Cassidine Design

Imagine you are a hungry ant out and about for an evening snack and you come across something shiny and colorful, yet kind of “turtle-ey”. It looks small and delicious, yet you can’t quite tell where it’s head or legs are, and can’t seem to pry the little sucker off it’s leaf! Now you are very angry. You go home and write bad poetry in your tiny journal.

What you have just experienced is an aptly-named Tortoise Beetle (Subfamily Cassidinae). Tortoise Beetles get their common name from the peculiar structure of their elytra (hardened wing covers), which have a flattened ridge outlining the body, concealing the head and legs much like a tortoise.When threatened tortoise beetles hunker down, tucking in their legs and head, forming a shield against many would-be predators.

The Cassidine Beetles belong to the Chrysomelidae, or leaf beetles, a huge family of beetles with over 37,000 species represented. They are worldwide in distribution but are particularly speciose in the neotropics (new world tropics). Here are some mighty fine Cassidines!

Omocerus casta coeruleopuncta, Polychalca variolosa, and Omocerus spp., respectively.

In a few species of tortoise beetles, live specimens differ drastically from their dried museum specimen counterparts. This difference is illustrated in the Golden Tortoise Beetle (Charidotella sexpunctata, previously known as Metriona bicolor). Nicknamed “goldenbugs”, these beetles can be found throughout North America on their favorite food plants such as sweet potato and morning glory. Let’s first take a look at a dried museum specimen:

Not terribly golden! However, take a look at a live specimen:

Gold-mania! The ephemeral nature of the golden tortoise beetle’s iridescence is actually caused by an optical illusion. Their transparent shell contains three tiers, each tier containing tightly packed layers covered in nano-sized grooves. When the nanogrooves are filled with liquid, they give the layers a smooth surface which perfectly reflects light like a mirror. With death and dessication this fluid and therefore iridescence is lost, revealing the brownish color of the bottom layer.

While alive, golden tortoise beetles can actually control the intensity of their iridescence by widening or contracting the spaces between the layers of cuticle and forcing liquid out of the nanogrooves. This change happens when they are disturbed by predators or agitated in some way. For example, here is what a golden tortoise beetle looks like when it is very very upset:

Can you see the rage??! This sort of rapid color-change is very rare in insects, but also occurs in the Panamanian Tortoise Beetle, and the grasshopper genus Kosciuscola (which changes color in response to ambient temperature). And if you are out collecting golden tortoise beetles keep in mind that they also change color depending on the availability of the liquid layer, so that in the fall and winter, the beetles become less lustrous and are more orange and bronze with flashes of iridescence.

Just one more reason insect design is endlessly amazing!

A big thank you to Jay Cossey for allowing me to use his image of an agitated golden tortoise beetle! More of his spectacular images can be found here (make sure to look at his invertebrate portraits and insect egg gallery) :



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