The kiss of death: Deceptive predatory tactics of assassin bugs

Assassin bugs (Reduviidae) belong to the Hemipteran order, sometimes referred to as “true bugs.” Hemipterans also include aphids, leafhoppers, and cicadas. Like all Hemipterans, assassin bugs feed using a specialized proboscis, called a rostrum. However, unlike their vegetarian, sap-sucking cousins, assassins use their rostrum for extracting fluids from living prey.

Though most assassin bugs feed on other insects and arachnids, some species predate mammals such as bats (video) and humans. In fact, certain species of assassins are the primary vector of Chagas disease in humans. Assassin bugs are evolutionarily specialized for their predatory lifestyles in an astonishing number of ways:

  • Stylets and digestive venom: Integrated with the rostrum, assassin bugs have specialized serrated stylets for tearing into crevices in animal tissue. Once inside, the assassin injects a digestive saliva that breaks down the unlucky prey’s innards into a nutrient rich slurry; which the assassin then sucks back up through the stylet.
  • Raptorial forelegs: Thread-legged assassin bugs (Emesinae) have beefed-up forelegs equipped with sharp spines for grabbing, impaling, or pinning their prey. Liken this adaptation to the forelegs of praying mantids and “spearer” mantis shrimp.
  • The specialized foreleg of thread-legged assassin bugs (Redei, 2007; artour_a).

  • Sticky hairs: Assassin bugs of the genus Zelus also use their forelegs to capture prey. However, instead of sharp spines, they use fine hairs coated in a glue like substance. It is not known if the bugs produce their own glue, or if they obtain it from plant sap. Regardless, they use it to ambush and immobilize prey as they begin to liquidate their insides.
  • Zelus longipes has fine hairs (electron micrograph, right panel) on its forelegs which are coated with a viscous, glue-like material. This is used to immobilize prey. Click the image for a much larger version. (Photo by: Chuck Ulmer; SEM image adapted from Werner and Reid, 2001)

  • Lure signals: Feather-legged assassin bugs (Holoptilinae) lure ants to their doom with visual signals and pheromones produced in a special organ, called a trichome. Read more at Myrmician.

Wow, I really got sidetracked in that introduction. I had no idea how awesome assassin bugs were. Every bit of research I completed for this post led me to another exciting factoid.

Regardless, I need to get to the point of this post, which is a new paper about some disturbingly sinister predatory tactics in the assassin bug species, Stenolemus bituberus. This assassin bug has its work cut out for itself, as it predominately stalks some truly dangerous quarry, arachnids. The assassin bug faces the challenge of obtaining an advantageous position on the spider; from which it can launch a swift, fatal strike. The researchers found that these sneaky assassins use more than one technique to outsmart and turn the tables on their cunning prey.

The Australian based researchers placed assassin bugs on the webs of five species of spider. Through tedious observation they discovered that S. bituberus uses two contrasting methods to get the drop on web-building spiders. Both methods involve manipulation of the spider’s own web.

First, in a stalking behavior, the assassin sneaks up on the spider. In order to accomplish this, the bug walks over the web with an irregular pattern of footsteps. The spider does not notice arrhythmic motions, and the assassin is able to get within striking distance. This technique is also used by web-invading jumping spiders (and is useful when you want to avoid drawing the attention of colossal sandworms while crossing the deserts of Arrakis). In addition, the assassin bugs also make use of natural “smokescreens” such as strong gusts of wind on the webs in order to advance on the unwitting spiders.

The researchers also noted a second predatory behavior in which the assassin bugs bait and lure the spiders. They accomplish this by plucking the web in such a way as to mimic the struggles of a helplessly trapped insect. When the spider comes over to inspect and process its captive, it instead gets an carapace-full of rostrum, as the assassin bug pounces it. Also, considering the ant-luring techniques of the feather-legged assassins described above, one must wonder if chemical attractants are involved in this case as well. Watch a video of the luring and striking behavior, here.

As an interesting aside, the researchers also noted that the assassin bugs habitually tapped the spiders with their antenna just prior to the strike. This behavior is seen in other predatory arthropods, however its purpose is not clear. It is possible that the assassin bug is getting last-second distance, orientation, and identity information about the spider before launching its attack. Another possibility is that the assassin bug is hypnotizing the spider, habituating it to stimuli, so that it is less likely to respond violently when the assassin strikes for real.

Damn, these bugs are awesome.

References:

  • Wignall, A., & Taylor, P. 2010. Predatory behaviour of an araneophagic assassin bug. Journal of Ethology. DOI: 10.1007/s10164-009-0202-8
  • Wolf, K., & Reid, W. 2001 Surface Morphology of Legs in the Assassin Bug (Hemiptera: Reduviidae): A Scanning Electron Microscopy Study with an Emphasis on Hairs and Pores. Annals of the Entomological Society of America, 94(3), 457-461. DOI: 10.1603/0013-8746(2001)094[0457:SMOLIT]2.0.CO;2
  • Redei, D. 2007. New and little-known thread-legged assassin bugs from Australia and New Guinea (Heteroptera: Reduviidae: Emesinae). Acta Zoologica Academiae Scientiarum Hungaricae. 53 (4), 363–379.

7 Comments

  1. Steve Ziegler June 13, 2010 10:55 pm

    I am wondering if you can help with an ID on an assassin bug. Unfortunately, I did not get a picture, but here is the info.
    Spotted in South Eastern PA in early June.
    Looks very much like the assassin in the first picture of this post. Very similar in the abdomen especially. That abdomen was flattened an curled up a bit.
    But the way it is different from the first picture if this post was what really intrigued me about it. The legs were much thicker than that bug. In fact, there was little doubt, by seeing the dark coloration, with light colored thick legs,coupled with a robotic gait, that this assassin bug was most certainly trying to mimic a jumping spider. Was it trying to ward off predators, since jumping spiders have a nasty bite? Did it feed on them? I don’t know, but I am intrigued. Please let me know if you have any ideas. Thanks!

  2. Mike Bok June 25, 2010 6:44 am

    Steve,

    Sorry for the slow reply, I was in the field in Australia with limited internet access.

    Unfortunately, this is a little out of my area of expertise. I would probably be able to help with crustacean IDs, but researching this article was essentially the first hard look I took at assassin bugs. I would suggest checking out the Reduviidae section on Bug Guide. Check out some photos and descriptions of the various subfamilies of assassins and see if any of the images and localities match up. IF you can’t get it you can submit your query to one of their experts.

    As far as mimicking jumping spiders; these bugs have a tremendous diversity of predatory tactics (many more than I could include in the article), and it would be unsurprising if certain species mimicked their target prey.

    However, I think assassins are more commonly preyed on by jumping spiders than vice versa. Check out this article about assassins trying to camouflage themselves from jumping spiders by decorating themselves with the corpses of ants. So yeah, its a possibility that the bug you saw was using a defensive mimicry tactic.

    http://arthropoda.wordpress.com

  3. myrmician July 29, 2010 12:37 am

    Nice post, and thanks for the plug.

    http://myrmician.wordpress.com

  4. Ted C. MacRae October 6, 2010 12:44 am

    Great post, but I love the seamless Dune reference!

    http://beetlesinthebush.wordpress.com

    • Michael Bok October 6, 2010 9:50 am

      I try to sneak Dune references in whenever possible.