Mayflies are least attracted to vertical polarization: A polarotactic reaction helping to avoid unsuitable habitats
Introduction
Since the pioneering work of Schwind [22], [23] and the extended successive research (reviewed by [9], [12]) it has been known that aquatic insects have positive polarotaxis, that is, they are attracted to horizontally polarized light, because they find their aquatic habitats by means of the horizontal polarization of light reflected from the water surface. Mayflies, as typical aquatic insects, are positively polarotactic as well, because they also find water by means of the horizontally polarized water-reflected light [17], [18], [19]. In the case of mayfly species swarming immediately above the water surface, such as Ephoron virgo [Olivier 1791] (Fig. 1) and Palingenia longicauda [Olivier 1791], their positive polarotaxis is partly responsible for keeping them above water during their whole flying activity [9], [21], [26], while other mayflies may leave the water bodies up to a distance of 1 km [5]. In the latter case, positive polarotaxis guides the females back to water to oviposit.
In this work we study the behavioural responses of E. virgo and Caenis robusta [Eaton 1884] mayflies to lamps emitting horizontally and vertically polarized and unpolarized light of the same spectrum. We selected these species for our experiments, because they belong to two different mayfly (Ephemeroptera) families (E. virgo: Polymitarcidae, C. robusta: Caenidae) and inhabit different habitats. The larvae of E. virgo develop only in rivers [14], while the larvae of C. robusta occur in slow-flowing streams, still waters and rivers [2], [16], [20]. There are similarities between their behaviours: They start to swarm after sunset [2], [26] and do not leave the vicinity of the water surface [5]. At the beginning of swarming, the male subimagos of E. virgo emerging from exuviae land on the riverbank, moult to imagos, and then fly back to the river surface [13]. The male imagos fly rapidly in a straight line at a height of 2.5–5.0 cm directly above the water surface and mate with females. A typical event in the swarming behaviour of these mayflies occurs when they are approaching the bank, and before reaching it they suddenly reverse their direction of flight and fly back to the river mid-line, in order to keep their position above the water surface during swarming [26]. At the beginning, females fly above the water surface together with males (Fig. 1), where they copulate. After copulation, the females increase their altitude and begin their upstream-directed compensatory flight, which ends in oviposition onto the water surface [14]. The swarming behaviour of E. virgo is also typical for P. longicauda inhabiting rivers [18], [21]. C. robusta mayflies swarm above the water surface, where they form groups including several hundred individuals comprising both males and females. In these congregations, the number of males is 4–6 times greater than that of females [5].
Málnás et al. [21] showed an example where mayflies were influenced by an artificial object: the upstream-directed compensatory flight of P. longicauda females was interrupted by a bridge and its mirror image and shadow on the river surface. The latter formed an optical barrier displaying a weakly and vertically polarized reflection-polarization signal. Therefore, the continuous highly and horizontally polarized signal of the river surface, guiding the flight of mayflies above water, was broken up by the vertically polarized mirror image and shadow of the bridge crossing the river. Imaging polarimetric measurements of Horváth and Varjú [11], Bernáth et al. [3], [4] and Málnás et al. [21] and the reflection-polarization patterns presented here show that a weakly and non-horizontally (mainly vertically) polarized area is also formed along the riverside where the mirror image and shadow of the riparian vegetation are observable on the water surface. May this weakly and non-horizontally polarized signal keep flying mayflies away from the edge regions of water bodies and keep them above the open water surface? If yes, this would be an additional behaviour that could control the stability of mayfly swarming above the water surface, beside the well-known positive polarotaxis induced by horizontal polarization. In field experiments we tested this possibility.
Section snippets
Experiment 1
We observed the mass swarming of E. virgo (Fig. 1) in Tahitótfalu (47° 75′ N, 19° 08′ E, Hungary) every evening (from 21:00 to 23:00 h = local summer time = GMT + 2 h) between 15 August and 2 September 2013 at a bridge overarching the river Danube. On 23, 24, 27 and 28 August 2013 between 21:00 and 23:00 h (GMT + 2 h) we performed field experiments to examine the attractiveness of light sources with three different polarization characteristics (unpolarized, horizontally polarized, vertically polarized with
Results
Fig. 2A–C shows the colour photograph and the polarization characteristics of the linearly polarizing and depolarizing filters used in experiment 1 in the blue (450 nm) part of the spectrum. Fig. 2E–J shows the patterns of the degree of linear polarization d (Fig. 2E,F,G), and the angle of polarization α (clockwise from the vertical, Fig. 2H,I,J) of the three different light traps used in experiment 3 emitting horizontally polarized (d = 97.4 ± 2.6%, α = 92.3° ± 0.6°), vertically polarized (d = 98.0 ± 2.0%,
Discussion
Mayflies, like many other water-seeking insects, actively move towards the source of horizontally polarized light being associated with water [9], [12], [23]. In daylight, they do not react to unpolarized ambient light: they are neither attracted to, nor repelled by such light. For terrestrial insects (e.g. migrating desert locusts Schistocerca gregaria, [24]), it can be important to detect water by means of the horizontal polarization of reflected light to avoid water, since they may perish if
Acknowledgements
We thank the organizational help of Györgyi Antoni (director, Center for Innovation and Grant Affairs, Eötvös University, Budapest). Many thanks to Tibor Csörgő, who allowed our experiments on the Ócsa Bird Observatory in Hungary. Gábor Horváth thanks the German Alexander von Humboldt Foundation for an equipment donation. The financial support from the Lendület Project received by András Báldi (supervisor of György Kriska) from the Hungarian Academy of Sciences (grant number: LP2011_014) is
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