Balázs Bernáth

Dragonflies Find Crude Oil Visually More Attractive than Water: Multiple-Choice Experiments on Dragonfly Polarotaxis

Table 1. Row 1: The total number and sex (F: female, M: male) of dragonflies (Sympetrum vulgatum, Ischnura pumilio, Enallagma cyathigerum) trapped by the crude-oiland water-filled trays during the first choice experiment. Rows 2–4: The relative brightness, degree of polarization and direction of polarization of light reflected from the trays and measured by video polarimetry in the blue spectral range (lp470 nm) from a direction of view of 707 with respect to the vertical. The trays are designated by S1 and S2 as in Fig. 2

Positive polarotaxis in a mayfly that never leaves the water surface: polarotactic water detection in Palingenia longicauda (Ephemeroptera)

Tisza mayflies, Palingenia longicauda (Olivier 1791), swarm exclusively over the river Tisza (from which the name of the mayfly was derived). This river is bordered by a high vertical wall of trees and bushes, which hinder P. longicauda to move away horizontally from the water. During swarming, Tisza mayflies fly immediately above the river in such a way that their cerci touch the water frequently or sweep its surface. This continuous close connection with water and the vertical wall of the shore and riparian vegetation result in that Tisza mayflies never leave the water surface; consequently, they need not search for water. Several Ephemeroptera species move away far from water and return to it guided by the horizontal polarization of water-reflected light. To reveal whether also P. longicauda is or is not polarotactic, we performed a field experiment during the very short swarming period of Tisza mayflies. We show here that also P. longicauda has positive polarotaxis, which, however, can be observed only under unnatural conditions, when the animals are displaced from the water and then released above artificial test surfaces. P. longicauda is the first species in which polarotactic water detection is demonstrated albeit it never leaves the water surface, and thus, a polarotactic water detection seems unnecessary for it. The polarotactic behaviour of Tisza mayflies explains the earlier observation that these insects swarm above wet asphalt roads running next to river Tisza.

Why is it worth flying at dusk for aquatic insects? Polarotactic water detection is easiest at low solar elevations

SUMMARY Using 180° field-of-view imaging polarimetry, we measured the reflection-polarization patterns of two artificial surfaces (water-dummies) in the red, green and blue spectral ranges under clear and partly cloudy skies at different solar elevations. The dummies consisted of a horizontal glass pane with a matt black or matt light grey cloth underneath, imitating a dark or bright water body, respectively. Assuming that polarotactic water insects interpret a surface as representing water if the degree of linear polarization of reflected light is higher than a threshold and the deviation of the direction of polarization from the horizontal is lower than a threshold, we calculated the proportion, P, of the artificial surfaces detected polarotactically as water. We found that at sunrise and sunset P is maximal for both water-dummies and their reflection-polarizational characteristics are most similar. From this, we conclude that polarotactic water detection is easiest at low solar elevations, because the risk that a polarotactic insect will be unable to recognize the surface of a dark or bright water body is minimal. This partly explains why many aquatic insect species usually fly en masse at dusk. The daily change in the reflection-polarization pattern of water surfaces is an important visual ecological factor that may contribute to the preference of the twilight period for habitat searching by polarotactic water insects. Air temperature at sunrise is generally low, so dusk is the optimal period for polarotactic aquatic insects to seek new habitats.

Polarotaxis in non-biting midges: female chironomids are attracted to horizontally polarized light.

Non-biting midges (Chironomidae, Diptera) are widely distributed aquatic insects. The short-living chironomid adults swarm in large numbers above water surfaces, and are sometimes considered a nuisance. They are vectors of certain bacteria, and have a key-role in benthic ecosystems. Optical cues, involving reflection-polarization from water, were found to be important in the habitat selection by three Mediterranean freshwater chironomid species. In this work we report on our multiple-choice experiments performed in the field with several other European freshwater chironomid species. We show that the investigated non-biting midges are positively polarotactic and like many other aquatic insects their females are attracted to horizontally polarized light. Our finding is important in the visual ecology of chironomids and useful in the design of traps for these insects.

Polarotaxis in egg-laying yellow fever mosquitoes Aedes (Stegomyia) aegypti is masked due to infochemicals

Aquatic and water-associated insects need to locate suitable bodies of water to lay their eggs in and allow their aquatic larvae to develop. More than 300 species are known to solve this task by positive polarotaxis, relying primarily on the horizontally polarized light reflected from the water surface. The yellow fever mosquito Aedes (Stegomyia) aegypti has been thought to be an exception, locating its breeding habitats by chemical cues like odour of conspecifics, their eggs, or water vapour. We now demonstrate through dual-choice experiments that horizontally polarized light can also attract ovipositing Ae. aegypti females when the latter are deprived of chemical cues: water-filled transparent egg-trays illuminated by horizontally polarized light from below gained a 94.2% higher total number of eggs than trays exposed to unpolarized light, but only when no chemical substances capable of functioning as cues were present. Ae. aegypti is the first known water-associated insect in which polarotaxis exists, but does not play a dominant role in locating water bodies and can be constrained in the presence of chemical cues.

