Scientists Solve Mystery of How Caterpillars Heal Their Wounds in Seconds

Insect blood is very different from our own. It lacks hemoglobin and platelets, and uses amoeba-like cells called hemocytes to protect the immune system instead of red blood cells.

This insect equivalent to blood – called hemolymph – also functions very differently. Most notably, hemolymph can clot within a matter of seconds after an insect is wounded, drastically limiting potential blood loss.

This rapid action is supposed to give insects, which are vulnerable to dehydration, the greatest chance of survival after sustaining an injury. But until now, scientists did not understand exactly how hemolymph manages to clot so quickly outside of the body. Understanding this mechanism would be an important first step in assessing whether a similar process could be developed and used for human medicine and trauma response.

In a new paper, published in Frontiers in Soft Matter, materials scientists at Clemson University have fully described how the feat is performed by caterpillars of the Carolina sphinx moth for the first time.

The very tricky caterpillar

Studying insect hemolymph is a difficult process. In their new study, the Clemson University researchers examined fully-grown tobacco hornworms. These caterpillars are around 7.5-10 centimeters long at this stage in development, but still only contain tiny amounts of hemolymph.

In order to study the action of the caterpillars’ hemolymph, the researchers restrained each caterpillar in a plastic sleeve. Through a small cut-out in the sleeve, they made a small wound in one of the caterpillar’s pseudo-legs to draw hemolymph out of the body. Immediately afterwards, the researchers placed a metal ball against the droplet of hemolymph and pulled it away, forming a “bridge” of hemolymph. As the ball was pulled away, the “bridge” narrowed and broke, casting off more tiny droplets of hemolymph. This entire action was recorded by the researchers using a high frame rate camera equipped with a macro lens.

Even with this setup, the hemolymph was tricky to study; the researchers noted a failure rate of up to 95% for some of the more complex manipulations. However, after many attempts, the team was able to closely examine the transformation that occurs in hemolymph after it is spilled.

“Here we show that these caterpillars, called tobacco hornworms, can seal the wounds in a minute. They do that in two steps: first, in a few seconds, their thin, water-like hemolymph becomes ‘viscoelastic’ or slimy, and the dripping hemolymph retracts back to the wound,” explained senior author Dr. Konstantin Kornev, a professor in the Department of Materials Science and Engineering at Clemson University.

“Next, hemocytes aggregate, starting from the wound surface and moving up to embrace the coating hemolymph film that eventually becomes a crust sealing the wound.”

Injuries spur near-instantaneous changes in insect blood

This change in the hemolymph happens incredibly quickly. In the first five seconds after wounding, the hemolymph appeared to flow like water. But after another 10 seconds had passed, the consistency of the hemolymph changed to form a much thicker, more viscous texture. This thicker substance is known as a viscoelastic fluid.

“A good example of a viscoelastic fluid is saliva,” said Kornev. “When you smear a drop between your fingers, it behaves like water: materials scientists will say it is purely viscous. But thanks to very large molecules called mucins in it, saliva forms a bridge when you move your fingers apart. Therefore, it’s properly called viscoelastic: viscous when you shear it and elastic when you stretch it.”

Kornev and colleagues confirmed the viscoelastic nature of freshly spilled hemolymph by placing a nickel nanorod inside a droplet of fresh hemolymph and studying how it responded when exposed to a rotating magnetic field. They saw that, at first, the nanorod would spin well. But after mere seconds, the hemolymph would thicken to the point of causing significant lag between the nanorod spin and the magnetic field’s rotation, caused by the nanorod struggling to push through a more viscous environment.

Around 60-90 seconds after caterpillars were initially wounded, the researchers noted that bleeding completely ceased, with a crust already having formed over the site of the wound.

Could this reaction help humans one day?

Using optical phase-contrast and polarized microscopy, X-ray imaging and other materials science modeling techniques, the researchers looked into the cellular processes that might lead to this quick crust formation in Carolina sphinx moths and their caterpillars. A similar analysis followed for 18 other insect species.

They found that all of the hemolymph from these species reacted similarly to shear forces. However, there was a clear difference in material properties between the caterpillars and cockroaches that have hemocyte-rich hemolymph versus the butterflies and moths that were generally hemocyte-poor. For the latter, the familiar hemolymph “bridge” was never observed after injury.

“Turning hemolymph into a viscoelastic fluid appears to help caterpillars and cockroaches to stop any bleeding, by retracting dripping droplets back to the wound in a few seconds,” said Kornev. “We conclude that their hemolymph has an extraordinary ability to instantaneously change its material properties. Unlike silk-producing insects and spiders, which have a special organ for making fibers, these insects can make hemolymph filaments at any location upon wounding.”

Based on their observations, the researchers believe that these hemocytes are likely the main driver of caterpillar and cockroaches’ unique two-stage clot-forming strategy.

Understanding this behavior could have important implications for human medicine, the researchers believe. While human blood has a very different makeup, there may be some aspects of this mechanism that could be translated or adapted for modern medical applications.

“Our discoveries open the door for designing fast-working thickeners of human blood,” Kornev said. “We needn’t necessarily copy the exact biochemistry, but should focus on designing drugs that could turn blood into a viscoelastic material that stops bleeding. We hope that our findings will help to accomplish this task in the near future.”

Reference: Aprelev P, Brasovs A, Bruce TF, Beard CE, Adler PH, Kornev KG. To seal a wound, caterpillars transform blood from a viscous to a viscoelastic fluid in a few seconds. Front Soft Matter. 2024;4. doi: 10.3389/frsfm.2024.1341129

This article is a rework of a press release issued by Frontiers. Material has been edited for length and content.