Wavelength-specific negatively phototactic responses of the burrowing mayfly larvae Ephoron virgo.

Mayflies are typically negatively phototactic during larval development, whereas the adults possess positive phototaxis. However, no extensive research has been done into the wavelength dependence of phototaxis in any mayfly larvae. We measured the repellency rate of Ephoron virgo larvae to light as a function of wavelength in the 368-743 nm spectral range. We established that the magnitude of repellence increased with decreasing wavelength and the maximal responses were elicited by 400 nm violet light. This wavelength dependence of phototaxis is similar to the recently reported spectral sensitivity of positive phototaxis of the twilight-swarming E. virgo adults. Negative phototaxis not only facilitates predation evasion: avoidance of the blue-violet spectral range could also promote the larvae to withdraw towards the river midline in the case of a drop in the water level, when the underwater light becomes enriched with shorter wavelengths as a result of the decreasing depth of overhead river water.

Spectral sensitivity transition in the compound eyes of a twilight-swarming mayfly and its visual ecological implications.

Aquatic insect species that leave the water after larval development, such as mayflies, have to deal with extremely different visual environments in their different life stages. Measuring the spectral sensitivity of the compound eyes of the virgin mayfly (Ephoron virgo) resulted in differences between the sensitivity of adults and larvae. Larvae were primarily green-, while adults were mostly UV-sensitive. The sensitivity of adults and larvae was the same in the UV, but in the green spectral range, adults were 3.3 times less sensitive than larvae. Transmittance spectrum measurements of larval skins covering the eye showed that the removal of exuvium during emergence cannot explain the spectral sensitivity change of the eyes. Taking numerous sky spectra from the literature, the ratio of UV and green photons in the skylight was shown to be maximal for θ ≈ -13° solar elevation, which is in the θmin = -14.7° and θmax = -7.1° typical range of swarming that was established from webcam images of real swarmings. We suggest that the spectral sensitivity of both the larval and adult eyes are adapted to the optical environment of the corresponding life stages.

Polarization neutral point pairs of the solar corona and the lunar disc observed during the total solar eclipse on 11 August 1999 in Hungary.

During the total solar eclipses on 11 August 1999 in Kecel (Hungary) and on 29 March 2006 in Side (Turkey), two Hungarian groups performed full-sky imaging polarimetric measurements of the eclipsed sky. They observed the spatiotemporal change of the celestial polarization pattern and detected three polarization neutral points as well as two points with maximal polarization of the sky. Parallel to these studies, the polarization pattern in front of the lunar disc, the solar corona, and the surrounding sky have also been measured. During the total solar eclipse on 21 August 2017 in the USA (Rexburg-Idaho, Madras-Oregon), three American/international groups have measured the polarization characteristics of the full sky and the solar corona. The first group observed the spatiotemporal variation of the celestial polarization pattern, and the second group detected three polarization neutral points of the sky and observed two neutral points of the solar corona. The latter were named as Minnaert and van de Hulst neutral points. The third group observed two neutral points of the lunar disc. We have reanalyzed the earlier polarization patterns of the lunar disc, solar corona, and the surrounding sky measured during the Hungarian total eclipse on 11 August 1999. In these reanalyzed polarization patterns, all four neutral points observed during the eclipse on 21 August 2017 in the USA occur: the Minnaert/van de Hulst neutral point pair above/below the eclipsed Sun, where coronal polarization is canceled by sky polarization, and the northern and southern (unnamed) neutral points of the lunar disc, where the directions of polarization of coronal light and foreground skylight are perpendicular to each other with the same polarized intensity. We name the latter two polarization neutral points after Coulson and Vorobiev.

Horsefly reactions to black surfaces: attractiveness to male and female tabanids versus surface tilt angle and temperature.

