room from the outside – in that case, the bowl takes on the function of a window and its translucence is the essential property. Alternatively, it can be used as a vase for flowers, in which case what is essential is its concavity. The point is that in a similar way organisms can, in the course of evolution, repurpose almost all of their traits, structures, or organs, be it at a molecular or macroscopic level. By reassigning the meaning of a given structure based on its various aspects (such as the bowl’s translucence or shape), almost any structure can be semiotically co-opted for a new purpose and thus endowed with a new meaning or, if you will, a new adaptive role (Kleisner 2008; Kleisner 2011; Maran and Kleisner 2010). This process is based on a re-use of some pre-existing property of the object. Importantly, though, what turns a neutral or non-adaptive structure into an adaptive organ is this shift in meaning, that is, the exploitation of some previously unused but already present feature of the structure under specific circumstances that may have newly arisen and triggered an adaptive response in the local population of organisms.

Uexküll’s Umwelt theory in a sense complements the empirical and experimental endeavour of the early pioneers of UV light perception and reflection, such John Lubbock, Karl von Frisch, or Frank E. Lutz, who were mentioned already in the introduction. Uexküll’s empirical and theoretical framework was probably the very first to explicitly introduce a multispecies perspective of biological reality.

3.2 How to Read UV Photography

If we take our attempt to understand the Umwelten of animals whose receptors are sensitive to UV light seriously, we should also bear in mind that they are not sensitive only to UV. Depending on their species-specific sensitivity, their visual perception includes also other wavelengths, so that UV light is only part of the overall percept. One can roughly imagine such visual perception as if our own vision was enriched by an additional colour, a colour that is often structural in nature, meaning that the intensity of its reflection depends on the angle relative to the observer and the incoming light. It thus exhibits characteristics similar to iridescent colorations with a metallic shine (as in case of Colias crocea male hindwing, see Fig. 5).

An effective way of combining UV and visible light spectra is false-colour UV photography (Lunau et al. 2021), which combines UV and colour images to create a false-colour representation based on the trichromatic vision of bees. It is especially effective for rapid screening of large numbers of flowers for structures and surface textures which are visible to bees but cannot be detected by standard spectrophotometry (Lunau et al. 2021).

3.3 Advantages and Limitations of UV Photography Compared to Spectrophotometry

Poorly specified methods propagate error throughout the analytical pipeline. For example, Brunton and Majerus (1995) used UV spectrophotometry to compare Colias and Gonepteryx butterflies but omitted angular and spatial sampling controls: the resulting spectra were indecipherable because UV reflectance radically changes depending on patch orientation (Wilts et al. 2011). Similarly, studies that used incorrect filtration, consumer‑grade CMOS sensors, ambient illumination, lossy JPEG encoding, or inconsistent white balance (Futahashi et al. 2019; Kodric-Brown and Johnson 2002) produced device‑dependent images that could not be quantitatively compared (Stevens et al. 2007). Such mistakes and omissions are not merely aesthetic: mischaracterised UV signals can lead to incorrect taxonomic diagnoses and misleading ecological or evolutionary interpretations by conflating UV, visible, and IR channels.

Moreover, conventional spectrophotometers provide only point samples, while an adequate characterisation of colour of a heterogenous object requires multiple samples across an appropriate sampling array, such as multiple transects (Crane 1954; Garcia et al. 2013). This places high demands not only on sampling time but also on information about the spatial relations between colours, which must be reconstructed from the geometry of the sampling array (Wilts et al. 2011). Even so, the spatial resolution is often crude. Spectrometry also requires a static subject, either because an array needs to be sampled or because the measuring probe needs to be close to or touch the colour patch, which is particularly challenging when working with delicate museum specimens (Stevens et al. 2007).

