Written by Nancy Miorelli
Many butterfly chrysalises glimmer in the sunlight with golden studs or gold leafing. Rather than looking like something living, these delicate pupa decorate the surrounding foliage like miniature ornaments. Their beauty and stunning brilliance has been noted throughout the ages. In fact, word ¨chrysalis¨is derived from the Greek word ¨chrysos¨meaning ¨gold.¨
So how do we make these colors? It depends on the butterfly but it can either be a pure structure or a combination of both structures and pigments working together.
Studded in Gold
Let’s start with Monarch butterflies (Danaus plexippus) – the one in the question – before we move onto butterflies in the same subfamily (Danainae) and others. The crown of golden points – the diadem – doesn’t sparkle until after the first 24 hours of the formation of the chrysalis. The shimmer of the diadem suggests that it’s at least partially created by a structure. The fact that after the butterfly emerges from the chrysalis and the diadem appears yellow suggests that there’s a pigment at play as well. Let’s break this down into their components.
Pigments are molecules that absorb light and appear to have the same color regardless of which angle you view the object. In this case, the gold of the monarch butterflies is in part created by a series of pigments called carotenoids . You can see carotenoids for yourself, that’s what make tree leaves yellow, orange, or red in the fall. If you haven’t seen this phenomenon, then you’ve eaten carotenoids in carrots. The caterpillar gets its carotenoids from the plants it’s eating – which in the case of monarchs is in the milkweed family. If the caterpillars are fed an artificial diet lacking carotenoids then the would-be golden crown develops as silver in the chrysalis.
The sheen is created by a structure. While pigments work by absorbing light structures are complicated formations of exoskeleton and sometimes water or air that work by reflecting light. The change in viewing angles is what give the structure its glimmer. The monarch butterflies have a structure were exoskeleton is raised in the ares of the diadem to create little mountains which reflect light at the peaks to create the sparkle.
Some butterfly pupae are completely covered in a golden or silvery draping. Taking a picture of them is so difficult that you just end up taking a selfie every time. How are they able to create these glassy hues? In some cases, the same way – with metallic structures backed carotenoid pigments. However, pigments are not always needed!
This is where things get METAL. Literally. With Multiple Endocuticular Thin Alternating Layers (METAL). So lets break this down a bit.
Insects have an exoskeleton. But there’s two parts the outside (exocuticle) and the inside (endocuticle). The endocuticle is more flexible and less prone to drying out so here is where you can pack in all those little layers filled with fluid.
These silver and gold pupae are broad band reflectors – to achieve the silver or gold you need to reflect back a wide spectrum of light. This is different than those of say, the morpho butterflies or others that appear to reflect only a single color. To reflect a bunch of wavelengths back out, you first need to make sure that a lot of light can get into the structure. This is a pretty difficult thing to do. The structures that the butterflies use is this regular multilayered structure alternating between chitin and fluid filled spaces of different thicknesses. Silver and gold insects have pretty much all developed the same structure. You can shift from a golden hue to a silvery hue by changing the slopes and thicknesses of the structure but the generalized structure remains the same.
Isn’t that a question that has many hypothesis attached to it.
One idea is that it’s for camouflage. The shiny chrysalises might look like water droplets on leaves. Or, they just might be so shiny that they reflect their surrounding areas like a mirror. There might be some evidence for this. Usually Aglais (Small Tortoiseshell Butterfly) chrysalises are gold (as in the above image), but during an important stage in their development, if you expose them to a white background the chrysalises come out silver indicated that camoflauge might be a reason for their color. However, it has not been thorougly tested.
Many butterflies are toxic, so it might be a way to show aposematic or warning coloration However, the Golden Tortoise Beetle (Charidotella sexpunctata) will turn colors from gold to red if provoked. Red is more traditionally thought of a warning color so as to why the beetle would display both colors is up for debate. Maybe the glittering dance of the colors is a way to startle predators but without a detailed experiment to show one way or another, we can’t say anything for certain about its affect on predators.
Or maybe the whole thing is an accident. Maybe the gold helps protect the insects from otherwise harmful sun rays. If it doesn’t have a negative effect on the insect, it’ll just continue to stay in the population. While it seems unlikely, it can’t be ruled out.
In monarch butterflies and a few others the gold is created by a coupling of a carotenoid pigment and a hill-like structure that reflects light from the peaks. Other chrysalises and insects use a multi layered structure called METAL to reflect a wide spectrum of wavelengths to create the stunning hue. The problem is, we still don’t know why they do it.
Neville AC. 1977. Metallic gold and silver colors in insect cuticles. Journal of Insect Physiology 23: 1267-1274.
Rothschild M, Gardnier B, Mummery R. 1978. The role of carotenoids in the “golden glance” of danaid pupae (Insecta: Lepidoptera). Journal of Zoology 186: 351-358.
Steinbrect RA. 1985. Fine structure and development of the silver and golden cuticle in butterfly pupae. Tissue & Cell 17 (5): 745-762.
Steinbrect RA, Mohren W, Pulker HK. Schneider D. 1985. Cuticular interference reflectors in the golden pupae of danaine butterflies. Proceedings of the Royal Society B 226: 1244.