Tabanid flies (Diptera: Tabanidae) are attracted to shiny black targets, prefer warmer hosts against colder ones and generally attack them in sunshine. Horizontally polarised light reflected from surfaces means water for water-seeking male and female tabanids. A shiny black target above the ground, reflecting light with high degrees and various directions of linear polarisation is recognised as a host animal by female tabanids seeking for blood. Since the body of host animals has differently oriented surface parts, the following question arises: How does the attractiveness of a tilted shiny black surface to male and female tabanids depend on the tilt angle δ? Another question relates to the reaction of horseflies to horizontal black test surfaces with respect to their surface temperature. Solar panels, for example, can induce horizontally polarised light and can reach temperatures above 55 °C. How long times would horseflies stay on such hot solar panels? The answer of these questions is important not only in tabanid control, but also in the reduction of polarised light pollution caused by solar panels. To study these questions, we performed field experiments in Hungary in the summer of 2019 with horseflies and black sticky and dry test surfaces. We found that the total number of trapped (male and female) tabanids is highest if the surface is horizontal (δ = 0°), and it is minimal at δ = 75°. The number of trapped males decreases monotonously to zero with increasing δ, while the female catch has a primary maximum and minimum at δ = 0° and δ = 75°, respectively, and a further secondary peak at δ = 90°. Both sexes are strongly attracted to nearly horizontal (0° ≤ δ ≤ 15°) surfaces, and the vertical surface is also very attractive but only for females. The numbers of touchdowns and landings of tabanids are practically independent of the surface temperature T. The time period of tabanids spent on the shiny black horizontal surface decreases with increasing T so that above 58 °C tabanids spent no longer than 1 s on the surface. The horizontally polarised light reflected from solar panels attracts aquatic insects. This attraction is adverse, if the lured insects lay their eggs onto the black surface and/or cannot escape from the polarised signal and perish due to dehydration. Using polarotactic horseflies as indicator insects in our field experiment, we determined the magnitude of polarised light pollution (being proportional to the visual attractiveness to tabanids) of smooth black oblique surfaces as functions of δ and T.

How does the water springtail optically locate suitable habitats? Spectral sensitivity of phototaxis and polarotaxis in Podura aquatica.

Optical detection of horizontally polarized light is widespread among aquatic insects. This process usually occurs in the UV or blue spectral ranges. Recently, it was demonstrated that at least one collembolan species, the water springtail (Podura aquatica) also possesses positive polarotaxis to horizontally polarized light. These hexapods are positively phototactic, live on the surface of calm waters and usually accumulate close to the riparian vegetation. In laboratory experiments, we measured the wavelength dependence of phototaxis and polarotaxis of P. aquatica in the 346-744 nm and 421-744 nm ranges, respectively. According to our results, the action spectrum of phototaxis is bimodal with two peaks in the blue (λ1=484 nm) and green-yellow (λ2=570 nm) ranges, while polarotaxis operates in the blue spectral range. For the first time, we show that collembolan polarotaxis functions in the same spectral range as the polarotaxis of many aquatic insects. We present our experiments and discuss the possible ecological significance of our findings.

Method to reduce motion artifacts of sequential imaging polarimetry: long enough exposures minimize polarization blurs of wavy water surfaces.

Researchers studying the polarization characteristics of the optical environment prefer to use sequential imaging polarimetry, because it is inexpensive and simple. This technique takes polarization pictures through polarizers in succession. Its main drawback is, however, that during sequential exposure of the polarization pictures, the target must not move, otherwise so-called motion artifacts are caused after evaluation of the polarization pictures. How could these disturbing motion artifacts be minimized? Taking inspiration from photography, our idea was to take the polarization pictures with an exposure that is long enough so that the changes of the moving/changing target can be averaged and, thus, motion artifacts are reduced, at least in a special case when the motion has a stable mean. In the laboratory, we demonstrated the performance of this method when the target was a wavy water surface. We found that the errors of the measured degree and angle of polarization of light reflected from the undulating water surface decreased with increasing exposure time (shutter speed) and converged to very low values. Although various simultaneous polarimeters (taking the polarization pictures at once) are available that do not suffer from motion artifacts, our method is much cheaper and performs very well, at least when the target is a wavy water surface.

Polarization sensitivity in Collembola: an experimental study of polarotaxis in the water-surface-inhabiting springtail Podura aquatica.

The ventral eye of the water-surface-inhabiting springtail Podura aquatica has six ommatidia with horizontal and vertical microvilli and perceives light from the ventral, frontal and frontodorsal regions, whereas the dorsal eye possesses two upward-looking ommatidia with vertical microvilli. The ventral eye may detect water by its polarization sensitivity, even if the insect is resting with its head slightly tipped down on a raised surface. The polarization sensitivity and polarotaxis in springtails (Collembola) have not been investigated. Therefore, we performed behavioural choice experiments to study them in P. aquatica We found that the strength of phototaxis in P. aquatica depends on the polarization characteristics of stimulating light. Horizontally and vertically polarized light were the most and least attractive, respectively, while unpolarized stimulus elicited moderate attraction. We show that horizontally polarized light attracts more springtails than unpolarized, even if the polarized stimulus was 10 times dimmer. Thus, besides phototaxis, P. aquatica also performs polarotaxis with the ability to measure or at least estimate the degree of polarization. Our results indicate that the threshold d* of polarization sensitivity in P. aquatica is between 10.1 and 25.5%.

