Why is the golden pupa golden?

Wow, golden legend!

What do you think of when you see the picture below? Is it a pendant made of gold, or a butterfly larva turning into a chrysalis?

Golden butterfly chrysalis

(Photo source: butterflyfarm.co.cr)

The answer may surprise everyone. This golden object is actually an insect pupa.

Perhaps many people would find it incredible that insect pupae could have such bright and domineering colors, as if they were coated with a layer of gold, shining, and even reflecting the surrounding scenery, making people marvel at the wonders of nature.

The eye-catching golden pupa

In fact, in nature, many butterfly pupae shimmer in golden color in the sun, and people have long noticed the beauty and amazing brilliance of insect pupae. The English word for insect pupa (especially butterfly or moth) is "chrysalis", which comes from the Greek "chrysos", which means "gold".

The golden-armored pupa in the picture above belongs to a butterfly called Tithorea tarricina, which belongs to the Nymphalidae family and is found in Mexico, Central America and South America, in lowland and hillside forest areas.

Compared with the domineering golden pupa, the adult of the Taxiaoxiao butterfly is a little bit "ordinary". The pattern of the adult's wings varies greatly. Usually the back of the forewings is black as a whole, decorated with white spots, while the hind wings are orange with black edges. Just looking at the picture of the adult, it would be difficult for everyone to connect the two.

Taxiao Xiaodie

(Photo source: butterflyfarm.co.cr)

In addition to the Taxiao butterfly, the pupae of some butterflies are also golden, such as the Idea leuconoe. The Idea leuconoe also belongs to the Nymphalidae family and is mainly distributed in Taiwan, Diaoyu Islands, Malay Peninsula, Indonesia, Japan, etc. The Idea leuconoe flies slowly and gracefully, often gliding and rotating in the air on air currents, so it is also called the "paper kite butterfly".

Large silk butterfly

(Image source: bramanswanderings.com)

The pupa of the great silk butterfly also shines with golden light, and there are many delicate black polka dot patterns on the pupa. However, as the pupa emerges as an adult, the empty shell loses its former luster and is no longer dazzling.

Golden chrysalis of the great silk butterfly

(Photo source: animal.photos)

Compared with the above two, the pupa of the monarch butterfly (Danaus plexippus) is much more low-key.

Despite the impressive name, the monarch butterfly's pupa is not golden all over, but has a "golden edge" on the top. Some people call this golden edge the monarch butterfly's "crown". In addition to the shining "crown", there are also some scattered "gold dots" on the monarch butterfly's pupa, which can be said to be both low-key and luxurious.

Monarch butterfly chrysalis

(Image source: bugguide.net)

After seeing so many golden insect pupae, are you curious about a question: How do butterflies dress their pupae in golden clothes?

The Secret of Turning Pupa into Gold

Different butterflies have different answers to the above questions.

Specifically, the monarch butterfly uses pigments to perform the high-end operation of "turning its pupae into gold", while the Taxiaoxiao butterfly relies on "structural color" to cover its pupae with golden armor. Therefore, although the pupae of both of them are golden, they have their own characteristics.

Let’s start with the “crown” on the monarch butterfly chrysalis.

As early as the 1970s and 1980s, scientists could not contain their curiosity and conducted relevant research on monarch butterflies. They found through observation that after the monarch butterfly pupae transformed into adults, the golden edges on the pupa shells still existed, and the golden color remained consistent no matter from which angle you looked at it. This phenomenon indicates that the golden color on the pupae may be caused by the accumulation of pigments.

The golden edges still exist on the chrysalis of the monarch butterfly

(Photo source: natureshealingimages.com)

So, in nature, what pigment is closest to this yellow? You may have guessed it, it is carotenoids, which are responsible for the leaves turning from green to yellow in autumn. Studies have found that monarch butterfly larvae obtain carotenoids from the plants they eat. When the larvae turn into pupae, the epidermis and cuticle at the "crown" position will accumulate more carotenoids, making these areas appear yellow. This color caused by pigment accumulation is called pigment color. Pigments cannot always exist stably, so after a period of time, the golden edges of the monarch butterfly pupa will gradually fade.

