Assassin Bugs Use Corpses to Avoid Predators

Not only can your prey give your nourishment and a full belly, it can also be a shield to protect you from predators! A recent article by Jackson & Pollard in the Journal of Zoology described an interesting study in which the ant-snatching assassin bug (Acanthaspis petax) makes a "backpack" of its dead prey to avoid being seen by its predator, the deadly jumping spider (Salticidae)! Masking to avoid predation is not a novel development. There are decorator crabs who will take just about anything and put it on its back (including other live critters), sea urchins will don mussel shells and some weevils will even grow fungi, algae or moss on its back. What I think is cool about this system is the use of "corpse camouflaging". Not only can it save you from starvation, it can save your butt too!

"...they capture prey by grabbing it with their legs, piercing it with their syringelike mouthparts and using the ‘syringe’ to inject digestive enzymes and paralysing saliva. The bug then proceeds to suck up the digested ant tissues from inside the ant’s exoskeleton. Once finished, the assassin bug places what remains of the ant, an empty shell, on its back. The ant carcass adheres because of fine adhesive threads that the bug secretes from its abdomen."-Jackson & Pollard (2007)

Now thats recycling! I think we can learn something here. Perhaps we should recycle our food waste into new and life-saving innovations. Hmm... what can we do with all those leftover lemon and orange rinds?

Figure 1 (from Jackson & Pollard (2007)): Two juvenile Acanthaspis petax: one without (left) and one with (right) a mask constructed of ant carcasses.

Jackson & Pollard (2007) demonstrated that Acanthaspis with corpse backpacks were significantly picked off less by jumping spiders. They tested this using 3 different jumping spiders: Hyllus sp., Plexippus sp. and Thyene sp. Salticids are vision-guided predators so they would not be able to detect the assassin bug by smell per se. Spiders were presented either with naked, or unmasked, bugs or bugs masked with the putrid remains of their last meals (see figure above).

The result is that hits by all three species of spiders significantly increased when bugs were unmasked, meaning that the corpse camouflage is a successful deterrent to predators. To test whether the size of the mask affected attacks, the authors made lures that either exceeded or were shorter than the length of the naked bug. The result was the same in all scenarios: unmasked bugs were attacked an order of magnitude more than masked bugs. Hence, prey size is not a factor and the disparity between masked and unmasked bugs is due to the shield of dead ant bodies in and of itself. The lures also helped to control for confounding variables of using the live bugs (motion, behavior and other sensory cues), lending strength to their study.

But why a mask constructed of ant corpses? Ants are not necessarily the best prey for many species and are well respected across the animal kingdom. Typically ants are chemically defended or behaviorally defended (think of the swarming behavior of some species of ants), making them a difficult prey item. Additionally, there's not a lot of meat there so the effort put in might not balance with nutrition received. The authors hypothesize that their data suggest

"... that the salticid readily detects the masked bug as an object separate from the background, but it fails to identify it as potential prey.[...] As most salticids may be averse to preying on or coming close to ants (Nelson & Jackson, 2006), the fact that the bug’s mask is normally made of ants may be important when the bugs encounter salticids."-Jackson & Pollard (2007)

It would be interesting to see how this behavior evolved, what genes turn on this behavior. Once the gene(s) is/are found, how does that gene's phylogeny map over more conservative (i.e. less selected) genetic markers like mitochondrial DNA? A cross comparison with other decorating species would be very interesting in understanding the evolutionary history that gave rise to decorating behavior and how that behavior became instinct that is rooted in the taxon.
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Jackson RR, Pollard SD (2007) Bugs with backpacks deter vision-guided predation by jumping spiders. Journal of Zoology 273:358-363. doi:10.1111/j.1469-7998.2007.00335.x

Nelson, X.J. & Jackson, R.R. (2006). Vision-based innate aversion to ants and ant mimics. Behav. Ecol. 17, 676–681. doi:10.1093/beheco/ark017