Yesterday, reader Roo sent me the Torygraph‘s photo of the day, which is an assassin bug. The caption is below (I’m not sure why they use the past tense):
These ruthless Assassin bugs hid from potential predators using a camouflage cloak – made from the bodies of ants they had killed. The deadly insects paralysed the ants by injecting them with a toxic enzyme before sucking them dry. They then piled the dried-out corpses on their sticky backs to act as a defence against other predators, such as jumping spiders. Picture: Guek Hock Ping/Photoshot/Solent News
Note that assassin bugs (unlike “ladybugs,” which are beetles in the order Coleoptera) really are bugs : they’re in the order Hemiptera, or “true bugs.” (If I want readers to learn anything from this site, it’s to use the word “bug” properly!) They’re also in the order Reduviidae, some of whose New World species—probably not the one above—carry the protozoans that cause Chagas disease, an often asymptomatic but sometimes fatal illness. For many years people thought that Darwin had been infected with Chagas on his Beagle voyage, accounting for his frequent and lifelong bouts of illness, including malaise and vomiting. We’ll never know for sure, for doctors have suggested many other causes, ranging from simple nervousness to the latest Darwin-illness fad, cyclical vomiting syndrome.
Assassin bugs are so called because they stick their snout (“rostrum,” if you want to be technical) into the prey, injecting a saliva that liquifies the prey’s insides. They then suck it dry.
It’s interesting to speculate how this evolved. This adaptation (and who can deny that it is one?) involves both a morphological trait (a sticky back) and a behavioral trait (the tendency to put the husks of your prey onto your back). Without that sticky back, you have no initial advantage, so I suspect that the evolution of this mimicry began simply because the bug had a back that could adhere to dead insects, perhaps because of cuticular lipids that served other functions, like desiccation resistance or attracting mates. Perhaps a prey accidentally adhered to one of the bugs with a particularly sticky back, and that individual gained an advantage, as it was simply harder to attack and eat. This would give an advantage to genes producing not only stickier backs, but also promoting any tendency to place sucked-out prey on your back. I am curious whether the ant carcasses are inherently sticky too—as they appear to adhere to each other—or whether the bug actually puts something on them to help them stick together.
But this is all speculation. What is on firmer ground is the idea (still probably not demonstrated through experiment) that this is a remarkable adaptation to deter predation. I wouldn’t call it “mimicry” (unless predators avoid piles of dead ants), for this ant-covered bug isn’t really deceiving the predator by “pretending” to be something else. It’s simply making it harder for predators to grasp and eat them.