Therefore, when scientists fed the larvae with artificial feed that did not contain carotenoids, the pupae they transformed into lost the shining "golden edge". The dazzling gold on the pupae disappeared, replaced by bright silver.

It is worth mentioning that the "crown" on the monarch butterfly pupa usually starts to sparkle 24 hours after the pupa is formed. Scientists believe that carotenoids only give the "crown" its color, and the flash is because the pupa forms a special protrusion-like structure during the hardening process, which can reflect light and make the "crown" sparkle.

The protrusion-like structure on the monarch butterfly chrysalis

(Source: References)

Compared to the monarch butterfly, the Taxiaoxiao butterfly adopts a different strategy. They do not act as "carriers" of pigments, but rely on the delicate structure of the pupa surface to produce dazzling colors.

Everyone must have seen scarab beetles. Their shells are not as smooth as they look. Instead, they are composed of multiple layers of periodically arranged structures. These structures can cause optical phenomena such as scattering and diffraction, producing the so-called "structural color."

Golden Christmas Beetle

(Image source: biologiavida-oficial.blogspot.com)

The golden color of the pupae of butterflies such as the Taxiaoxiao butterfly does not involve the accumulation of pigments, but is produced by special structures in the cuticle. These thin cuticles are actually not one layer, but are composed of many dense layers (D layer in the picture) and transparent layers (C layer in the picture) stacked alternately. Light will reflect a metallic luster in such a structure.

Microstructure of cuticle of insect pupa

(Source: References)

However, when the pupa turns into an adult, the pupal skin will become dry and the C layer will disappear, so the pupal skin we see will become transparent and lose its former brilliance.

Having said that, the golden pupa does look good, but in our eyes, the gold color is too conspicuous. Wouldn’t this put the pupa in danger?

Actually, it is not true! The chrysalis hides the great wisdom of butterflies~

Wisdom in the Chrysalis

In our eyes, those successfully camouflaged insect pupae should blend perfectly into the environment. In fact, many butterfly pupae do this.

For example, the pupa of the Hebomoia glaucippe looks like a green leaf. If you don’t look closely, you can’t tell it’s a pupa.

Pieris crested butterfly pupa

(Image source: butterflycircle.com)

Some are disguised as leaves, and naturally some are disguised as tree branches. It has to be said that the pupa of the Swallowtail butterfly is almost integrated with the dead branch. If it weren’t for the "fuse" on the waist of the pupa, could everyone recognize that this is an insect pupa?

Swallowtail butterfly pupa

(Image source: butterflycircle.blogspot.com)

Compared to these masters of camouflage, how do those glittering insect pupae protect themselves?

There are generally two views on this issue. One view is that they are also camouflaged, and the shiny pupae are like a drop of water on a leaf or tree trunk. Or, the shiny pupae can reflect the surrounding scenery like a mirror, making it difficult for natural enemies to find them. Studies have found that the pupae of the nettle butterfly are golden under normal circumstances, but if their larvae are exposed to a white background at an important stage of development, their pupae will change to silver in response to the background.

Another view is that the golden background with black spots is often used as a warning color in nature. The pupae of butterflies like the Great Marionette butterfly have a yellow background with black spots. They may want to tell predators not to come near me.

The pupa of the great silk butterfly is really creepy after seeing it for a long time.

(Photo source: animal.photos)

So, what do you think is the more likely reason why the insect pupa is covered with golden armor?

(Note: Latin parts in the text should be italicized)

References:

Rothschild, M., Gardiner, B., & Mummery, R. (1978). The role of carotenoids in the “golden glance” of danaid pupae (Insecta: Lepidoptera). Journal of Zoology, 186(3), 351-358.

reflectors in the golden pupae of danaine butterflies. Proceedings of the Royal society of London. Series B. Biological sciences, 226(1244), 367-390.

Neville, AC (1977). Metallic gold and silver colors in some insect cuticles. Journal of Insect Physiology, 23(10), 1267-1274.

Steinbrecht, RA (1985). Fine structure and development of the silver and golden cuticle in butterfly pupae. Tissue and Cell, 17(5), 745-762.

Produced by: Science Popularization China

Author: EVEE (School of Life Sciences, Peking University)

Producer: China Science Expo