On top of that, spectrophotometry does not yield data suitable for analysing the three-dimensional nature of many patterns (Troscianko and Stevens 2015). Even digital UV photography is relatively challenging, but it has many advantages over spectrophotometry, most notably its ability to use powerful and complex image processing algorithms to analyse entire spatial patterns without the need to reconstruct topography from point samples. More obviously, the process is relatively fast and allows for a rapid collection of large quantities of data from unrestrained targets and with minimal equipment (Pike 2011). Imaging programs can be used to obtain various forms of data, including colour patch size and distribution measures, various ‘brightness’ and colour metrics, or broadband reflection values. Moreover, digital technology can be used to manipulate stimuli for use in behavioural experiments.

Jack J. Windig, a pioneer in digital image analysis of lepidopteran wing patterns, had stated that collection of imaging data and image analysis should meet three criteria (Windig 1991): completeness, repeatability, and speed. What he meant is that the studied trait should be quantified comprehensively, with respect to all its characteristics, the procedure should be repeatable, and the process should be fast relative to other available methods. Although digital photography products are available, fast, and they have already transformed the study of coloration, patterns, and shapes in biology, one should proceed with caution and make suitable calibrations before and after each image is taken. An English saying states that ‘a picture is worth a thousand words’ – in this context, we should say ‘a standardised picture is worth a thousand words’.

3.5 Concluding Remarks: Towards an Integrative Study of UV Umwelten

A thorough understanding of UV communication requires integration across multiple levels: (i) the physical level of UV generation, propagation, and perception; (ii) the developmental level of scale nanostructure formation and pigment synthesis; (iii) the behavioural level of mate choice, display, and spatial orientation; and (iv) the semiotic level, where UV patterns become sign-vehicles embedded within species-specific Umwelten. Contemporary approaches that combine digital UV imaging (Stella et al. 2016, 2018), geometric morphometrics (Pecháček et al. 2014, 2019), and spectral modelling (Wilts et al. 2011) enable a reconstruction of these multilevel relationships. By situating empirical measurements within a biosemiotic and ecological framework, one can start investigating how UV signals function and for whom and within what Umwelt they are meaningful. While in some taxa, such as butterflies and birds, there is a relatively large body of literature on both reflection and perception of UV light, in other taxa research is minimal. There are thus many groups of organisms, especially invertebrates (including the otherwise often studied groups of holometabolous insects, such as beetles and dipterans), where UV research is still in infancy and very surprising discoveries can be made (Pope and Hinton 1977).

In the present work, our goal was to offer researchers practical means for investigating the expression of UV patterns as components of organismal communication systems. We have examined some structural, ecological, and perceptual determinants of UV reflectance and evaluated how UV-reflective patterns contribute to the formation of a species’ UV Umwelt. Our approach thus brings together observations regarding the biophysical nature of UV reflectance and photographic imaging and a theoretical interpretation rooted in biosemiotics, which investigates how meaning emerges from the interplay between appearance, perception, and environment.

Author Contribution

K.K. and D.S. wrote the main manuscript text, prepared figures, and reviewed the manuscript.

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No competing interests reported.

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Ultraviolet Umwelten: Exploring Semantic Organs Beyond the Visible Spectrum

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Abstract

Figures

1. Introduction

2. Materials and Methods: Acquisition of Photography in UV Light

2.1 General Properties of UV Reflectance and Its Photographic Image

2.2 Standardised Workflow for UV Reflectance Imaging

2.2.1 Equipment

2.2.1.1 Camera Body

2.2.1.2 Lens

2.2.1.3 Filters

2.2.1.4 Light Sources and Reference Standards

2.2.2 Specimen Preparation

2.2.3 Image Acquisition

2.2.4 Calibration and Image Processing

2.2.5 Notes on Field Applications

2.2.6 Data Archiving

3. Discussion

3.1 The Umwelt Theory, Semiotic Co-option, and the Evolution of UV Semantic Organs

3.2 How to Read UV Photography

3.3 Advantages and Limitations of UV Photography Compared to Spectrophotometry

3.5 Concluding Remarks: Towards an Integrative Study of UV Umwelten

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Author Contribution

References

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Status:

Version 1