Flight of mayflies towards horizontally polarised and unpolarised light

Abstract

Light in the optical environment is not just coloured, but it is polarised. The visual system of many insect species is polarisation sensitive which allows them to use the polarisation pattern of skylight for navigation purposes. There is also increasing evidence that mayflies and other aquatic and semiaquatic insects are able to detect water bodies on the base of the water-reflected horizontally polarised light using it as an environmental cue to detect suitable habitats. In a pilot experiment we constructed and tested light trap pairs emitting unpolarised and horizontally linearly polarised light with the same intensity and spectrum to record potential polarisation-sensitive species from various insect orders. Aquatic insects are assumed to use horizontally polarised light for habitat detection. Thus, we expected that these species will be captured in significantly greater numbers by the trap emitting horizontally polarised light than by the trap emitting unpolarised light. Trappings were carried out at each night from April to October during 2001 and 2002 at two sites in Hungary. One of the trapping sites was at the edge of the flood basin of a river with some smaller ponds. The other trapping place situated in a protected sand dune area scattered with ephemeral alkali lakes. The same mayfly species were collected by the traps emitting polarised and unpolarised light. From the recorded five species four species were caught in greater numbers and more frequently by the trap emitting horizontally polarised light. Caenis horaria and Cloeon dipterum mayflies were attracted significantly stronger to the polarised trap, thus they are likely to use positive polarotaxis in habitat detection. Ephoron virgo and Caenis macrura also showed a tendency for polarotaxis, but they were represented with too low numbers of individuals in the samples to produce significant differences.