I’m shamelessly stealing this story from Alex Wild’s great Scientific American website, Compound Eye. His latest post describes a paper from the Proceedings of the Royal Society B (link below) by Wignali and Taylor, who show that assassin bugs from Australia (Stenolemus bituberus; these are true bugs in the order Hemiptera) kill spiders by entering their webs and producing vibrations that lure the spider by mimicking either the vibrations made by normal prey trapped in the web. This shows that the evolution of mimicry need not involve any change in appearance but simply a change in behavior: in this case natural selection has favored those assassin bugs who are able to vibrate spider webs with the proper frequency.
Assassin bugs are usually called “thread-legged bugs” for obvious reasons:
They’re cryptic, too; as Alex notes: “In the field the insect looked like so little I thought it merely debris in a disorganized spider’s web. I didn’t see the faint outline of a young assassin bug until the debris shuddered, ever so slightly.” (Remember that some spiders, and of course bug-hunting birds, have keen vision.)
I hope my readers are now biology-savvy enough to understand the paper’s abstract:
Assassin bugs (Stenolemus bituberus) hunt web-building spiders by invading the web and plucking the silk to generate vibrations that lure the resident spider into striking range. To test whether vibrations generated by bugs aggressively mimic the vibrations generated by insect prey, we compared the responses of spiders to bugs with how they responded to prey, courting male spiders and leaves falling into the web. We also analysed the associated vibrations. Similar spider orientation and approach behaviours were observed in response to vibrations from bugs and prey, whereas different behaviours were observed in response to vibrations from male spiders and leaves. Peak frequency and duration of vibrations generated by bugs were similar to those generated by prey and courting males. Further, vibrations from bugs had a temporal structure and amplitude that were similar to vibrations generated by leg and body movements of prey and distinctly different to vibrations from courting males or leaves, or prey beating their wings. To be an effective predator, bugs do not need to mimic the full range of prey vibrations. Instead bugs are general mimics of a subset of prey vibrations that fall within the range of vibrations classified by spiders as ‘prey’.
Here’s another photo showing the bug entering a spider’s web for nefarious purposes:
Finally, a video (taken from the original paper via Alex) showing an assassin bug luring a spider to its death:
Some assassin bugs also kill spiders not by mimicking prey vibrations, but by sneaking up on them and stabbing them with their mouthparts (ergo their name). An earlier BBC report notes that, when using this latter tactic, assassin bugs are most likely to move toward their spider prey when the wind is blowing, masking any vibrations produced by their movement. They’re like ninja cats! This was demonstrated in clever experiments using fans to mimic the vibrations of spider webs produced by wind.
Wignali, A. E. and P. W. Taylor. 2013. Assassin bug uses aggressive mimicry to lure spider prey. Proc. R. Soc. B 7 May 2011 vol. 278 no. 1710 1427-143, Published online October 27, 2010 doi: 10.1098/rspb.2010.2060
See also: Wignall, A.E. . & Taylor, P.W. (2008). Biology and life history of the araneophagic assassin bug Stenolemus bituberus including a morphometric analysis of the instars (Heteroptera, Reduviidae).. Journal of Natural History 42: 59-76. (pdf here)