Phototaxis and polarotaxis hand in hand: night dispersal flight of aquatic insects distracted synergistically by light intensity and reflection polarization.

Based on an earlier observation in the field, we hypothesized that light intensity and horizontally polarized reflected light may strongly influence the flight behaviour of night-active aquatic insects. We assumed that phototaxis and polarotaxis together have a more harmful effect on the dispersal flight of these insects than they would have separately. We tested this hypothesis in a multiple-choice field experiment using horizontal test surfaces laid on the ground. We offered simultaneously the following visual stimuli for aerial aquatic insects: (1) lamplit matte black canvas inducing phototaxis alone, (2) unlit shiny black plastic sheet eliciting polarotaxis alone, (3) lamplit shiny black plastic sheet inducing simultaneously phototaxis and polarotaxis, and (4) unlit matte black canvas as a visually unattractive control. The unlit matte black canvas trapped only a negligible number (13) of water insects. The sum (16,432) of the total numbers of water beetles and bugs captured on the lamplit matte black canvas (7,922) and the unlit shiny black plastic sheet (8,510) was much smaller than the total catch (29,682) caught on the lamplit shiny black plastic sheet. This provides experimental evidence for the synergistic interaction of phototaxis (elicited by the unpolarized direct lamplight) and polarotaxis (induced by the strongly and horizontally polarized plastic-reflected light) in the investigated aquatic insects. Thus, horizontally polarizing artificial lamplit surfaces can function as an effective ecological trap due to this synergism of optical cues, especially in the urban environment.

Seasonality and daily activity of male and female tabanid flies monitored in a Hungarian hill-country pasture by new polarization traps and traditional canopy traps.

Blood-sucking female tabanid flies cause serious problems for animals and humans. For the control of tabanids, the knowledge about their seasonality and daily activity is of great importance. Earlier, only traditional traps capturing exclusively female tabanids have been used to survey tabanid activity. The data of such temporal trapping do not reflect correctly the activity of male and female tabanid flies. Our major aim was to monitor the trapping numbers of male and female tabanids during a 3-month summer survey in Hungary. We used (i) conventional canopy traps with liquid traps on the ground beneath the canopy and (ii) L-shaped sticky traps with vertical and horizontal components. Our other goal was to compare the efficiencies of the two components of each trap type used. We observed two greater peaks of the trapping number of tabanids. These peaks started with increased catches of female tabanids captured by the canopy traps and the vertical sticky traps and ended with a dominance of male and female tabanids caught by the liquid traps and the horizontal sticky traps. The swarming periods were interrupted by rainy/cool days, when the number of tabanids decreased drastically. Among the 17 species, six dominated and composed 89.4% of the captured tabanids: Haematopota pluvialis, Tabanus tergestinus, Tabanus bromius, Tabanus maculicornis, Tabanus bovinus and Atylotus loewianus. The number of water-seeking male and female tabanids rose up to 12-13 h and then decreased but had a secondary peak at about 17 h. The stochastic weather change and the communities of different species resulted in large standard deviations of the averaged number of tabanids in the course of a day. The horizontally polarizing (liquid and horizontal sticky) traps captured both male and female specimens and were about three times more efficient than the canopy and vertical sticky traps that caught only females. The results of the horizontal sticky traps corresponded to those of the liquid traps, while the catches of the vertical sticky traps corresponded to those of the canopy traps. The catches of the used trap types reflected well the species and water/host-seeking composition of tabanids.

Orientation with a Viking sun-compass, a shadow-stick, and two calcite sunstones under various weather conditions.

It is widely accepted that Vikings used sun-compasses to derive true directions from the cast shadow of a gnomon. It has been hypothesized that when a cast shadow was not formed, Viking navigators relied on crude skylight polarimetry with the aid of dichroic or birefringent crystals, called "sunstones." We demonstrate here that a simple tool, that we call "shadow-stick," could have allowed orientation by a sun-compass with satisfying accuracy when shadows were not formed, but the sun position could have reliably been estimated. In field tests, we performed orientation trials with a set composed of a sun-compass, two calcite sunstones, and a shadow-stick. We show here that such a set could have been an effective orientation tool for Vikings only when clear, blue patches of the sky were visible.

Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes.

The characteristic striped appearance of zebras has provoked much speculation about its function and why the pattern has evolved, but experimental evidence is scarce. Here, we demonstrate that a zebra-striped horse model attracts far fewer horseflies (tabanids) than either homogeneous black, brown, grey or white equivalents. Such biting flies are prevalent across Africa and have considerable fitness impact on potential mammalian hosts. Besides brightness, one of the likely mechanisms underlying this protection is the polarization of reflected light from the host animal. We show that the attractiveness of striped patterns to tabanids is also reduced if only polarization modulations (parallel stripes with alternating orthogonal directions of polarization) occur in horizontal or vertical homogeneous grey surfaces. Tabanids have been shown to respond strongly to linearly polarized light, and we demonstrate here that the light and dark stripes of a zebra's coat reflect very different polarizations of light in a way that disrupts the attractiveness to tabanids. We show that the attractiveness to tabanids decreases with decreasing stripe width, and that stripes below a certain size are effective in not attracting tabanids. Further, we demonstrate that the stripe widths of zebra coats fall in a range where the striped pattern is most disruptive to tabanids. The striped coat patterns of several other large mammals may also function in reducing exposure to tabanids by similar mechanisms of differential brightness and polarization of reflected light. This work provides an experimentally supported explanation for the underlying mechanism leading to the selective advantage of a black-and-white striped coat pattern.

New kind of polarotaxis governed by degree of polarization: attraction of tabanid flies to differently polarizing host animals and water surfaces.

Aquatic insects find their habitat from a remote distance by means of horizontal polarization of light reflected from the water surface. This kind of positive polarotaxis is governed by the horizontal direction of polarization (E-vector). Tabanid flies also detect water by this kind of polarotaxis. The host choice of blood-sucking female tabanids is partly governed by the linear polarization of light reflected from the host's coat. Since the coat-reflected light is not always horizontally polarized, host finding by female tabanids may be different from the established horizontal E-vector polarotaxis. To reveal the optical cue of the former polarotaxis, we performed choice experiments in the field with tabanid flies using aerial and ground-based visual targets with different degrees and directions of polarization. We observed a new kind of polarotaxis being governed by the degree of polarization rather than the E-vector direction of reflected light. We show here that female and male tabanids use polarotaxis governed by the horizontal E-vector to find water, while polarotaxis based on the degree of polarization serves host finding by female tabanids. As a practical by-product of our studies, we explain the enigmatic attractiveness of shiny black spheres used in canopy traps to catch tabanids.

An unexpected advantage of whiteness in horses: the most horsefly-proof horse has a depolarizing white coat.

White horses frequently suffer from malign skin cancer and visual deficiencies owing to their high sensitivity to the ultraviolet solar radiation. Furthermore, in the wild, white horses suffer a larger predation risk than dark individuals because they can more easily be detected. In spite of their greater vulnerability, white horses have been highly appreciated for centuries owing to their natural rarity. Here, we show that blood-sucking tabanid flies, known to transmit disease agents to mammals, are less attracted to white than dark horses. We also demonstrate that tabanids use reflected polarized light from the coat as a signal to find a host. The attraction of tabanids to mainly black and brown fur coats is explained by positive polarotaxis. As the host's colour determines its attractiveness to tabanids, this parameter has a strong influence on the parasite load of the host. Although we have studied only the tabanid-horse interaction, our results can probably be extrapolated to other host animals of polarotactic tabanids, as the reflection-polarization characteristics of the host's body surface are physically the same, and thus not species-dependent.

Optics of sunlit water drops on leaves: conditions under which sunburn is possible.

