Why do red and dark-coloured cars lure aquatic insects? The attraction of water insects to car paintwork explained by reflection-polarization signals

Abstract

We reveal here the visual ecological reasons for the phenomenon that aquatic insects often land on red, black and dark-coloured cars. Monitoring the numbers of aquatic beetles and bugs attracted to shiny black, white, red and yellow horizontal plastic sheets, we found that red and black reflectors are equally highly attractive to water insects, while yellow and white reflectors are unattractive. The reflection-polarization patterns of black, white, red and yellow cars were measured in the red, green and blue parts of the spectrum. In the blue and green, the degree of linear polarization p of light reflected from red and black cars is high and the direction of polarization of light reflected from red and black car roofs, bonnets and boots is nearly horizontal. Thus, the horizontal surfaces of red and black cars are highly attractive to red-blind polarotactic water insects. The p of light reflected from the horizontal surfaces of yellow and white cars is low and its direction of polarization is usually not horizontal. Consequently, yellow and white cars are unattractive to polarotactic water insects. The visual deception of aquatic insects by cars can be explained solely by the reflection-polarizational characteristics of the car paintwork.

Figure 1

Insects associated with water landing on the roof of a red car. (a) A mayfly, Baetidae sp. (b) Another mayfly, Ecdyonuridae sp. (c) A water beetle, Hydrochara caraboides. (d) A water bug, Sigara striata. The insects were observed and photographed in April and May of 2005 in Hungary on the roof of the same red car (Daewoo Matiz).

Figure 2

Statistical dissimilarity in insects captured on differently coloured shiny horizontal plastic sheets based on cluster analyses of (a) abundance data compared by the Bray–Curtis-index (complete linkage method) and (b) species composition compared by the Sørensen index (complete linkage method).

Figure 3

Reflection–polarization patterns of a (a) black, (b) white, (c) red and (d) yellow car (Suzuki Swift) measured by imaging polarimetry in the red (650 nm), green (550 nm) and blue (450 nm) parts of the spectrum under a clear sky at a solar zenith angle of 42°. The cars were illuminated from the left-hand side by the sun. The long axis of the cars and the viewing direction of the polarimeter were perpendicular to the solar meridian. The angle of declination of the optical axis of the polarimeter was −20° from the horizontal. (i) Colour picture of the cars. The rectangles show the areas (bonnets) for which the mean and standard deviation of the degree of linear polarization p and angle of polarization α in table 3 are given. Patterns of p (ii) and α (iii) of light reflected from the car-bodies.