I often lament the fact that humans are freakishly gargantuan next to nearly all other animals, and thus unable to appreciate the beauty and complexity of the majority of smaller life forms that share the world with us. Yet, at the same time I am thankful that we do not need to contend with the likes of tiger beetles or solifugids, and that ambush bugs will never succeed in luring us into their deadly grip with their imitation of the sounds that humans make. Compared to even the most vicious and dangerous mammals, the world of arthropods is orders of magnitude more advanced in the art of killing – if a lion were to be shrunk to the size of an average insect it would last no more than a few minutes before being dismembered by the first praying mantis or a spider, who would laugh at its puny claws, soft and squishy body, and the complete lack of any real weapons.
And although I prefer to study the more peaceful aspects of invertebrate behavior, such as their courtship and love songs, I cannot help but be impressed by the sophistication of weaponry used by arthropods to kill, or exact revenge on those who try.
On my last night in Mozambique, when I should have been packing and getting some sleep before a long journey home, I suddenly realized that I had almost missed the opportunity to solve the mystery of how ectrichodiine assassin bugs hunt their favorite prey, the giant African millipedes. I had caught a beautifully metallic, wingless assassin bug (Glymmatophora sp.) a few days earlier, and now all I needed was its prey. Luckily, I was still in Gorongosa, where most biological questions can be answered by taking a slow walk and paying attention, and within a few minutes I managed to find a large millipede.
For a docile, seemingly harmless animal, millipedes (Diplopoda) have surprisingly few enemies. As is often the case among small animals, an innocent demeanor hides powerful and quite unexpected defensive strategies. Millipedes have two major lines of defense, both effective enough to deter virtually all predators. First, the chitinous base of their exoskeleton is composed of up to 70 percent calcium and magnesium carbonate, making it much harder than that of most terrestrial arthropods. Few predators are strong enough to crack it, although mongooses have been reported to toss millipedes against rocks to break their shells. For smaller predators such as spiders, the hard exoskeleton is an almost impenetrable barrier. But if one tries its luck anyway, a rich arsenal of chemical warfare stops the predator in its tracks, literally. Some millipedes produce substances that act as sedatives, forcing the spider into a state of suspended animation that lasts for many hours. Interestingly, this substance is virtually identical to the drug quaalude, a synthetic sedative once widely used in medicine and as a recreational drug. But licking millipedes to get high is not a good idea as many species produce cyanide, the most powerful inorganic poison known to man.
Others exude substances that cause vomiting, headaches, and other unpleasant effects. And yet, few millipede species have warning coloration, which is typical of organisms that produce repellant chemicals. It is believed that the extreme effectiveness of their chemical weapons, combined with the very long time they have been around (terrestrial millipedes date back to the Carboniferous), made predators develop a genetically based aversion to these animals, and the millipede’s shape alone is a sufficient warning.
Assassin bugs (Reduviidae: Ectrichodiinae) are some of the very few animals that are able to hunt millipedes. They overcome the millipedes’ mechanical defenses by injecting them with a fast acting venom through a soft membrane between the calcified plates that cover their body. They also avoid being poisoned by their prey’s chemical defenses thanks to the way they feed – rather than chewing food, and thus potentially ingesting some of the toxic compounds, they inject their prey with digestive enzymes and then drink the liquified tissue through a long, sharp rostrum. All this is well known to biologists, of course, but surprisingly there exist very few documented observations of how the hunt unfolds, and what part of the millipede’s body is targeted by the assassin bug. On my last night in Gorongosa I had a chance to find out.
I placed the two animals on the white stage of my field photo studio and waited. After a few minutes the assassin bug noticed the millipede and started to approach it stealthily. With imperceptibly slow steps it climbed the millipede’s body and positioned itself almost upside down, with the head and rostrum facing the ventral side between the millipede’s legs. It was clearly aiming for the softest part of the body. And then, with an amazing speed, it pierced the millipede with its stiletto-like rostrum. The millipede’s reaction was dramatic – its body twisted and uncoiled like a spring, nearly sending it up into the air. After a few seconds the body relaxed, and yellow, acrid droplets started oozing from between its plates. This was the moment when the assassin bug decided to walk away.
I was already feeling bad for staging this drama, but the bug’s behavior made me feel even more guilty. “You ain’t going nowhere, buddy”, I thought as I placed a large container over the predator and its prey, “finish the job and eat it.” I simply could not understand why the assassin bug, which clearly must have been hungry, would suddenly abandon its dinner. And then, within a couple of minutes, the assassin bug was dead.
It all now made sense – the assassin bug was not abandoning its kill, it was just walking away to avoid being poisoned by the deadly benzoquinones dripping from the body of its victim. It would have undoubtedly come back to feed after the millipede had died and most of the volatile chemicals had blown away. By placing a container over the two animals I created a gas chamber that killed them both. I still feel awful about it but, alas, this is how biological mysteries are sometimes solved. I strongly suspect that the scent of benzoquinones, which are only emitted by an injured or dying millipede, acts as a long-range attractant to other assassin bugs that are often seen feeding communally on large millipedes, but at the same time the animal that made the kill must temporarily move away to avoid death from the same compounds. Aren’t invertebrates amazing?