Imaging polarimetry of forest canopies: how the azimuth direction of the sun, occluded by vegetation, can be assessed from the polarization pattern of the sunlit foliage

Radiance, color, and polarization of the light in forests combine to create complex optical patterns. Earlier sporadic polarimetric studies in forests were limited by the narrow fields of view of the polarimeters used in such studies. Since polarization patterns in the entire upper hemisphere of the visual environment of forests could be important for forest-inhabiting animals that make use of linearly polarized light for orientation, we measured 180 degrees field-of-view polarization distributions in Finnish forests. From a hot air balloon we also measured the polarization patterns of Hungarian grasslands lit by the rising sun. We found that the pattern of the angle of polarization alpha of sunlit grasslands and sunlit tree canopies was qualitatively the same as that of the sky. We show here that contrary to an earlier assumption, the alpha-pattern characteristic of the sky always remains visible underneath overhead vegetation, independently of the solar elevation and the sky conditions (clear or partly cloudy with visible suns disc), provided the foliage is sunlit and not only when large patches of the clear sky are visible through the vegetation. Since the mirror symmetry axis of the alpha-pattern of the sunlit foliage is the solar-antisolar meridian, the azimuth direction of the sun, occluded by vegetation, can be assessed in forests from this polarization pattern. Possible consequences of this robust polarization feature of the optical environment in forests are briefly discussed with regard to polarization-based animal navigation.

Polarized light and oviposition site selection in the yellow fever mosquito: no evidence for positive polarotaxis in Aedes aegypti.

Aquatic insects and insects associated with water use horizontally polarized light (i.e., positive polarotaxis) to detect potential aquatic or moist oviposition sites. Mosquitoes lay their eggs onto wet substrata, in water, water-filled tree/rock holes, or man-made small containers/bottles/old tyres containing water. Until now it has remained unknown whether mosquitoes are polarotactic or not. The knowledge how mosquitoes locate water would be important to develop new control measures against them. Thus, we studied in dual-choice laboratory experiments the role of horizontally polarized light in the selection of oviposition sites in blood-fed, gravid females of the yellow fever mosquito, Aedes aegypti. On the basis of our results we propose that Ae. aegypti is not polarotactic. Thus the yellow fever mosquito is the first known water-associated insect species that does not detect water by means of the horizontally polarized water-reflected light. This can be explained by the reflection-polarization characteristics of small-volume water-filled cavities/containers preferred by Ae. aegypti as oviposition sites.

An alternative interpretation of the Viking sundial artefact: an instrument to determine latitude and local noon

An eleventh century artefact, a fragment of a compass dial found at Uunartoq in Greenland, is widely accepted as proof of the ability of Vikings to navigate with sun-compasses. The artefact is half of a wooden compass dial bearing deliberately incised lines that were interpreted as gnomonic lines valid on the day of equinox and near the summer solstice at the 61st latitude. Supposed loose markings of cardinal directions and several unexplained scratches are visible on this find. We offer here a new possible interpretation that some of these scratches might be fundamental lines of a geometrical construction process used for forming the gnomonic lines. Our hypothesis renders the cardinal directions to be precisely marked in the dial and assigns exact dates to both gnomonic lines. We reinterpret the artefact as a combination of sun-compass and sun shadow board designed for appointing local solar noon and the length of the noon shadow at open sea, playing a role analogous to that of a late-mediaeval backstaff. Greenland occurrence of geometrical construction of gnomonic lines, a known cultural asset of ancient European people, may denote that mediaeval Norse people not only shared in European culture, but used its achievements, even in the utmost frontiers.

Imaging polarimetry of the rainbow

Using imaging polarimetry, we measured the polarization patterns of a rainbow on the shore of the Finnish town of Oulu in July 2001. We present here high-resolution color-coded maps of the spatial distributions of the degree and angle of linear polarization of the rainbow in the red (650 +/- 30 nm), green (550 +/- 30 nm), and blue (450 +/- 30 nm) ranges of the spectrum. The measured polarization characteristics of the investigated rainbow support earlier theoretical and computational results and are in accordance with previous qualitative observations. To our knowledge, this is the first imaging polarimetric study of rainbow polarization.

How could the Viking Sun compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment

Vikings routinely crossed the North Atlantic without a magnetic compass and left their mark on lands as far away as Greenland, Newfoundland and Baffin Island. Based on an eleventh-century dial fragment artefact, found at Uunartoq in Greenland, it is widely accepted that they sailed along chosen latitudes using primitive Sun compasses. Such instruments were tested on sea and proved to be efficient hand-held navigation tools, but the dimensions and incisions of the Uunartoq find are far from optimal in this role. On the basis of the sagas mentioning sunstones, incompatible hypotheses were formed for Viking solar navigation procedures and primitive skylight polarimetry with dichroic or birefringent crystals. We describe here a previously unconceived method of navigation based on the Uunartoq artefact functioning as a ‘twilight board’, which is a combination of a horizon board and a Sun compass optimized for use when the Sun is close to the horizon. We deduced an appropriate solar navigation procedure using a twilight board, a shadow-stick and birefringent crystals, which bring together earlier suggested methods in harmony and provide a true skylight compass function. This could have allowed Vikings to navigate around the clock, to use the artefact dial as a Sun compass during long parts of the day and to use skylight polarization patterns in the twilight period. In field tests, we found that true north could be appointed with such a medieval skylight compass with an error of about ±4° when the artificially occluded Sun had elevation angles between +10° and −8° relative to the horizon. Our interpretation allows us to assign exact dates to the gnomonic lines on the artefact and outlines the schedule of the merchant ships that sustained the Viking colony in Greenland a millennium ago.