Scientists discover how caterpillars can stop their own bleeding in seconds

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Tobacco hornworm, ie. late instar caterpillar of the Carolina sphinx moth. Credit: Konstantin Kornev


Tobacco hornworm, ie. late instar caterpillar of the Carolina sphinx moth. Credit: Konstantin Kornev

Blood is an extraordinary material: it must remain liquid inside the blood vessels, but must clot as quickly as possible outside them to stop the bleeding. The chemical cascade that makes this possible is well understood for vertebrate blood. But hemolymph, the equivalent of blood in insects, has a very different composition, lacking red blood cells, hemoglobin, and platelets, and having amoeba-like cells called hemocytes instead of white blood cells for immune defense.

Like blood, hemolymph clots quickly outside the body. How it does this has long remained a mystery. Now, materials scientists have shown in Limits on Soft Matter how this feat is managed by Carolina sphinx moth caterpillars. This discovery has potential applications for human medicine, the authors said.

“Here we show that these caterpillars, called tobacco worms, can close wounds in under a minute. They do it in two steps: first, in seconds, their thin, water-like hemolymph becomes ‘viscoelastic.’ or slippery, and the dripping hemolymph is drawn back into the wound,” said senior author Dr. Konstantin Kornev, a professor in Clemson University’s Department of Materials Science and Engineering.

“Hemocytes then accumulate, starting at the surface of the wound and moving up to embrace the layer of hemolymph layer that eventually becomes a crust that seals the wound.”

Challenging to study

Fully grown tobacco worms, ready to pupate, are between 7.5 cm and 10 cm long. They contain only a small amount of hemolymph, which usually clots within seconds, making it difficult to study with conventional methods.

For these reasons, Kornev and colleagues had to develop new techniques for the current study and work quickly. However, the failure rate for the most complicated manipulations was high (up to 95%), requiring a lot of effort.

They restrained individual hornworms in a plastic sleeve and made a light wound on one of each caterpillar’s pseudolegs through a window in the sleeve. They then touched the dripping hemolymph with a metal ball, which retracted, creating a hemolymph ‘bridge’ (about two millimeters long and hundreds of micrometers wide) that then narrowed and broke, producing satellite droplets. Kornev and team filmed these events with a high frame rate camera and macro lens to study them in detail.

Tobacco hornworm, ie. late instar caterpillar of the Carolina sphinx moth. Credit: Konstantin Kornev


Tobacco hornworm, ie. late instar caterpillar of the Carolina sphinx moth. Credit: Konstantin Kornev

Instant property change

These observations suggested that during the first approximately five seconds after the onset of flow, the hemolymph behaved similarly to water: in technical terms, as a low-viscosity Newtonian fluid. But within the next 10 seconds, the hemolymph underwent a noticeable change: it now did not break immediately, but formed a long bridge after the fall. Typically, bleeding stops completely after 60 to 90 seconds, after a crust forms over the wound.

Kornev and colleagues further studied the flow properties of hemolymph by placing a 10-micrometer-long nickel nanorod in a drop of fresh hemolymph. When a rotating magnetic field caused the nanorod to rotate, its delay relative to the magnetism provided an estimate of the hemolymph’s ability to hold the rod back through viscosity.

They concluded that within seconds of leaving the body, the caterpillar’s hemolymph changes from a low-viscosity fluid to a viscoelastic fluid.

“A good example of a viscoelastic fluid is saliva,” Kornev said. “When you smear a drop between your fingers, it behaves like water: materials scientists would say it’s just viscous. But thanks to very large molecules called mucins in it, the saliva forms a bridge when you remove your fingers. So it’s just right it is called viscoelastic: viscous when sheared and elastic when stretched”.

The scientists further used phase and polarized optical contrast microscopy, X-ray imaging and materials science modeling to study the cellular processes by which hemocytes aggregate to form a crust over a wound. They did this not only in Carolina sphinx moths and their caterpillars, but also in 18 other insect species.

Hemocytes are key

The results showed that the hemolymph of all studied species reacted similarly to shearing. But its response to stretching differed drastically between the hemocyte-rich hemolymph of caterpillars and cockroaches on the one hand and the hemocyte-poor hemolymph of adult butterflies and moths on the other: the droplets stretched to form bridges for the former , but immediately broke for the latter.

“Turning the hemolymph into a viscoelastic fluid seems to help the caterpillars and cockroaches stop any bleeding, drawing the drippings back into the wound in seconds,” Kornev said.

“We conclude that their hemolymph has a remarkable ability to instantly change its material properties. Unlike insects and silk-producing spiders, which have a special organ for making fibers, these insects can make hemolymph threads in any place after injury.”

The scientists concluded that hemocytes play a key role in all these processes. But why caterpillars and cockroaches need more hemocytes than butterflies and adult moths is still unknown.

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

More information:
To seal a wound, caterpillars convert blood from a viscous fluid to a viscoelastic fluid in seconds. Limits on Soft Matter (2024). DOI: 10.3389/frsfm.2024.1341129. … fm.2024.1341129/full

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