These Beetles Withstand Cyanide Blasts to Eat Millipedes
By Ed Ricciuti
A fast, vicious predator leaps upon the armored back of its would-be prey, ripping and rending with powerful jaws. The victim twists like a pretzel, rolling and thrashing, trying to dislodge its tormentor. This epic battle that exploded right under the noses of two University of California–Berkley scientists was worthy of a clash between two Mesozoic monsters (think “raptor pouncing on ankylosaurus”) but really raged in a condiment cup four inches across.
The scientists were exploring why the two species that constitute the beetle genus Promecognathus can shrug off a knockout blast of cyanide gas, the go-to defensive weapon deployed by many flat-backed millipedes (Polydesmida) against predators. They found that the beetles, which specialize in targeting millipedes, do not use avoidance behavior or any other observable countermeasures, but survive the gas attack because of a high tolerance to cyanide. Their findings are reported in a new study published this month in Annals of the Entomological Society of America.
A few predators besides Promecognathus have evolved tactics that enable them to avoid the cyanide weapons of millipedes, some of which produce 18 times the dose that can kill the average pigeon. Glowworm beetles (Phengodidae), for example, inject millipedes with a fluid that paralyzes them before they can discharge cyanide.
As well, some herbivorous insects manage to eat plants containing cyanide by enzymatically detoxifying it, but, say the authors, “There are no documented cases of a predator that has an enzyme-based resistance to cyanide. … This is the first recorded instance of cyanide tolerance in predatory insects, or any predatory animal as far as we know.” What they do not know is what enables the tolerance, although it may be some sort of detoxification mechanism or other adaption so far undetected.
Learning more about the beetles’ resistance to cyanide could not only further entomological knowledge, but also prove beneficial to human medicine. Further study of the biochemistry of Promecognathus could open doors to new treatment of cyanide poisoning in humans, suggest the researchers, Kipling W. Will, Ph.D., and his student, Brandt P. Weary, at the University of California Berkeley.
Will and Brandt collected the two beetle species, P. laevissimus and P. crassus, along with their inch-long millipede prey, Xystocheir dissecta, in the San Francisco Bay Area. The beetles inhabit a restricted range that spans from southern British Columbia to northern California along the coast and coastal mountain ranges. They are especially abundant in oak woodlands, which cover a vast amount of landscape in the region. Their activity is moisture driven, peaking in late winter to early spring depending on the winter rains. At higher elevations and to the north, the colder winters push activity to later in the spring and early summer. When it is dry, they hunker down and are hard or impossible to find. The millipede has an even smaller range than the beetles, mostly in and around the San Francisco Bay Area, and their seasonal activity correlates with that of the beetles.
To check whether the beetles use avoidance tactics, the researchers placed them with millipedes in a cup, one on one, to watch their hunting behavior. Beetles that successfully killed millipedes jumped atop their prey, repeatedly biting with their long, thin, pointed mandibles.
The attack techniques of the beetles have two modes. If the assault mounts from the rear end of the millipede, the beetle clambers on the prey’s back and bites repeatedly, clamping down its mandibles well short of the head. In a head-on attack, the beetle climbs atop the millipede, bites and turns so that it faces the same direction as its prey, again short of the head. It inserts its mandibles between the thick plates protecting the millipede’s segmented body and bites with a scissoring action. Millipedes that succumbed—after struggling for up to 45 minutes—died from bleeding and exhaustion.
“We did not, however, find any evidence that Promecognathus uses this behavior to minimize its exposure to cyanide,” say the researchers. “The predation events were protracted and the millipedes produced their chemical defense in profuse amounts, enough to adversely affect a human observer.” Indeed, the millipedes released so much cyanide that the researchers were able to smell it from a meter away. Says Brandt, “While observing the beetles and millipedes in an enclosed room, I developed eye pain and a dull headache, which lasted for several hours afterwards. In the future, I made sure to always observe millipedes in a well-ventilated room, and I never experienced problems again. It is possible that the cyanide vapors irritated my eyes.”
The beetles, on the other hand, seemed impervious to the poisonous gas. “Cyanide did not seem to affect the behavior of Promecognathus in any way, and therefore we propose that cyanide tolerance in this millipede specialist is likely part of its suite of adaptations to millipede hunting,” say the authors.
Each millipede responded to a beetle attack violently, twisting and turning in an effort to literally get the enemy off its back and taking advantage of its natural body armor by curling into a tight little ball, shielding its underside. For most attackers, this defense works (especially when combined with cyanide), but it does not deter Promecognathus.
The millipede predators belong to a huge family of beetles, the ground beetles (Carabidae), which look like just about every person’s concept of a typical beetle. When the researchers placed other species of carabids with live millipedes, the beetles pretty much shunned them, although they did eat pre-killed millipedes that could not produce cyanide. The observations support the unique ability of Promecognathus to circumvent the cyanide defense.
Their prey are members of the Polydesmida, an order of 3,500 species, many of which are able to synthesize and store chemical precursors to cyanide, such as benzaldehyde cyanohydrin, in glands strung in pairs along the side of each segment. The glands are not just factories, but reaction chambers in which enzymes catalyze the precursors, which break down into poisonous hydrogen cyanide gas and almond-smelling benzaldehyde. Under duress, sometimes only at a touch, the millipede squirts the emission through a microscopic opening in each gland.
The researchers compared the reactions of Promecognathus and the other carabids to both cyanide and benzaldehyde under laboratory conditions. Promecognathus gobbled with gusto dog food heavily dosed with benzaldehyde, while the others pretty much ignored it. The almond-smelling chemical, the researchers suggest, may be an effective warning or deterrent that keeps most potential predators at bay. When hit with doses of cyanide far above what millipedes produce, Promecognathus beetles were up and about even after two hours of exposure, while other carabids were quickly knocked down.
Will and Brandt theorize that Promecognathus probably evolved exceptional cyanide tolerance, long mandibles, a unique prey-handling behavior, and a similar seasonal life cycle to overcome its prey’s arsenal of defenses and take advantage of a food source for which few other animals compete. They hope that further study will solve the mystery of how the beetles survive a poison that can be so deadly.
“My dream,” says Brandt, “is that this study paves the way for a wave of research on millipede-specialists. By studying Promecognathus and other insects, we may learn how to cure or prevent a variety of poison-related illnesses. For millions of years, natural selection has solved problems that humans have only just begun to tackle.”
Annals of the Entomological Society of America
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.