*It is a widespread belief that plants must not be watered in the midday sunshine, because water drops adhering to leaves can cause leaf burn as a result of the intense focused sunlight. The problem of light focusing by water drops on plants has never been thoroughly investigated. *Here, we conducted both computational and experimental studies of this phyto-optical phenomenon in order to clarify the specific environmental conditions under which sunlit water drops can cause leaf burn. *We found that a spheroid drop at solar elevation angle theta approximately 23 degrees, corresponding to early morning or late afternoon, produces a maximum intensity of focused sunlight on the leaf outside the drop's imprint. Our experiments demonstrated that sunlit glass spheres placed on horizontal smooth Acer platanoides (maple) leaves can cause serious leaf burn on sunny summer days. *By contrast, sunlit water drops, ranging from spheroid to flat lens-shaped, on horizontal hairless leaves of Ginkgo biloba and Acer platanoides did not cause burn damage. However, we showed that highly refractive spheroid water drops held 'in focus' by hydrophobic wax hairs on leaves of Salvinia natans (floating fern) can indeed cause sunburn because of the extremely high light intensity in the focal regions, and the loss of water cooling as a result of the lack of intimate contact between drops and the leaf tissue.

Reducing the maladaptive attractiveness of solar panels to polarotactic insects.

Human-made objects (e.g., buildings with glass surfaces) can reflect horizontally polarized light so strongly that they appear to aquatic insects to be bodies of water. Insects that lay eggs in water are especially attracted to such structures because these insects use horizontal polarization of light off bodies of water to find egg-laying sites. Thus, these sources of polarized light can become ecological traps associated with reproductive failure and mortality in organisms that are attracted to them and by extension with rapid population declines or collapse. Solar panels are a new source of polarized light pollution. Using imaging polarimetry, we measured the reflection-polarization characteristics of different solar panels and in multiple-choice experiments in the field we tested their attractiveness to mayflies, caddis flies, dolichopodids, and tabanids. At the Brewster angle, solar panels polarized reflected light almost completely (degree of polarization d ≈ 100%) and substantially exceeded typical polarization values for water (d ≈ 30-70%). Mayflies (Ephemeroptera), stoneflies (Trichoptera), dolichopodid dipterans, and tabanid flies (Tabanidae) were the most attracted to solar panels and exhibited oviposition behavior above solar panels more often than above surfaces with lower degrees of polarization (including water), but in general they avoided solar cells with nonpolarizing white borders and white grates. The highly and horizontally polarizing surfaces that had nonpolarizing, white cell borders were 10- to 26-fold less attractive to insects than the same panels without white partitions. Although solar panels can act as ecological traps, fragmenting their solar-active area does lessen their attractiveness to polarotactic insects. The design of solar panels and collectors and their placement relative to aquatic habitats will likely affect populations of aquatic insects that use polarized light as a behavioral cue.

Bioreplicated coatings for photovoltaic solar panels nearly eliminate light pollution that harms polarotactic insects.

Many insect species rely on the polarization properties of object-reflected light for vital tasks like water or host detection. Unfortunately, typical glass-encapsulated photovoltaic modules, which are expected to cover increasingly large surfaces in the coming years, inadvertently attract various species of water-seeking aquatic insects by the horizontally polarized light they reflect. Such polarized light pollution can be extremely harmful to the entomofauna if polarotactic aquatic insects are trapped by this attractive light signal and perish before reproduction, or if they lay their eggs in unsuitable locations. Textured photovoltaic cover layers are usually engineered to maximize sunlight-harvesting, without taking into consideration their impact on polarized light pollution. The goal of the present study is therefore to experimentally and computationally assess the influence of the cover layer topography on polarized light pollution. By conducting field experiments with polarotactic horseflies (Diptera: Tabanidae) and a mayfly species (Ephemeroptera: Ephemera danica), we demonstrate that bioreplicated cover layers (here obtained by directly copying the surface microtexture of rose petals) were almost unattractive to these species, which is indicative of reduced polarized light pollution. Relative to a planar cover layer, we find that, for the examined aquatic species, the bioreplicated texture can greatly reduce the numbers of landings. This observation is further analyzed and explained by means of imaging polarimetry and ray-tracing simulations. The results pave the way to novel photovoltaic cover layers, the interface of which can be designed to improve sunlight conversion efficiency while minimizing their detrimental influence on the ecology and conservation of polarotactic aquatic insects.

Striped bodypainting protects against horseflies.

