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Lamp-Lit Bridges as Dual Light-Traps for the Night-Swarming Mayfly, Ephoron virgo: Interaction of Polarized and Unpolarized Light Pollution

Fig 4

Estimated numbers of Ephoron virgo mayflies attracted to polarized and unpolarized light sources placed above the Danube river on four dates in 2012.

Number (mean ± standard deviation) of mayflies attracted to horizontally polarized (continuous line) and unpolarized (dashed line) light as a function of time (= local summer time = GMT + 2 hours, where GMT = Greenwich Mean Time). Each estimate is based on 10 photographs (see subsection Experiments with linear polarizers of the Materials and Methods). Comparisons of total detections over the course of each test indicate that more individuals were attracted to polarized versus unpolarized light sources of the same intensity: (A) 23 August, N = 11582, U = 181, Z = 10.302, p < 0.00001; (B) 24 August, N = 22786, U = 674.5, Z = 11.388, p < 0.00001; (C) 27 August, N = 5425, U = 303.5, Z = 8.945, p < 0.00001; (D) 28 August, N = 93935, U = 8682, Z = 10.465, p < 0.00001, where N is the number of total mayfly detections, U is the parameter giving the sum of ranks used in the non-parametric method, Z is the standard deviation of data for a given p, and p is the level of significancy (p < 0.05 means significant).

Fig 4

doi: https://doi.org/10.1371/journal.pone.0121194.g004