Figure 3

<div><p>The Perception of Colored Leaves by an Aphid</p> <p>(A) A green leaf reflects the light from the sun and is seen by an aphid with a green (G), blue (B), and ultraviolet (U) photoreceptor [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050187#pbio-0050187-b029" target="_blank">29</a>]. Evidence from behavioral studies [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050187#pbio-0050187-b031" target="_blank">31</a>] indicates that a COM processes the input from the photoreceptors. (B) Reflectance spectra of three leaves from the bird-cherry, Prunus padus. To humans, these leaves appear green, red, and yellow. For measurement methods see [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050187#pbio-0050187-b026" target="_blank">26</a>]. (C) Tentative spectral sensitivities of the green peach aphid's UV, blue, and green photoreceptors. (D) Relative excitation of the UV, blue, and green photoreceptors produced by the green, yellow, and red bird-cherry leaves shown in (B). For methods of calculation, see [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050187#pbio-0050187-b048" target="_blank">48</a>]. (E) Excitation spectrum of a COM inferred from behavioral data [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050187#pbio-0050187-b026" target="_blank">26</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050187#pbio-0050187-b031" target="_blank">31</a>]. The mechanism is fed by the three photoreceptors in (C), with positive input from the green and negative input from the other two receptors. Based on behavioral data, a mechanism of this kind is presumably what many insect herbivores use as a “greenness detector.” (F) Excitation of the COM by the three bird-cherry leaves and a blue-purple flower (Ajuga genevensis) on a one-dimensional scale. Note how the yellow leaf produces an even more positive signal than the green leaf.</p>