The Translucent Jewel Caterpillar

Posted on the blog of the Scientific American on May 9, 2012, this article documents the amazing Translucent Jewel Caterpillar. The orginal article is available at:

http://blogs.scientificamerican.com/observations/2012/05/09/the-biology-of-the-translucent-jewel-caterpillar-the-nudibranch-of-the-forest/

The Biology of the Translucent Jewel Caterpillar, the Nudibranch of the Forest

By Ferris Jabr | May 9, 2012 |  

Acraga coa larva (Credit: Daniel Janzen)

Yesterday, stunning photos of a semi-translucent, gelatinous caterpillarspread quickly across the Internet—probably setting a new speed record for larvae of all kinds. Scuba instructor and amateur wildlife photographer Gerardo Aizpuru spotted the creature in early April on a mangrove tree leaf near Cancun, Mexico. He submitted his pictures to Project Noah, a user-created database of geotagged wildlife photos, where various commenters identified the species as the larva of a fuzzy orange moth called Acraga coa.

The adult Acraga coa moth (Credit: Daniel Janzen)

Although it’s not 100 percent certain that the “jewel caterpillar” Aizpuru photographed is Acraga coa, it almost definitely belongs to the same family of moths, known as Dalceridae. Scientists have identified around 84 different species of Dalceridae moths, whose larvae are sometimes called “slug caterpillars” because they are so gooey. If you search for “Dalceridae” in Google Images, you’ll see different larvae with the same roly poly bug shape and gumdrop spines, but different colors and patterns. Dalceridae larvae reminded me immediately ofnudibranchs, a group of strikingly colored mollusks whose appearance is perhaps best summarized as “trippy.”

Unidentified Dalceridae larva (Credit: artour_a, flickr)

Nudibranchs and many other animals—including many caterpillars—use vivid pigments to advertise their toxicity and keep predators away. But so far biologists have not figured out why some Dalceridae larvae are so colorful. Daniel Janzen, a biologist at the University of Pennsylvania, has raised Dalceridae in captivity. He says that although the larvae tend to be bright and conspicuous and spend a lot of time walking on the tops of leaves—as though they did not fear birds and other predators—he has no evidence that Dalceridae are poisonous and he knows that they do not sting, unlike some of their cousins.

Acraga hamata larva, also of the Dalceridae family (Credit: Daniel Janzen)

Biologists do have some ideas about the function of larvae’s gumdrop spines, however. The glutinous cones break off extremely easily—one can gently tweeze them off or even pull them off by accident—suggestive of the way some lizards’ tails snap off in a predator’s mouth. Janzen says this trick might help the larvae escape from hungry insects and birds, but researchers have not yet confirmed this. In one telling experiment, however, Marc Epstein—an insect biosystematist at the California Department of Food & Agriculture—and his colleagues placed Dalcerides ingenita larvae in glass Petri dishes and introduced a few ants (Camponotus floridanus) to each dish. Many ant species devour caterpillars and other plump grub if they get the chance. Once inside the Petri dish, the ants inspected the larvae with their antennae, but most backed off without trying to take a bite. The few ants that chomped down got their mouths temporarily stuck in the larvae’s jelly coat or pulled away quickly and cleaned the gunk off their mandibles. In subsequent tests, Epstein found no evidence of toxic chemicals in the larvae’s goo, suggesting that it deters ants purely because of its stickiness.

Another Dalceridae larva (Credit: Daniel Janzen)

Perhaps “jewel caterpillar” is an apt name after all—you can look, but you can’t touch.

The Golden Tortoise Beetle

An amazing article from Becky Crew writing in the Scientific American Blog on the Golden Tortoise Beetle, which changes colour when mating! That first photo is remarkable! Just like a blob of molten gold! Be sure to check out the orginal article at:

http://blogs.scientificamerican.com/running-ponies/2012/07/04/glad-you-ditched-the-anal-fork-golden-tortoise-beetle/ 

Glad you ditched the anal fork, Golden Tortoise Beetle

By Becky Crew | July 4, 2012 |  

Charidotella sexpunctata Credit: Chimetsetan (projectnoah.org/users/chimetsetan)

This pretty little molten gold beetle has been doing the rounds of the Internet lately, because not only does it look like nothing else on Earth, but it can also completely change colours. And it’s just as pretty when it does.

This is golden tortoise beetle (Charidotella sexpunctata, previously known asMetriona bicolor), a tiny, metallic North American insect that belongs to the leaf beetle family, Chrysomelidae, which includes flea beetles, asparagus beetles and longhorn beetles. Nicknamed ‘goldenbugs’, golden tortoise beetles grow to around 5.0 to 7.0 mm in length and favour foods such as sweet potato and morning glory.

