Female presence is required for male sexual maturity in the nematode Steinernema longicaudum

following nest exits. Every 2 to 3 days, nest position was changed in the horizontal plane on a 4 m long platform to make positional cues unreliable. The choice criterion for a returning bee was defined as the first colour landmark that the bee hovered in front of, or landed on. Thus, if the bee chose the training colour landmark, the response was considered correct. Data from all bees were pooled, and G-tests were used to determine whether choice distributions differed significantly from chance. With fewer than 20 choices, binomial tests were used. In Test 1, bees chose darker and brighter yellows (Y1, Y2) as often as the training yellow (T) showing that they neglected intensity-related cues, while grey was chosen only once Even when Test 2 was performed under the darkest conditions with the sun more than 12° below the horizon, and less than 10% of the moon visible (luminance of white stimuli between 10 –3 and 10 –5 cd m –2), the bees chose the training yellow (T) (Figure 1E, 'dark'). Bees from a third nest trained to a grey landmark, and tested with four yellow and green shades, chose grey in 29 of 31 trials (G = 79.40, P < 0.0001). When the training grey was substituted with a darker grey (Gr3 in Supplemental Figure S2), bees always chose the correct colour (n = 8, binomial test, P < 0.01). Thus, the bees discriminated the landmarks using colour rather than intensity-related ('brightness') cues even at light intensities much lower than the human colour vision threshold. This is the first evidence of nocturnal colour vision in an animal with apposition compound eyes and the first demonstration of nocturnal colour vision in an animal's natural environment. X. tranquebarica is an active nocturnal forager on flowers that vary significantly in colour, and may rely on colour vision to distinguish them. Colour vision is useful for reliable object discrimination at night because crepuscular shifts in illumination colour [6] make 'brightness' unreliable: yellow (T) is brighter than grey (Gr1) in starlight but darker in twilight. The colours, however, remain the same (Figures 1A, B and Supplemental Figure S2). Colour vision in starlight by bees with apposition eyes was unexpected — in dim light, diurnal bees apparently pool signals from the different spectral classes of photoreceptors to sacrifice colour vision in favour of an improved signal-to-noise ratio [2]. Exactly how nocturnal colour vision …