Mayflies are least attracted to vertical polarization: A polarotactic reaction helping to avoid unsuitable habitats

Highlights

• Ephoron virgo and Caenis robusta mayflies are attracted less to vertically polarized than to unpolarized light.

• Both species were attracted more to horizontally polarized than to unpolarized light.

• This polarotactic behaviour helps mayflies to avoid unsuitable habitats.

• The attractiveness of mayflies to differently polarized light depends on intensity and species.

• The mirror image of riparian vegetation reflects weakly and non-horizontally polarized light.

• These may facilitate the stability of mayfly swarming above water surfaces.

Abstract

Like other aquatic insects, mayflies are positively polarotactic and locate water surfaces by means of the horizontal polarization of water-reflected light. However, may vertically polarized light also have implications for the swarming behaviour of mayflies? To answer this question, we studied in four field experiments the behavioural responses of Ephoron virgo and Caenis robusta mayflies to lamps emitting horizontally and vertically polarized and unpolarized light. In both species, unpolarized light induces positive phototaxis, horizontally polarized light elicits positive photo- and polarotaxis, horizontally polarized light is much more attractive than unpolarized light, and vertically polarized light is the least attractive if the stimulus intensities and spectra are the same. Vertically polarized light was the most attractive for C. robusta if its intensity was about two and five times higher than that of the unpolarized and horizontally polarized stimuli, respectively. We suggest that the mayfly behaviour observed in our experiments may facilitate the stability of swarming above water surfaces. Beside the open water surface reflecting horizontally polarized light, the shadow and mirror image of riparian vegetation at the edge of the water surface reflect weakly and non-horizontally (mainly vertically) polarized light. Due to their positive polarotaxis, flying mayflies remain continuously above the water surface, because they keep away from the unpolarized or non-horizontally polarizing edge regions (water surface and coast line) of water bodies. We also discuss how our findings can explain the regulation of mayfly colonization.

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.