The power that natural selection has to sculpt both the appearance and the behaviours of creatures so that they intricately and precisely fit their respective environments is for me a source of endless fascination and wonder. Some of the most impressive examples of natural selection’s power lie among the mimics of the insect and spider world where a huge diversity of body forms are to be found, from insects which look uncannily like leaves or moss, to spiders that look just like ants. The benefits of these disguises vary from species to species. For many blending seamlessly into the background provides some protection against predators, while for others it allows them to creep up on their prey unnoticed or lure victims to their demise.
The south-east Asian orb-web spider known as Cyclosa ginnaga is a perfect example of how mimicry may be used to conceal an animal from its predators, in this case highly aggressive predatory wasps. Although by themselves individuals of this species are conspicuously silver in colour and not all that well disguised, they are able to spin white circular silk decorations which they stand on in the centre of their webs as a way of concealing themselves. That might not sound like a great way to hide but the size, shape and colour of the spider when viewed against the white background of its decoration look remarkably like a bird dropping which, of course, is of no interest to predators. This type of mimicry, in which animals mimic inanimate objects, is termed masquerading and the details of this particular case were recently published in a new paper by Min-Hui Liu and colleagues.
Liu and colleagues wanted to know if the decoration of C. ginnaga really did function as an anti-predator masquerade. To test this the researchers first used a technique called spectral reflectance imaging to examine how the spider and its decoration appears through the eyes of its predators. After all, what looks like a bird dropping to us may look completely different to a wasp. This method compared the way that light reflects from the body of the spider and its decoration to what is known about the sensitivity of insect eyes. The results were unequivocal, wasps cannot see the difference between bird droppings and the masquerade display of C. ginnaga.
The crucial test however, was to show that mimicking a bird dropping really does reduce the predation risk for the spiders and lead to real fitness benefits for individuals. To do this the researchers divided 39 wild caught spiders into three groups. To one group they coloured the bodies of the spiders black while leaving their decorations untouched, to another they coloured the decorations black while leaving the spiders themselves untouched, and to the final group they coloured both the bodies of the spider and their decorations black. They then observed the frequency of predator attacks on each group over 13 days and compared this to the predation rate on untouched spiders. From these three groups a huge increase in predation was observed on those spiders that had only their decorations blackened. This suggests strongly that having a white decoration really does help C. ginnaga to hide itself from predators.
As shown in the figure above, when both the spider and its decoration were blackened no increase in the frequency of predator attacks was observed. This is not so surprising as in this case the black spider is likely to be well camouflaged against its black background. What is surprising however, is that when the spiders body was blackened but the decoration was not there was also no increase in predator attacks. This seems strange since a black body on a white background might be expected to stand out very clearly to predators. It may be that the wasps recognise only silver spiders as their target species and so don’t see the black coloured spiders as potential prey. It could also be that when a black spider is on a white background it still looks like a bird dropping since bird droppings often have black bits in them. The authors don’t discuss this anomaly in their paper but it does cast doubt on the idea that the silver spider in combination with the white decoration together form a masquerade which reduces the risk of predation. Nonetheless it is clear from this study that the web decoration does substantially reduce predator attacks, why that is so remains an interesting question that is open for discussion.
Liu MH, Blamires SJ, Liao CP, & Tso IM (2014). Evidence of bird dropping masquerading by a spider to avoid predators. Scientific reports, 4 PMID: 24875182