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When tobacco hornworm caterpillars feel pain, they don't wriggle or try to escape. Instead, according to the latest research, they dial down their pain response by adopting a posture known as the "sphinx" state, so called because it resembles an ancient sphinx statue.

Most animals react to pain by standing their ground to defend themselves or by running away, the fight or flight response. But a research team from Tufts University in Massachusetts found that tobacco hornworm caterpillars do the opposite. They curl their head and thorax downward and remain still.

First, the researchers wanted to see what would make the caterpillars enter this pose and tried a variety of mechanical stimuli. These included stroking the caterpillars with a paintbrush, tapping the material they were resting on and moving it. In every case, their response was the same, and they entered the sphinx state.

The researchers also tested the insects' response to painful stimuli while in this state. They found that when they subjected different parts of the caterpillar's body to heat, the insects showed reduced responsiveness.

To work out how the sphinx state is triggered, the researchers performed surgery on the cerebral ganglion, the main part of the insect brain. They discovered that the brain region that controls movement and sensory input cannot activate the state automatically. In other words, it is not a passive response but one that the caterpillars actively choose to do.

Typically, the reaction to pain is an automatic reflex in animals known as nociception. However, the caterpillars' behavior is different, as the researchers note in their study in Biology Letters.

"This discovery reveals a novel instance of active behavioral modulation in insects and highlights the flexibility of nociceptive responses, challenging the notion of nociception as strictly hard-wired and stereotyped."

Pest management and pain relief

Insights from this research could lead to a host of practical applications. Understanding how insects actively manage pain and defensive behaviors could lead to new strategies for pest control. Instead of using broad-spectrum pesticides, which can harm other organisms, scientists may be able to develop more targeted solutions that make pests unable to feed or reproduce.

There are also significant implications for pain research in humans. Although our brains are much more complex than those of insects, studying how they turn off their pain signals could reveal more about pain suppression. The relative simplicity of a caterpillar's brain makes it easier for researchers to see the exact regions involved in a conscious decision to suppress a painful sensation.