Colouration in crab spiders: substrate choice and prey attraction

Abstract

SUMMARY Australian crab spiders Thomisus spectabilis ambush pollinating insects, such as honeybees (Apis mellifera) on flowers, and can change their body colour between yellow and white. It is traditionally assumed that the spiders change their colour to match the flower colour, thus rendering them cryptic to insect prey. Here, we test this assumption combining state-of-the-art knowledge of bee vision and behavioural experiments. In the field, yellow spiders are only found on yellow daisies(Chrysanthemum frutescens), whereas white spiders are found on yellow and white daisies. These field patterns were confirmed in the laboratory. When given the choice between white and yellow daisies, yellow spiders preferred yellow daisies, whereas white spiders showed only a slight but non-significant preference for white flowers. Thus, T. spectabilis select background colours according to their own body colour. When viewed from a distance, bees use an achromatic signal produced by their green receptors for target detection. Through this visual channel, white spiders on white flowers, and yellow spiders on yellow flowers are virtually undetectable. From a closer distance of a few centimetres, when bees evaluate colour contrast, the combination of spider colour against different flower backgrounds affected the response of honeybees, but not in ways predicted by a classical crypsis/conspicuousness interpretation. Yellow spiders on yellow flowers are not perfectly matched when interpreted through the colour vision of a honeybee. Nevertheless, honeybees showed indifference to the presence of a spider, equally landing on vacant or spider-occupied flowers. Likewise, white spiders are poorly hidden on white flowers, as white spiders reflect ultraviolet light strongly, while white flowers do not. Surprisingly, bees are attracted to this contrast, and significantly more honeybees preferred white flowers occupied by white spiders. White spiders on yellow flowers produce the highest colour contrast and bees again preferred spider-occupied flowers. Yellow spiders on white flowers were the only pairing where bees rejected spider-occupied flowers, especially in cases where the contrast between the two was relatively strong. Thus, T. spectabilis select flower colours adaptively in a way that deceives honeybees, or at least does not deter them.

The functional morphology of color changing in a spider: development of ommochrome pigment granules

“…Hypothesis 3: the color of ommochromes is believed to be used in signalling, mimicry and crypsis. This is the hypothesis supported by most of the community working on color changing insects such as stick insects and mantids (FuzeauBraesch, 1985), including Mantis religiosa, Sphodromantis viridis and Locusta migratoria (Vuillaume, 1968), and spiders (Rabaud, 1918;Rabaud, 1919;Gabritschevsky, 1927;Chittka, 2001;Schmalhofer, 2000;Théry and Casas, 2002;Heiling et al, 2003;Heiling et al, 2005;Théry et al, 2005;Théry, 2007).…”

Function of bright coloration in the wasp spider Argiope bruennichi (Araneae: Araneidae)

“…For example, the Australian crab spider, Thomisus spectabilis, closely matches flower colour within the human visibility spectrum, but contrasts strongly under UV, which bees can also see. Studies have concluded that this UV colour contrast is a sensory trap that results in more frequent visits to flowers occupied by crab spiders (Chittka 2001;Heiling et al 2003Heiling et al , 2005. Similarly, Tso et al (2002) and Tso et al (2004) found that naturally occurring melanics of the wood spider Nephila maculata, on which UV-reflective markings are absent, have lower rates of insect interception and catch fewer insects than do brightly coloured morphs.…”

First report of predation on floral visitors by crab spiders on Croton selowii Baill. (Euphorbiaceae)

“…Studies show that these Thomisidae spiders (including species of Misumenoides F.O.P-Cambridge 1900 and Misumenops F.O.P-Cambridge 1900) can modify, gradually and reversibly, their body color to resemble the fl owers, a behavior that avoids their recognition by fl oral visitors (Schmalhofer 2000), and even may enhance fl ower attraction (Heiling et al 2005). Some studies suggest that the forelimbs seem to be the spider trait recognized by pollinators (mainly hymenopterans), thus both the spider position in the plant and the apparent use of fl oral substances observed here seem to constitute strategies to camoufl age the forelimbs (Gonçalves-Souza et al 2008).…”