Bodypainting is widespread in African, Australian and Papua New Guinean indigenous communities. Many bodypaintings use white or bright yellow/grey/beige stripes on brown skin. Where the majority of people using bodypainting presently live, blood-sucking horseflies are abundant, and they frequently attack the naked brown regions of the human body surface with the risk of transmitting the pathogens of dangerous diseases. Since horseflies are deterred by the black and white stripes of zebras, we hypothesized that white-striped paintings on dark brown human bodies have a similar effect. In a field experiment in Hungary, we tested this hypothesis. We show that the attractiveness to horseflies of a dark brown human body model significantly decreases, if it is painted with the white stripes that are used in bodypaintings. Our brown human model was 10 times more attractive to horseflies than the white-striped brown model, and a beige model, which was used as a control, attracted two times more horseflies than the striped brown model. Thus, white-striped bodypaintings, such as those used by African and Australian people, may serve to deter horseflies, which is an advantageous byproduct of these bodypaintings that could lead to reduced irritation and disease transmission by these blood-sucking insects.

Spatiotemporal change of sky polarization during the total solar eclipse on 29 March 2006 in Turkey: polarization patterns of the eclipsed sky observed by full-sky imaging polarimetry.

Using 180 degrees field-of-view (full-sky) imaging polarimetry, we measured the spatiotemporal change of the polarization of skylight during the total solar eclipse on 29 March 2006 in Turkey. We present our observations here on the temporal variation of the celestial patterns of the degree p and angle alpha of linear polarization of the eclipsed sky measured in the red (650 nm), green (550 nm), and blue (450 nm) parts of the spectrum. We also report on the temporal and spectral change of the positions of neutral (unpolarized, p = 0) points, and points with local minima or maxima of p of the eclipsed sky. Our results are compared with the observations performed by the same polarimetric technique during the total solar eclipse on 11 August 1999 in Hungary. Practically the same characteristics of celestial polarization were encountered during both eclipses. This shows that the observed polarization phenomena of the eclipsed sky may be general.

Imaging polarimetry of glass buildings: why do vertical glass surfaces attract polarotactic insects?

Recently it was observed that the Hydropsyche pellucidula caddis flies swarm near sunset at the vertical glass surfaces of buildings standing on the bank of the Danube river in Budapest, Hungary. These aquatic insects emerge from the Danube and are lured to dark vertical panes of glass, where they swarm, land, copulate, and remain for hours. It was also shown that ovipositing H. pellucidula caddis flies are attracted to highly and horizontally polarized light stimulating their ventral eye region and thus have positive polarotaxis. The attraction of these aquatic insects to vertical reflectors is surprising, because after their aerial swarming, they must return to the horizontal surface of water bodies from which they emerge and at which they lay their eggs. Our aim is to answer the questions: Why are flying polarotactic caddis flies attracted to vertical glass surfaces? And why do these aquatic insects remain on vertical panes of glass after landing? We propose that both questions can be partly explained by the reflection-polarization characteristics of vertical glass surfaces and the positive polarotaxis of caddis flies. We measured the reflection-polarization patterns of shady and sunlit, black and white vertical glass surfaces from different directions of view under clear and overcast skies by imaging polarimetry in the red, green, and blue parts of the spectrum. Using these polarization patterns we determined which areas of the investigated glass surfaces are sensed as water by a hypothetical polarotactic insect facing and flying toward or landed on a vertical pane of glass. Our results strongly support the mentioned proposition. The main optical characteristics of "green," that is, environmentally friendly, buildings, considering the protection of polarotactic aquatic insects, are also discussed. Such "green" buildings possess features that attract only a small number of polarotactic aquatic insects when standing in the vicinity of fresh waters. Since vertical glass panes of buildings are abundant in the man-made optical environment, and polarotactic aquatic insects are spread worldwide, our results are of general interest in the visual and behavioral ecology of aquatic insects.

Method to improve the survival of night-swarming mayflies near bridges in areas of distracting light pollution.

Numerous negative ecological effects of urban lighting have been identified during the last decades. In spite of the development of lighting technologies, the detrimental effect of this form of light pollution has not declined. Several insect species are affected including the night-swarming mayfly Ephoron virgo: when encountering bridges during their mass swarming, these mayflies often fall victim to artificial lighting. We show a simple method for the conservation of these mayflies exploiting their positive phototaxis. With downstream-facing light-emitting diode beacon lights above two tributaries of the river Danube, we managed to guide egg-laying females to the water and prevent them from perishing outside the river near urban lights. By means of measuring the mayfly outflow from the river as a function of time and the on/off state of the beacons, we showed that the number of mayflies exiting the river's area was practically zero when our beacons were operating. Tributaries could be the sources of mayfly recolonization in case of water quality degradation of large rivers. The protection of mayfly populations in small rivers and safeguarding their aggregation and oviposition sites is therefore important.