#panarthropod

Onychophorans, also known as velvet worms, look almost like a cross between a caterpillar, a millipede, and a worm, but they actually belong to their own unique phylum.

Above: a velvet worm of the genus Eoperipatus

The velvet worm's fleshy antennae, chubby little feet, and gleeful expression might make it seem kind of cute, at least at first glance, but these creatures are ferocious predators that prey on terrestrial arthropods like crickets, cockroaches, and spiders. In order to immobilize their prey, they shower their victim with a remarkably strong and quick-drying adhesive.

Above: another Eoperipatus species

As this article explains:

The velvet worm, a squishy little predator that looks like the stretch-limo version of a caterpillar, has a whimsical MO: it administers death by Silly String.

In the leaf litter of tropical and temperate forests around the world, velvet worms stalk the night on dozens of stubby legs. The pocket-size predator—whose species range from less than half an inch to eight inches long—can barely see, so it bumbles around, hoping to literally bump into an edible bug such as a cricket or a woodlouse. When it finds one, the velvet worm uses nozzles on either side of its face to shoot jets of sticky slime at its victim.

Above: genus Peripatoides and genus Eoperipatus

Velvet worm slime is ejected as a liquid, but it rapidly hardens into a gel as it soars through the air, forming fibers that are as strong as nylon. The substance then solidifies into glassy adhesive fibers as soon as it hits the target, trapping the victim in an inescapable net.

The slime can hit its target from up to 1.5 feet away.

Above: close-up of a velvet worm spraying its slime

The slow-moving velvet worm then approaches its prey, pierces the victim's exoskeleton with a pair of blade-like jaws, uses its saliva to dissolve the insect's innards, and then quickly devours its meal.

Above: the velvet worm's horrifying face as it prepares to feed

Velvet worm slime is quick-setting, strong, dissolves in water, and can also be reconstituted into new fibers. The mechanisms that produce these properties were unknown until just 16 years ago, when an Australian scientist discovered that the slime contains "chaotic proteins:"

The proteins are loaded with amino acids that repel one another, and they’re short of the water-repelling ones that help other proteins to establish a solid core. Rather than folding, they adopt open and random structures that are extensively coated with water molecules. Their watery sheaths prevent the protein molecules from interacting with one another. They can only do so when the water disappears. And that’s exactly what happens when the slime hits its target.

Insects are covered in waxy, water-repellent shells, but the velvet worm’s slime contains fat and detergent molecules that break past this defence. These chemicals, and the sheer force with which the slime is shot, means that it spreads all over the victim. The insect’s struggles seal its fate by drawing the slime into threads. Spread over a large surface area, the water in the slime quickly evaporates, unsheathing the proteins and leaving them to mingle for the first time. They form tight chemical bonds with one another and the once-liquid slime hardens.

Above: Eoperipatus feeding on an unknown arthropod and Peripatoides suteri feeding on a harvestman

Velvet worms are some of the oldest terrestrial animals on Earth, dating back to nearly 540 million years ago, when most of the world's creatures were still confined to the oceans. They're older than dinosaurs, trees, sharks, and even horseshoe crabs.

Surprisingly, their morphology has changed very little in the last 400 million years or so -- their fossilized ancestors look remarkably similar to the velvet worms that are still roaming the earth today.

Above: assorted velvet worms

The world's smallest species of velvet worm is Ooperipatellus nanus, with a length of just 5mm (0.2 inches); the largest is Mongeperipatus solorzanoi, which can grow to a length of 22cm (8.7 inches). Velvet worms can have between 13 and 43 pairs of feet, depending on the species

Above: the tiny feet of a velvet worm

The term Onychophora literally means "claw-bearer," which is a reference to the hooked claw at the tip of each foot. Together, these claws allow the worm to travel across uneven terrain, but they can also retract as it moves onto smoother surfaces. When the claws are retracted, the worm simply walks on the stubby little pads of its feet.

Above: Epiperipatus barbadensis

Sources & More Info:

Phys.org: Velvet Worm Slime

National Mag Lab: Cracking the Chemical Code of the "Silly String of Death"

National Geographic: Scientists Uncover Secret of the Velvet Worm's Quick-Setting Slime

Scientific American: Velvet Worm Slime Reveals its Sticky Secrets

Journal of the American Chemical Society: Peculiar Phosphonate Modifications of Velvet Worm Slime Revealed

Onychophoran Website: Onychophora

ZooKeys: An Updated World Checklist of Velvet Worms

iNaturalist: Velvet Worms

Cambroraster falcatus was a radiodont that lived during the Cambrian, about 508-505 million years ago, in what is now western Canada. Part of the hurdiid family of radiodonts, it reached sizes of up to 30cm long (~12") – making it one of the largest animals known from the Burgess Shale fossil deposits.

Its distinctively-shaped massive head shield led to it being nicknamed "the spaceship" during collection and study of numerous fossil specimens. This was then referenced in its species name of "falcatus", inspired by its resemblance to the fictional Millennium Falcon from the Star Wars franchise.