More than 30 years ago, golden tortoise beetles became the first known insect species with the ability to rapidly change colour during copulation. They also do it when disturbed or agitated by predators. Since then, other species of tortoise beetle, such as the Panamanian tortoise beetle (Charidotella egregia), have been found to do the same. Reversible colour change is extremely rare, but not unheard of in insects – in Australia, male members of the dragonfly genus Austrolestes and the grasshopper genus  Kosciuscola  can switch from black to a brilliant blue due to changes in temperature. What sets the colour-changing tortoise beetles apart, however, is that the colour change is controlled by them, in response to specific events in their environment, such as being poked by a curious human or stumbling upon a willing mate.

Publishing in The Coleopterists Bulletin  in 1979, professor of biology Edward M. Barrows from Georgetown University described the results of his investigation into the mating and colour change of the golden tortoise beetle. Barrows collected a bunch of tortoise beetles from Washington and housed them in petri dishes in his lab, feeding them, breeding them and observing their sexual habits. Not only did he find that golden tortoise beetle copulation could last anywhere between 15 to 583 minutes, but he also observed that they would change colour as quickly as two minutes into it. Those beetles that started off a brilliant gold would turn to a goldish orange with black spots and then to a brownish orange with black spots, and those that started out a duller orange would turn golden. The same changes occurred when Barrows gently applied pressure to the beetles when holding them between his fingers. Other reports have these beetles turning from golden to a shimmering red when copulating or agitated.

"Here's two of them f---ing", said the Internet last week. Credit: magickcanoe.com

The golden tortoise beetle, and some other species in its subfamily, is able to change colours due to an optical illusion. Barrows noted how previous observations of golden tortoise beetle colours from other scientsists were extremely varied, ranging from brownish and purplish to bright orange or gold. “Metriona bicolor sometimes looked greenish gold in the field, and highlights of its colouration are probably related to reflected light from its substrate and other nearby objects. Also, depending on the inclination of the line of vision, the color varies from gold to green or even blue,” he wrote in his paper.

Just how this illusion could be produced was discovered in 2007 by researchers from University of Numar in Belgium, who studied the very similar gold-to-red colour change in the Panamanian tortoise beetle. Using scanning and transmission electron microscopes, they saw that the transparent shell of these beetles contains a three-tier structure, each tier made up of several tightly packed layers covered in patches of nanosized grooves. The tiers run from thickest at the bottom to thinnest at the top, and beneath them sits a layer of liquid red pigment. When the nanogrooves are filled with the red liquid, they give the layers a smooth surface, which perfectly reflects the light to give the Panamania tortoise beetle its metallic golden appearance. When the red fluid is drained from the grooves, in response to a stimulus such as agitation or copulation, it “destroys the optical properties” of the shell, leaving an “unobstructed view of the deeper-lying, pigmented red substrate”, the researchers reported in Physical Review E . It’s likely the way golden tortoise beetles manage to change colour and appear differently depending on the light they are exposed to is related to how Panamanian tortoise beetles do it.

Not much is known about why these beetles change colour the way they do, but Barrows suggested it has to do with defence and/or sexual signalling. He suggested for the beetles that changed from dull and spotty to golden, they could be signalling to the opposite sex that they are ready to mate, as beetles in his experiment that were not mature enough to produce the golden colouration did not mate. He also suggested that the metallic quality of the gold could make them more difficult for birds to see, due to the glare. Barrows also suggested that for the beetles that changed from golden to orange and spotty, they could be mimicking ladybeetles, giving them a ‘safety in numbers’ form of defence, as birds can’t tell the difference. This was even true for the golden tortoise beetles that changed from gold to orange without any spots, because in some habitats, Barrows found them living side by side with the spotless ladybeetle (Cycloneda munda). When he tested a bird in the lab that had never been in contact with either insect before, the bird found the golden tortoise beetles to be delicious, but the spotless ladybeetles to be distasteful, so perhaps this colour change is an example of the Batesian mimicry we see in Heliconius numata butterflies , where one species mimics a different, bad-tasting species to fool predators into leaving them alone.

Oh and did I mention that golden tortoise beetles, as larvae, protect themselves by sticking old skin and faecal matter to their anal forks – otherwise known as faecal parasols – to form a shield? How did something with an anal fork get to be so pretty?

For similary pretty insects, check out Ferris Jabr’s post on translucent jewel caterpillars. They’re incredible.