Exploding Aphids Plaster Holes in Their Home With Bodily Fluids
In an extreme version of the clotting process, the bugs suicidally erupt to save their nest.
If you’ve ever complained about DIY home repairs, spare a thought for the colonial aphid, Nipponaphis monzeni, for whom the task of fixing the house can be spectacularly fatal. It fixes holes in its nest by suicidally erupting and, in its death throes, plastering its bodily fluids over the openings.
Each of these aphids is a white bead, just half a millimeter across. In large numbers, they can compel Japanese trees to form large, hollow spheres called galls—roomy mansions in which hundreds or thousands of them can live. Like ants, bees, and termites, aphids divide their labor: Adults reproduce, while immature nymphs act as both workers and soldiers. If moth caterpillars tunnel their way into the galls, the nymphs stab these intruders to death, using the sharp mouthparts that they normally use to suck sap from trees. That deals with the caterpillar, but what about the huge hole that it leaves in the gall?
The aphid’s solution, discovered in 2003, is dramatic. Dozens or hundreds of the young soldiers will gather around a hole and discharge fluid from a pair of tubes on their backsides. This isn’t a gentle leak but a violent eruption, which drains the nymphs so thoroughly that they shrivel down to just a third of their initial volume. As they dry and die, they also use their legs to mix the fluids over the holes. These harden within an hour, sealing the gap and sometimes entombing the suicide plasterers.
These acts of sacrifice save the colonies. In 2009, Mayako Kutsukake from the National Institute of Advanced Industrial Science and Technology showed in a study that when she halted the aphids’ repairs by absorbing the plasterers’ bodily fluids with tissue, the galls almost always die. If she carried out the repairs herself with glue, the galls and the colonies within them survived. An open gall is vulnerable to predators and desiccation. “Letting the plants heal the galls naturally takes a long time and is very risky for the aphid colony,” says Takema Fukatsu, who led the study.
Back then, Kutsukake and Fukatsu compared the aphids’ repairs to the clotting process that we and other animals use to heal open wounds. Now, after a decade of work, they’ve realized that the analogy is more fitting than they could have imagined.
If an insect is wounded, cells called hemocytes rush to the breach and burst, releasing fats that quickly coagulate into a soft plug. Those bursting cells also release an enzyme called phenoloxidase, which cross-links molecules in the insects’ blood into a hard, reinforced mesh—a scab.
This is almost exactly what happens when the soldier aphids sacrifice themselves at a gall hole. Hemocytes in their bodily fluids rupture, releasing fats that quickly form a plug, which phenoloxidase slowly reinforces. All the components are the same; the soldiers just have a lot more of them. By exaggerating their own immune defenses, they’ve evolved a way of defending their entire colonies. “It’s clotting, just not of the body,” says Nancy Moran from the University of Texas at Austin. “They’re clotting the house that they live in.”
That discovery was hard-won. The aphids can’t be reared in the lab, and in the field, “the gall-repairing soldiers are available only for a few months every year,” Kutsukake says. Her work had to proceed in fits and starts, which is why it took a decade to divine the details of the process.
There are many examples where a colony’s defenses mirror an individual’s: Some ants, for example, will kill infected larvae to stop diseases from spreading, just as their immune systems will destroy infected cells. But Kutsukake’s research “shows that these parallels can even exist at the level of the molecules,” says Sylvia Cremer from the Institute of Science and Technology Austria. In the social aphids, “the same cells and molecules responsible for individual wound healing have been co-opted for colony-level wound healing.”
Many social animals will give up their life to protect their genetically related colony mates—honeybee workers famously die after stinging. But the brand of suicidal altruism where individuals sacrificially rupture themselves has a special name—“autothysis.”
One termite species commits autothysis to release a chemical weapon: When it bursts, blue crystals in its back mingle with chemicals in its salivary glands to create a toxic sludge that kills its opponents. Only older individuals do this; their jaws are too worn to be useful in combat, and, to misquote Neil Young, it’s better to blow up than to fade away.
Meanwhile, several species of exploding ants defend their colonies by flexing so hard that they rip apart their abdomens, unleashing sticky, toxic, corrosive chemicals onto their attackers. To maximize the effect of these chemicals, some workers will stick their abdomens into the mouths of their opponents before letting rip. New species of exploding ants are being discovered all the time; one, which was identified last year, is aptly named Colobopsis explodens.
None of these examples, however, established a clear link between the self-sacrificing behavior and the insect’s own immune system. The social aphids are the first, and they should prompt scientists “to revisit in more detail whether the immune system in those other species is also involved in self-explosion,” says Rebeca Rosengaus from the Northeastern University College of Science.