Its short frontal appendages bore long curved spiny blades that formed a basket-like structure. Originally these were thought to be used to sift through seafloor sediment, but more recently they've been proposed as filter-feeding structures instead.

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🍬 Sour velvet worm 🐛 and Gummy water bear

Worm of the day: Velvet Worm from real life!

Retro vs Modern #09: Hallucigenia sparsa

If just one single species had to represent how our reconstructions of prehistoric animals can drastically change, it would have to be Hallucigenia sparsa.

1970s

First discovered in the 1910s in the Canadian Burgess Shale fossil deposits, specimens of Hallucigenia were initially categorized as being a species of the early polychaete worm Canadia. It wasn't until the 1970s that they were recognized as being something else entirely, and the first reconstruction of this tiny animal was bizarre.

It was depicted as a long-bodied creature with a single row of tentacles along its back, and several pairs of long sharp spines that were interpreted as being stilt-like "legs" used to walk. The tentacles were thought to catch food from the water and pass it forwards to the bulbous "head" – and at one point it was even proposed that all the tentacles had their own additional "mouths" at their tips!

It's easy to look back on this version now and laugh at how ridiculous and obviously wrong it was, but it's important to remember the historical context here. This was coming from a point when the incredible animal diversity of the Cambrian Explosion was only just starting to be understood, revealing a range of poorly-understood bizarre and alien-looking forms like Opabinia – "weird wonders" that were considered to be representatives of previously unknown ancient branches of life.

At the time, Hallucigenia's utter weirdness and impractical body plan seemed to almost make sense as a unique evolutionary "failed experiment" that had left no living relatives.

1990s

Discoveries of legged-and-armored lobopodian "worms" in the Chinese Chengjiang fossil deposits during the 1980s prompted a re-interpretation of Hallucigenia in the early 1990s. Speculatively reconstructing it as a lobopodian with the spines on its back and with the tentacles as a set of paired clawed legs started to make it seem a lot less alien and a lot more like a real velvet-worm-like animal – and just a year later the "missing" other half of the leg pairs was confirmed to be present in some of the fossil specimens.

But it was still unclear which end was actually the head, and whether the large blob-like structure was a real part of Hallucigenia's anatomy or just an artifact of the fossilization process.

2020s

New research in the mid-2010s finally settled the head problem and clarified a lot of Hallugicenia's anatomy, discovering that the slender elongated end had a pair of simple eyes and a mouth with a throat ringed with tiny teeth.

We now know Hallucigenia sparsa lived all around the world during the mid-Cambrian, about 518-508 million years ago, with body fossils known from Canada and China and isolated spines found in numerous other similarly-aged locations. Instead of an evolutionary dead-end "weird wonder" it was actually an early member of the vast arthropod lineage, just one of a highly diverse collection of successful Cambrian lobopodians, and its closest living relatives are probably velvet worms and tardigrades.

It grew up to about 5cm long (2") and had seven pairs of long sharp defensive spines along its back, covered with a microscopic surface texture of tiny triangular "scales". It had seven pairs of clawed walking legs, with most of its feet tipped with two claws each but the final two pairs having just one, and its body ended right at the final pair of limbs – the "blob" structure in some fossils was actually just an artifact the whole time, formed by Halligenia's innards being forcefully squeezed out during its burial in the seafloor sediment.

Its neck region bore three pairs of long delicate tendril-like limbs, which may have been covered in feathery hair-like structures for filter-feeding similar to some other lobopodians. A small pair of velvet-worm-like antennae may also have been present on its head, and could have been a sexually dimorphic feature.

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Radiodonts were early arthropods with specialized frontal appendages, disc-like mouths, complex compound eyes, and swimming flaps along the sides of their bodies. Once considered to be bizarre "weird wonders" of the Cambrian Explosion that represented a failed evolutionary experiment, we now know that they were actually a highly diverse and successful lineage that lasted for at least 120 million years.

While some radiodonts were the largest animals of their time periods, Stanleycaris hirpex here was one of the smallest known members of the group – although at around 10cm long (~4") it was still respectably big compared to most other Cambrian animals.

Discovered in the Canadian Burgess Shale deposits (~508 million years ago), it was originally known only from isolated frontal appendages and mouthparts, and had been assumed to be a fairly typical member of the hurdiid family. But the recent discovery of over 200 new fossils, including some exceptionally well-preserved full body specimens, has catapulted it directly from being poorly-known into now being one of the most completely known of all radiodonts.

And it had a very big surprise for us, right in the middle of its face.

It turns out that Stanleycaris had a huge third eye, unlike anything ever seen in a radiodont before. A large unpaired eye was also part of the five-eyed arrangement in opabiniids and Kylinxia, and finding a similar example in radiodonts too raises the possibility that this sort of well-developed "median eye" may have been more widespread in early arthropods than previously thought.

Along with the third eye, some of the Stanleycaris specimens preserve fine internal details of its nervous system and show that its brain was made up of two segments instead of the three seen in modern arthropods. It also had gills positioned on its underside, unlike most other radiodonts which had them on their backs.

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