Spiders have rich pigmentary and structural colour palettes

What is it about?

1. we show that spiders use carotenoids to produce yellow, suggesting that such colours may be used for condition-dependent courtship signaling. 2. we establish the Raman signature spectrum for ommochromes, facilitating the identification of ommochromes in a variety of organisms in the future. 3. we describe a potential new pigmentary/structural colour interaction that is unusual because it uses a shorter wavelength pigment in the production of red. 4. we present the first evidence for the presence of melanosomes in arthropods using both scanning and transmission electron microscopy, overturning the assumption that melanosomes are a synapomorphy of vertebrates.

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Why is it important?

Most current research on spider colouration focuses on answering functional questions through behavioral and/or ecological studies. However, the mechanistic approach to understand the biochemical basis of colours we describe here adds a novel dimension to these functional hypotheses. For example, it could help us understand the evolutionary origin of similar colour hues among related species, or help to determine that a particular colour is condition dependent and therefore useful as an honest signal in sexual selection. Understanding colour production mechanisms may also reveal potential non-visual functions. For example, finding eumelanin in spiders hints at functions like photoprotection, microbial resistance, and mechano-enhancement of cuticle sclerization. Similarly, Nephila’s golden web is sometimes hypothesized to attract insects to webs through visual cues. We found that Nephila’s golden silk contains carotenoids. While carotenoids certainly produce yellow colouration, they also degrade naturally through oxidative cleavage or by carotenoids cleavage dioxygenases (CCDs) to produce volatile apocarotenoids. Apocarotenoids are associated with the scents of flowers, ripe fruits, and the pheromones of Hymenopteran/Lepidoptera, hence they might attract insects to the webs as olfactory cues. While speculative, carotenoids cleavage dioxygenases are active under high light, and this could explain why Nephila sometimes weaves golden webs under high light intensity environments, but “normal” white webs in dim environments.