The caterpillar is all but invincible. It has bright warning colours that deter any birds which might want to eat it. It releases foul odours that deter hunters like beetle larvae, which rely more on scent. And it carries toxins that would make good on its threats. It’s a shame, then, that this caterpillar is dead. Its defences are the result of the creatures that killed it—an alliance between a parasitic worm and a glowing bacterium.
When the nematode worm Heterorhabditis bacteriophora burrows into an insect, it vomits out thousands of glowing bacteria, Photorhabdus luminescens. These release toxins that kill the insect and break its tissues into a nutritious soup, which the worms consume. The bacteria also make amino acids that the worms need to reproduce, and antibiotics that kill other microbes that might colonise the insect or decompose its corpse. (During the US Civil war, the same bacteria sometimes contaminated the wounds of soldiers, giving them an eerie blue shine while also protecting them from infections—they called it the “angel’s glow”.)
With the help of their bacterial allies, the worms grow within the dead insect, feeding, mating, and breeding. Eventually, their offspring burst out, suck up their own supply of killer bacteria, and head off to find their own hosts.
This takes around 20 days. During that time, the worms are exquisitely vulnerable: They will all die if a predator scavenges the insect host. And since the insect, being dead, is in no position to defend itself, the worms have to take over. They must protect the very body that they themselves killed. They must save it from being eaten from the outside so that they have enough time to eat it from the inside.
Again, their bacterial allies help. They produce toxins that can deter or kill ants, wasps, beetles, and other scavengers. And perhaps more importantly, they help to advertise these defences.
In 2011, Andy Fenton from the University of Liverpool found that the worms use pigments produced by the bacteria to paint their dead insects in warning colours, making them look as unpalatable as possible. As I wrote at the time, infected caterpillars start off as orange but soon take on a bright pink-red hue, which becomes more intense as the infections continue. This colour change was enough to deter robins, which hardly ever ate infected caterpillars but happily chomped down on healthy ones that had been dead for the same time.
Now, Fenton, together with Rebecca Jones and Michael Speed, have shown that the worms also release a pungent smell. They noticed it themselves: Even to their noses, infected insects stank in a way that uninfected individuals never did. Predatory ground beetles could tell the difference, too. When given a choice between the scents of infected and uninfected caterpillars, wafting out of separate jars, they almost always headed towards the uninfected smells.
It makes sense to produce warning smells as well as colours. The former can deter sight-oriented predators like birds, while the latter can put off smell-focused beetles, or foragers that operate at night. The colours also take several days to make, while the smells can be released soon after the infections begin, providing an earlier line of defence.
Indeed, there may be even earlier defences that the team haven’t discovered yet. Does the brief glow that the bacteria emit have a protective role? And since the bacteria produce light, toxins, off-putting odours, and warning colours, how do they prioritise between these different defences, given that each one takes energy to produce?